Food composition data

Editing, design and production

by the FAO Publishing Management Service

The designations employed and the presentation of material in this information product do not imply the expression of any opinion whatsoever on the part of the Food and Agriculture Organization of the United Nations concerning the legal or development status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries.

ISBN 92 5 104949 1

E-ISBN 978-92-5-108624-7 (MOBI)

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© FAO 2003

First edition published in 1992
by Elsevier Science Publishers

Contents

Foreword to the first edition

Preface to the second edition

Preface to the first edition

Acknowledgements

Introduction

Chapter 1Food composition data and food composition databases

Chapter 2Initiation and organization of a food composition programme

Chapter 3Selection of foods

Chapter 4Selection of nutrients and other components

Chapter 5Sampling

Chapter 6Choice of analytical methods and their evaluation

Chapter 7Review of methods of analysis

Chapter 8Assuring the quality of analytical data

Chapter 9Conventions and modes of expression of food composition data

Chapter 10Guidelines for the use of food composition data

Chapter 11Quality considerations in the compilation of a food composition data base

Chapter 12Current needs and future directions

Appendixes

Appendix 1INFOODS regional data centres

Appendix 2Calculation of sample numbers

Appendix 3Methods of preparation of foods for analysis

Appendix 4Examples of procedures for the preparation of analytical samples

Appendix 5Calculations of fatty acids in 100 g food and 100 g total fatty acids

Appendix 6Calculation of the composition of dishes prepared from recipes

Appendix 7Essential book list for food composition databases

Bibliography

Foreword to the first edition

About 20 years ago, there was a recognition in Europe that real benefits would flow from coordinating the manner in which food composition tables were produced in the various countries of Europe. Subsequent development of computerized nutritional databases has further highlighted the potential advantages of working together. Such cooperation could lead to improved quality and compatibility of the various European nutrient databases and the values within them. This realization was one of the driving forces behind the development of the EUROFOODS initiative in the 1980s when those people in Europe interested in data on food composition began working together. This initiative received further impetus with the establishment of the EUROFOODS-Enfant Concerted Action Project within the framework of the FLAIR (Food-Linked Agro-Industrial Research) Programme of the Commission of the European Communities.

It was quickly recognized that the draft guidelines for the production, management and use of food composition data, which had been prepared under the aegis of INFOODS (International Network of Food Data Systems, a project of the United Nations University), would be especially applicable to the objectives of the Concerted Action. The guidelines have been written by two recognized experts. Many people associated with FLAIR EUROFOODS-Enfant have added constructive criticism and advice to that offered previously by those associated with INFOODS. Thus the guidelines are backed by a consensus in the community of those responsible for the production and use of food composition tables and nutrient databases.

I am sure that the book will be regarded by those concerned with the production and use of nutritional compositional data as a lighthouse on an ocean with poor visibility, many hazards and shipwrecks. It will provide invaluable light not only for people in Europe but also for those on other continents across the oceans.

Clive E. West

Project Manager

FLAIR EUROFOODS-Enfant Project

Wageningen, February 1992

Preface to the second edition

The first edition of this book was used extensively in training food composition analysts and compilers around the world, commencing with the first Food Composition training course held in Wageningen, the Netherlands, in October 1992. Five courses have been held subsequently in Wageningen and the course has also been transferred to developing regions including one each in Chile for LATINFOODS countries, Jamaica for CARICOMFOODS countries, Thailand for ASEANFOODS and SAARCFOODS countries, and three in South Africa for the countries of ECSAFOODS.

The use of the book in the United Nations University/INFOODS training courses revealed that changes were required to update the text and figures, in particular to make the book more user-friendly on an international basis. Increasingly, as time went by, the huge explosion in methods of analysis meant that the book was becoming rapidly out-of-date. Further, the establishment of food composition programmes around the world increased the pool of experience available. However, revision was not feasible as a commercial project. Although some tertiary courses, largely in the industrialized countries, were able to draw on the book in teaching, the prohibitive cost of the first edition meant that purchases of the book were mainly for libraries rather than by individuals or for local food composition programmes. When the first edition became out of print, the copyright reverted to the original authors.

In 2001, Dr Barbara Burlingame, the Director of INFOODS (Food and Agriculture Organization of the United Nations [FAO]), proposed a rescue package – which was taken up eagerly by the authors. The proposal was for the authors to revise and update the first edition in the light of the comments of trainees in the course over the previous decade, and to incorporate improved methods of analysis (while not excluding those older methods, which were still being used satisfactorily in those parts of the world where access to sophisticated and costly instrumentation was limited). It was also proposed that FAO make the print edition of the book available at an affordable price, oversee its translation into the main languages of the United Nations Organizations and, further, place the book on the FAO Web site for worldwide access. The authors were pleased to accept this proposal since the original concept of the book had always been wide availability at a price that placed it within the reach of students and workers, particularly those in developing countries.

The second edition was largely prepared by means of electronic communication interspersed with occasional face-to-face meetings to establish the roles of the authors and FAO and identify the new or revised material to include. David Southgate worked from a very large literature database compiled by Heather Greenfield for the period 1990 to the present, together with his unparalleled experience in the compilation of the United Kingdom tables and discussions with trainees in courses held in the Netherlands and other parts of the world, to collate the first comprehensive draft of the revised edition, which included particular sections drafted by Heather Greenfield and inputs from members of the INFOODS mailing list.

A meeting of the authors with Barbara Burlingame in Norwich, United Kingdom, made possible an extensive review of the text, particularly to incorporate elements required by FAO. The draft chapters were reviewed by experts and the final version for publication was prepared through a long process of careful checking and revision conducted by Heather Greenfield, Barbara Burlingame and Ruth Charrondiere (FAO), working in collaboration by e-mail correspondence and, where possible, consultation with all the original sources of information. Barbara Burlingame oversaw the preparation of the final text for publication in various formats at FAO.

As in the first edition, the personal perspectives and prejudices of the authors doubtless show through. We believe that there is no a priori method of obtaining compositional data without analysis. The book recognizes that analytical facilities and resources are limited in virtually all countries and that, at the same time, there is a large amount of compositional data in the literature, in both published and unpublished sources and in other databases. It is essential to make proper use of this material. The book therefore devotes a considerable amount of attention to the evaluation of this published material to ensure that it is of the appropriate quality to use in combination with directly analysed values. We trust that this book, used in combination with other INFOODS texts, will be a key to the improved quality of food composition data worldwide.

Preface to the first edition

In 1972 a working party of the Group of European Nutritionists met in Zurich (Switzerland) to consider the principles that should be used in preparing national tables of food composition. A small book based on a working paper for this conference and describing guidelines for the preparation of such tables was subsequently published (Southgate, 1974).

During those discussions it became clear that in the future more tables providing international coverage (e.g. for all of Europe) would be needed. Since then, widespread advances in computer techniques have made the creation of such international databases technically feasible; their development is impeded, however, by the variable analytical quality, the incompatibilities, and even the unknown provenance of existing compositional data. Furthermore, large areas of the world remain where little information on food composition is available.

In 1983 a conference was held at Bellagio (Italy), under the auspices of the United Nations University, to identify the tasks that needed to be carried out in order for internationally valid, consistent and usable food composition data to become available. During the discussions the creation of an International Network of Food Data Systems (INFOODS) was proposed (Rand and Young, 1983).

One of the first tasks for INFOODS was to revise and extend the earlier Southgate (1974) guidelines, which addressed issues relevant to the central problem of data quality and compatibility. Accordingly, one of us (HG) spent four months as an INFOODS Fellow working with the original guidelines’ author (DATS) at the Food Research Institute in Norwich (United Kingdom). This initial work, continued and completed by correspondence, drew information from production and management of food composition data in the United Kingdom and United States and from Australian experience of producing data. In January 1985, a partially completed version was reviewed by a working group in Washington, DC (United States). A revised version, prepared on the basis of this review, was reviewed again by a number of international authorities; their comments were used in the version prepared in 1986.

After reviews by experts in the computer field, and considerable inputs from participants in the FLAIR Concerted Action No. 12 EUROFOODS-Enfant Project, the final revised version was prepared by correspondence and meetings between the authors while one of us (HG) was a Visiting Scientist at the International Agency for Research on Cancer (IARC), Lyon (France) in connection with the Nutrition and Cancer Programme.

In preparing a document of this kind, personal feelings and prejudices inevitably emerge; they are the responsibilities of the authors alone, who nonetheless beg their readers to remember that these idiosyncrasies developed during lengthy consideration of nutritional compositional data, their production and use.

Acknowledgements

For the first edition

We are grateful to INFOODS (Dr N.S. Scrimshaw and Dr V.R. Young) for providing the initial impetus forthe project and for financial support which enabled its commencement. Thanks are also dueto Prof R.F. Curtis, AFRC Food Research Institute, Norwich (United Kingdom) for administrative support of the first phase of the project. In addition, thanks are due to the many people who contributed ideas, skills or information for the initial draft. They include: the INFOODS review committee members, N-G. Asp, R. Bressani, M. Deutsch, H. Herstel, J.C. Klensin, J. Pennington, W.M. Rand, R. Sawyer, W. Wolf, V.R. Young. In the United Kingdom: A. Broadhurst, D.H. Buss, J.R. Cooke, K.C. Day, R.M. Faulks, A.A. Paul, L. Stockley, G. Mason, E.M. Widdowson. In the United States: G. Beecher, F. Hepburn, J. Holden, B. Perloff, K.K. Stewart. In Italy: F. Fidanza, J. Perissé, W. Polacchi. In the Netherlands: R. Breedveld, A.E. Cramwinckel, M.B. Katan, M. van Stigt Thans, C.E. West. In Indonesia: D. Karyadi. In Thailand: A. Valyasevi, K. Tontisirin. In India: K. Pant, K. Doesthale, B.S. Narasinga Rao. In Australia: K. Cashel, R. English, G. Hutchison, A.R. Johnson, J.H. Makinson, A.S. Truswell, R.B.H. Wills, M. Wootton. In Sweden: Å. Bruce, L. Bergström.

We are particularly grateful to Dr C.E. West and the FLAIR Concerted Action No. 12 EUROFOODS-Enfant Project for enabling the completion and publication of this book and to Dr L. Tomatis (Director) and Dr E. Riboli (Head, Nutrition and Cancer Programme) of the International Agency for Research on Cancer for administrative support for completion of the book for publication. Our thanks are due to the participants of the FLAIR EUROFOODSEnfant Concerted Action for reviewing the final draft: A. Amorin Cruz (Portugal), W. Becker (Sweden), H.K. Hendrickx (Belgium), P. Hollman (Netherlands), M.T. Fernández Muñoz (Spain), I. Martins (Portugal), D.L. Massart (Belgium), M.L. Ovaskainen (Finland), A.H. Rimestad (Norway), I. Torelm (Sweden) and C.E. West (Netherlands). We are very grateful for their comments, which have been extremely valuable in preparing the final version. Thanks are due also to W. Horwitz for comments on Chapter 5. We also acknowledge the advice of J. Cheney, B. Hémon and M. Friesen (IARC).

For the second edition

The authors would like to express their deep gratitude to B. Burlingame, Director of INFOODS (Food and Agriculture Organization of the United Nations [FAO]/United Nations University) for initiating and resourcing the second edition under the aegis of FAO. They also acknowledge the work of B. Burlingame and R. Charrondiere (FAO) for revisions and updates to the manuscript.

For this edition, the authors and editors are grateful to the following people for their reviews: W. Schüep (Switzerland), H. Schonfeldt and L. Smit (South Africa), S. Gilani (Canada), P.J.M. Hulshof (Netherlands), A. Sinclair (Australia), P. Finglas (United Kingdom), H. Boon (Australia) and the members of the INFOODS Food Composition mailing list for their responses to surveys. We also acknowledge the work of G. di Felice (FAO) and S. Debreczevi (UNSW) for secretarial assistance.

Introduction

A knowledge of the chemical composition of foods is the first essential in dietary treatment of disease or in any quantitative study of human nutrition.

(McCance and Widdowson, 1940)

This statement is as true now as it was in 1940, when it formed the first sentence in the introduction to the book that has now evolved into the United Kingdom National Nutritional Database (Food Standards Agency, 2002a).

The source of information on the composition of foods was, traditionally, printed food composition tables; these are now being replaced by computerized compositional databases from which the printed versions are usually produced. The information is widely used in the health, agriculture and trade sectors.

The data are used in research studies of the effects of diets on health, reproduction, growth and development. Data are also used for devising diets with specific nutrient composition in clinical practice, in the formulation of ration scales and in the devising of emergency food supplies. Nationally and internationally, compositional data are used in the assessment of the nutritional value of the food consumed by individuals and populations.

The recognition of the involvement of diet in the development of many diseases (McGovern, 1977) has led to an expansion in the number and range of studies of the relationship between diet and health and disease, which has led to a greater focus on nutrient data. Willett (1998) has drawn attention to this and to the need for databases to be reviewed regularly: “Diets of human populations are extremely complex … Maximal insight into the relation between diet and disease will usually be obtained by examining diets both as constituents and as foods. Calculations of intakes of nutrients and other constituents require a food composition database that is complete and current.”

The evidence that has emerged from these epidemiological studies has led to a growth in the production of national and international guidance on choosing a healthy diet. Composition data provide the foundations for the development of education programmes on choosing healthy diets. As part of this guidance to consumers, many governments have implemented the nutrition labelling of foods. Some countries require the producers of food products to provide their own analytical data on the composition of their products.

However, in appropriate cases, most regulations allow the use of compositional data taken from an authoritative compilation, such as a national food composition database, as a substitute for direct analysis. This development has added a quasi-regulatory role to food composition databases and strengthens the need for maintenance of data quality in terms of both the representativeness of the samples and the quality of the analytical data.

Establishing the composition of foods often has advantages for the trade in foods because importing countries with nutrition-labelling regulations prefer (and may require) that imported foods conform to the standards expected of locally produced foods.

Computerized databases have substantial advantages over printed food composition tables: they can contain a greater volume of information and the data can be used in calculations much more easily. The information can also be reformulated in different ways relatively easily to accommodate the needs of different users.

These advantages of calculation from computerized databases are especially important for nutritional epidemiologists, who frequently have to work with very large numbers of subjects and a large number and variety of food consumption records.

The power of epidemiological studies can be greatly enhanced when they are implemented at the international scale. For this to be effective requires, first, compatible records of food consumption and, second, national databases that are compatible. Compatible in this context implies “capable of being used together”.

Achieving a worldwide system of compatible food composition databases lies at the heart of the INFOODS programme. INFOODS – the International Network of Food Data Systems – was established in 1984 on the basis of the recommendations of an international group, and it operates under the auspices of the Food and Agriculture Organization of the United Nations (FAO) and the United Nations University (UNU) (Scrimshaw, 1994). Its goal is to stimulate and coordinate efforts to improve the quality and availability of food analysis data worldwide and to ensure that anyone, anywhere, would be able to obtain adequate and reliable food composition data. It has established a framework for the development of standards and guidelines for the collection, compilation and reporting of food component data.

This book is a continuation of the INFOODS effort, building upon earlier books (Klensin et al., 1989; Rand et al., 1991; Klensin, 1992; Greenfield and Southgate, 1992). The principles and guidelines contained in this book are intended to aid individuals and organizations concerned with the construction of food composition databases. The primary objective is to show how to obtain information that will meet the requirements of a database system that is compatible with systems that have already been, or are being, developed worldwide.

The book focuses on the areas of information-gathering that are critical in determining data quality and must therefore be closely controlled.

It is important to recognize that the term “guidelines” is not used in a prescriptive sense but in the sense of the “principles” of preparing databases. These principles draw on and are a result of experience gained in the preparation of databases over many years and in different countries. The guidelines do not set out detailed sampling or analytical protocols but provide examples of approaches that have been used successfully. In many countries, the protocols that should be followed are set out within a legal framework that must, of course, be followed. However, by discussing and setting out the available options the guidelines may suggest where established programmes might be revised.

The nutritional and analytical sciences are developing continuously and these developments may indicate better approaches than those set out in these guidelines. It is expected that these principles will serve as a framework for the future development of food composition data programmes.

The structure of the book follows the stages in an idealized programme of work in preparing a food composition database. Chapter 1 describes the variety of uses of a food composition database that the compilers (those with executive responsibility for collecting and assessing the data to be used in the database and their presentation) have to meet. Chapter 2 describes the overall design of programmes for creating, or revising, a food composition database. Subsequent chapters deal with the selection of foods for inclusion (Chapter 3) and the selection of nutrients (Chapter 4). Chapter 5 describes the principles of sampling foods and Chapter 6 deals with the selection of analytical methods and their evaluation. Chapter 7 presents a review of the methods available for the nutrients, focusing on methods that have been shown to be compatible internationally. Chapter 8 describes the principles of assessing the quality of analytical data. Chapter 9 describes the presentation of data and the modes of expression that are central to producing compatible data. Chapter 10 discusses the compilation of data for inclusion in the computerized database. The processes and design of computerized systems for compositional databases lie beyond the scope of this book. Chapter 11 deals with the intrinsic limitations of nutrition databases that constrain their use. The chapter also provides guidance on the proper use of the food data. Finally, Chapter 12 discusses the future needs in the area of food composition.

Chapter 1

Food composition data and food composition databases

Early food composition studies were carried out to identify and determine the chemical nature of the principles in foods that affect human health. These studies were also concerned with the mechanisms whereby chemical constituents exert their influence and provided the basis for the early development of the science of nutrition (McCollum, 1957), and they continue to be central to the development of the nutritional sciences. Current knowledge of nutrition is still incomplete, and studies are still required, often at an ever-increasing level of sophistication, into the composition of foods and the role of these components and their interactions in health and disease.

Somogyi (1974) reproduced a page of the earliest known food composition table, dated 1818. Ever since, it has been customary to record food composition data in printed tables for use by both specialists and non-specialists. While printed tables will continue to be produced, computerized data systems have replaced them in some settings because of the ease with which data can be stored, and the facility with which the large amounts of data can be accessed and processed.

These systems are increasingly used to generate printed and computerized food composition tables and data files. Computerized and printed tables generally contain a subset of nutrients and foods and often no further documentation. A single computerized data system can generate a variety of tables and files, each containing specific subsets of numeric, descriptive and graphical information. Examples are the different user databases released by New Zealand (Burlingame, 1996).

Studies of the relationship between diet and health have led to increased interest in the range of biologically active constituents present in foods that accompany the nutrients, and data for these constituents are often required, as are data for additives and contaminants. A well-designed data system can accommodate non-nutrient data, although this should not detract from the primary objective of the database programme – the provision of data on the nutrient content of foods.

Methods of compiling food composition databases

Early food composition tables were based on analyses carried out in the laboratories of researchers such as Von Voit in Germany, Atwater in the United States of America and Plimmer in the United Kingdom (UK) (Somogyi, 1974; Atwater and Woods, 1896; Widdowson, 1974). Later, the United States moved towards compiling tables from scrutinized data produced by a number of laboratories. An element of this procedure was introduced into the UK tables, where the third edition of McCance and Widdowson (1940) included vitamin and amino acid values drawn from the literature. Southgate (1974) distinguished these two methods as the direct and indirect method of compiling tables. These methods, and other procedures for compiling food composition data, were described by INFOODS (Rand et al., 1991).

Direct method

The advantage of the direct method, in which all of the values are the results of analyses carried out specifically for the database being compiled, is that close control of the sampling, analysis and quality control procedures yields highly reliable data. Early UK food composition workers analysed different purchases of the same food separately, but without duplicate determinations, with the intention of gaining some limited information on nutrient variation in each food (McCance and Shipp, 1933). In subsequent versions of the UK tables, however, the various purchases of the food were combined, reducing costs and increasing the number of foods that could be analysed in a given period of time (McCance, Widdowson and Shackleton, 1936). Even with this procedure, the direct method remains costly and time-consuming, and imposes pressure on the analytical resources available in many parts of the world.

Indirect method

The indirect method uses data taken from published literature or unpublished laboratory reports. There is consequently less control over the quality of the data, which may be uneven. Great care must therefore be taken in their appraisal for inclusion in the database. In some cases, values are imputed, calculated (see below), or taken from other tables or databases, and it may be impossible to refer back to the original source; these values carry a lower degree of confidence. The indirect method is most commonly employed when analytical resources are limited, or the food supply is largely drawn from food imported from other countries where compositional data are available. Although the indirect method is clearly less demanding of analytical resources than the direct method, the level of scrutiny required often makes it time-consuming and costly.

Combination method

Most food composition databases nowadays are prepared by a combination of the direct and indirect methods, containing original analytical values together with values taken from the literature and from other databases as well as imputed and calculated values. This combination method is the most cost-effective and is particularly successful when staple foods are analysed directly, and data for less important foods are taken from the literature (including that from other countries, if necessary). However, minimization of the amount of imputed and calculated values in principle increases the reliability and representativeness of the database.

Types of food composition data

Food composition databases currently available contain compositional values of differing quality, reflecting the different ways in which they were obtained. If data are to be used internationally they must be of consistent and compatible quality so that they can be used in combination for collaboration between individuals and countries in nutritional research, nutrition education, food regulation, and food production and processing. Data types and sources can be identified in food composition databases by codes (USDA, 2003a; Burlingame et al., 1995a), as is done in many countries, and by reference (Wu Leung, Butrum and Cheng, 1972). In general order of preference, the sources of data are:

Original analytical values

These are values taken from the published literature or unpublished laboratory reports, whether or not they were from analyses carried out explicitly for the purpose of compiling the database. They may be assimilated into the database unmodified, or as a selection or average of analytical values, or as combinations weighted to ensure that the final values are representative. Original calculated values are included in this category (e.g. protein values calculated by multiplying the nitrogen content by the appropriate factor, or fatty acids per 100 g food calculated from fatty acid values per 100 g total fatty acids).

Imputed values

These data are estimates derived from analytical values obtained for a similar food (e.g. values for peas used for green beans) or for another form of the same food (e.g. values for “boiled” used for “steamed”). They may also be derived by calculation from incomplete or partial analyses of a food (e.g. carbohydrate or moisture by difference, sodium derived from chloride values or, more commonly, chloride calculated from the value for sodium). Similar calculations can be made by comparing data for different forms of the same food (e.g. “dried” versus “fresh” or “defatted” versus “fresh”).

Calculated values

These are values derived from recipes, calculated from the nutrient contents of the ingredients and corrected for preparation factors: loss or gain in weight, usually referred to as yields, and micronutrient changes, usually referred to as retention factors. Such values are only rough estimates, because the preparation conditions for recipes vary dramatically, such as cooking temperature and duration, which will significantly affect yield and retention. Another calculation method is the calculation of the nutrient values of cooked foods based on those of raw foods or foods cooked in a different way, using specific algorithms, retention and yield factors.

Borrowed values

These are values taken from other tables and databases where reference back to the original source may or may not be possible. Adequate reference to original sources is necessary to justify a borrowed value. In some cases, the borrowed values should be adapted to the different water and/or fat contents.

Presumed values

These are values presumed as being at a certain level or as zero, according to regulations.

Sources of food composition data

Foods are chemically analysed for a variety of purposes. Food composition databases rely on nutritional and toxicological analyses conducted by government, academia and industry to determine the potential contributions of foods to the diet, and to determine compliance with regulations concerning composition, quality, safety and labelling. Foods may also be analysed for the purpose of ongoing monitoring of the food supply (e.g. Bilde and Leth, 1990). All of these compositional studies produce data that can be considered for entry into a food composition database.

Nutritional evaluation of foods

In human nutrition studies, the composition of foods is investigated, ideally, in a research setting interacting with one or more other areas of nutrition research (Figure 1.1). The data are most useful when they represent foods in the forms generally consumed (see Chapter 5, Sampling).

In agriculture, factors such as disease resistance and yield, rather than nutritional value, have tended to dominate decision-making regarding policies and programmes. Similarly, in food technology economic considerations such as consumer appeal and profitability have been the major influences on product development. Attitudes are changing, however, and nutritional quality is now one of the factors considered in cultivar selection and the development of processed foods.

The production, handling, processing and preparation of foods profoundly affect their nutritional quality. Extensive literature covers agricultural practices (climate, geochemistry, husbandry, post-harvest treatments); processing methods (freezing, canning, drying, extrusion); and stages in food preparation (holding, cutting, cooking). Most nutritional studies in these areas, however, cover a limited range of nutrients (notably labile vitamins); very little information is provided on the broad range of nutrients (Henry and Chapman, 2002; Harris and Karmas, 1988; Bender, 1978; Rechigl, 1982). Nevertheless, data from these types of studies can often be useful in food composition databases, either as data per se, or in establishing relevant yield and retention factors for calculations (see Chapter 9).

Food regulations

Levels of certain nutrients, additives and contaminants in foods are monitored for several reasons. Some nutrients, for example, may react adversely under particular processing conditions, producing poor sensory quality or affecting the safety of the food (e.g. trans fatty acids). Labelling regulations also require prescribed levels of nutrients in specific foods (e.g. vitamins and minerals in fortified foods, polyunsaturated fat levels in margarine). Certain toxic substances are limited to prescribed levels and are monitored by government, industry and other laboratories. The nutrient content of manufactured foods is rarely made available in electronic format to compilers, and care must be exercised when compiling databases using information provided on food labels.

Management of food composition data

Food composition tables were, in the early development of nutrition, the major resource of food composition data; they are, however, constrained physically by the growing volume of compositional data, and their attendant documentation, or metadata. They are also expensive to update and thus older data can remain in use for longer than is desirable. The most significant disadvantage of food composition tables is that calculations made using the data they contain can only be made with considerable additional work. Computerized compositional databases do not suffer from these disadvantages and are often used instead of the printed tables as the primary sources of compositional data for foods. A comprehensive food composition database should be the repository of all numeric, descriptive and graphical information on the food samples.

This book is concerned with the production and assessment of food composition data intended for entry into a computerized database, but it is equally applicable to data intended for printed food composition tables, because the principles involved are virtually identical.

Food composition data can be managed at four different levels, which together provide an effective way of handling the data (Table 1.1). This approach has advantages for assessing the quality of the data. They form a sequence of stages.

Level 1: data sources

These are the published research papers and unpublished laboratory and other reports containing analytical data, together with their bibliographic references. Normally, the data sources are part of the reference database.

Level 2: archival data

These records (written or computerized) hold all data in the units in which they were originally published or recorded, and are scrutinized only for consistency as would be normal in the refereeing of scientific papers prior to publication. Foods should be coded or annotated to assist in identification, and values should be annotated to indicate unit, calculation, mode of sampling, numbers of food samples analysed, the analytical methods used and any quality assurance procedures in place. Any bibliographic references relevant to the data source are noted. At this stage it is possible to make a preliminary assessment of the data quality (see Chapter 8).

Such records should make it unnecessary to refer back to the original data sources whenever a query arises. Normally, the archival data are used in the preparation of the reference database.

Level 3: reference database

The reference database is the complete pool of rigorously scrutinized data in which all values have been converted into standard units and nutrients are expressed uniformly, but in which data for individual analyses are held separately. This database should include all foods and nutrients for which data are available, and provides links to sampling procedures and analytical methods, laboratory of origin, date of insertion and other relevant information, including bibliographic references to the data sources. The data will usually be expressed according to the conventions, units and bases adopted for the user databases (see Chapter 9).

The reference database will usually be part of a computer database management system, with computer programs or written protocols developed to calculate, edit, query, combine, average and weight values for any given food. It is from this database and its programs that the user databases can be prepared.

The database will be linked to records on analytical methods and records for other constituents, for example non-nutrient constituents such as biologically active constituents, additives and contaminants. Records of physical characteristics such as pH, density, non-edible portion or viscosity that are often collected in food technology papers should also be linked to the reference database. Conversion factors, calculations and recipes should also be stored.

Level 4: user database, printed and computerized tables

In general, the user database is a subset of the reference database, and the printed form often contains less information than the computerized form. Many professional users of food composition data would require the information recorded in the reference database, but most require only a database containing evaluated food composition data that, in some cases, have been weighted or averaged to ensure that the values are representative of the foods in terms of the use intended. Moreover, values for nutrients in each food may, if appropriate, be amalgamated (e.g. total sugars, ratios of the different classes of fatty acids) rather than shown as individual constituents. These databases may contain indications of data quality based on assessment of the sampling and analytical procedures.

These databases should include as many foods and nutrients as possible, with preference being given to complete data sets. Methods, sampling procedures and literature sources should be coded at nutrient level so the user can perform an independent evaluation or comparison with other databases. The data, of course, must be expressed in uniform, standard units (see Chapter 9). The defining feature of a user database may be considered as a database that gives one series of data per food item.

Simplified food composition database or tables

Simplified databases or tables can be produced from the main user database. In these, fewer nutrients are covered, and some reductions of food categories may be possible (e.g. for meat cuts data may appear only for “medium cooked,” omitting “rare” and “well cooked”). Values can appear as units per 100 g of food or per average serving, expressed in household units or portion sizes. Modified versions of the database can also be produced to assist manufacturers in food labelling. Various types of database or printed table can be produced from the same comprehensive database, ranging from a fairly extensive version for the professional user to a smaller version for consumers or for users involved in large-scale food preparation.

Special-purpose food composition tables and databases

Tables and databases restricted to selected nutrients can be produced for people with special dietary needs or interests (e.g. for diabetics, or for people with kidney disorders for whom a diet controlling protein, sodium and potassium is required, or for nutrition educators, or for people wishing to lose weight). Data may be presented per 100 g of food, or per portion size or common household measures. Such tables and databases might be produced showing foods with ranges of nutrients – high, medium and low levels, for example. Data could also be given in other useful units (e.g. sodium and potassium in millimoles for renal patients).

Types of food composition database programme

National

Ideally, each country should have an established programme to manage its own food composition data, the data being considered an important national resource, as important as any other national collection of data.

While the level of certain nutrients in some foodstuffs will vary little between countries (e.g. the amino acid composition of lean meats), other nutrients, even in foods that are available worldwide, will vary greatly because of differing cultivars, soils, climates and agricultural practices. Recipes for composite dishes with the same name vary between countries. Different technological practices are also used; flour, for example, is produced and used at different extraction rates and may be fortified to different levels with different nutrients (Greenfield and Wills, 1979). Some countries have unique foods, food products or processing procedures (Somogyi, 1974). For these and other reasons, it is essential to develop a national food composition database programme, and to ensure that such a programme draws on data from other countries only when those values are considered applicable to nationally consumed foods.

Although attempts are being made to develop common food standards (e.g. the Joint FAO–World Health Organization [FAO/WHO] Food Standards Programme, Codex Alimentarius (FAO/WHO, 2003a, b), differences in food descriptions will continue to occur between countries.

Regional

The preparation of regional food composition databases is of great importance. Many countries, particularly in the developing world, lack the resources needed for a full-scale national food composition programme, but share a similar food supply to that of neighbouring countries. Cooperation between United States government departments, the Institute of Nutrition of Central America and Panama (INCAP) and FAO has produced some early regional food composition tables for Latin America (Wu Leung and Flores, 1961), Africa (Wu Leung, Busson and Jardin, 1968), East Asia (Wu Leung, Butrum and Cheng, 1972) and the Near East (FAO, 1982). More recently, this cooperation with FAO/UNU/INFOODS has led to the publication of regional tables for Pacific island countries (Dignan et al., 1994), Latin America (LATINFOODS, 2000) and Southeast Asia (Puwastien et al., 2000).

Some countries are collaborating on food composition analyses among themselves – for example, those in the North European region and those in the South Pacific region (Becker, 2002; South Pacific Commission, 1982). Other regional programmes may be those serving participating countries in multicountry epidemiological studies (Slimani et al., 2000). Simplified national programmes can be derived from such international or regional programmes.

Criteria for a comprehensive food composition database

The current high level of interest in nutrition requires that food composition databases meet the following criteria:

1.Data should be representative

Values should represent the best available estimate of the usual composition of foods in the forms most commonly obtained or consumed. Ideally, some measure of variability in the composition of the food should be given.

2.Data should be of sound analytical quality

Original analytical data from rigorously scrutinized sources are the ideal. Values from other databases, and imputed or calculated data should be included only when original analytical data are not available or are known not to be of sufficient quality. High-quality analytical data are those produced by methods that have been shown to be reliable and appropriate to the food matrix and nutrient in question. These methods must be applied proficiently, and evidence of this proficiency is required to assure data quality. It is also desirable that the analyst and the laboratory satisfy criteria of good laboratory practice. Further, evidence is required to show that the food sample was representative and was collected and handled properly. However, for existing data, documentation on sampling, source or analytical method is often not available, at least electronically.

Chapters 5, 6, 7 and 8 contain more specific guidelines for sampling procedures, methods of analysis and assurance of data quality; these three areas should always be considered in determining the quality of analytical food composition data.

3.Coverage of foods should be comprehensive

The database should include all foods that form a major part of the food supply and as many as possible of the less frequently consumed foods. The selection of foods for inclusion in a database is discussed in Chapter 3.

4.Coverage of nutrients should be comprehensive

Values should be included for all of the nutrients and other components known or believed to be important to human health. National priorities regarding health will have a major role in deciding which nutrients should be included. The criteria for selecting nutrients to be covered are discussed in Chapter 4.

5.Food descriptions should be clear

To be easily identified, foods must be unambiguously named and described. (Food nomenclature is discussed by McCann et al. [1988]; Truswell et al. [1991]; Møller and Ireland [2000a, b]; and Unwin and Møller [2003].)

6.Data should be consistently and unambiguously expressed

The data should be unambiguous in mode of expression and consistent in the use of units, factors used in calculation, and procedures used in rounding values.

7.Origins of data should be provided at nutrient value level

Information should be given on the sources of the data, noting whether data are analytical, calculated or imputed, and, as appropriate, on the procedures of any calculation and imputation, and the methods of sampling and analysis. Confidence or quality codes for the values should also be supplied.

8.Tables and databases should be easy to use

In addition to having clear terminology and systematic expression, databases and computerized tables must be easily accessible and readily understood. Printed tables should be of clear legibility and manageable size and weight.

9.The content of different databases should be compatible

The descriptions of foods, modes of expression and derivations of values should conform as closely as possible to existing international standards (e.g. the INFOODS tagnames) and to other major comprehensive food composition databases. Scientific needs require computerized databases and tables to be constructed with a view to using them in combination with other such systems.

10.Database should have few missing data

It follows from the above that any food composition database or table should aim to have as few gaps as possible because missing data can significantly distort the resultant nutrient intake estimations. It may be better to include imputed or borrowed data, always clearly identified as such, than no data at all. On the other hand, practical considerations often dictate that an incomplete database or table be produced to meet immediate needs. Information besides nutrient data (e.g. data on toxic substances or additives), though useful, is not essential at this stage.

Uses of food composition data

Food composition data are used primarily for the assessment and the planning of human energy and nutrient intakes. In both cases, the approach is most useful when applied to groups rather than individuals. Assessment and planning can be divided into several subcategories for which the precise requirements of the database differ and for which additional information is required.

Assessment of nutrient intakes (nutritional analysis)

When the weights of consumed foods are known, food composition data permit the intake of each nutrient to be calculated by multiplying the weight of each food by the concentration of the nutrient in that food and then adding the results, according to the equation:

I = Σ (W_1C_1 + W_2C_2 + W_3C_3 + ........WnCn)

where: I = intake of the nutrient, W_1 = weight consumed of food 1, C_1 = concentration of the nutrient in food 1, etc.

Knowledge of nutrient intakes is required at several levels, as outlined below.

Individual level

A person’s nutrient intake can be calculated by the use of food composition data and food intake data (estimated from a dietary history or dietary recall or measured in a weighed intake study) (Cameron and van Staveren, 1988; Nelson, 2000). This information can show gross dietary adequacy or inadequacy, or dietary imbalance, and is important in the determination of dietary advice or in prescription of a therapeutic diet. The user must be aware, however, that due to the natural variability of foodstuffs, food composition data may not predict the composition of a single portion of any particular food with accuracy.

Group level

Foods consumed by populations can be measured by various techniques (Marr, 1971) and translated, by means of food composition data, into nutrients consumed. The results give one indication of the nutritional status of the group (Jelliffe and Jelliffe, 1989; Gibson, 1990) and may be used to explore the relationship of a diet to a variety of health indices – sickness and death patterns, growth rate, birth weight, measures of clinical nutritional status, physical performance, etc. Examples of groups usually studied in this way are:

a)physiological groups, such as growing children, pregnant and lactating women, elderly people;

b)socio-economic groups (e.g. racial, caste, income or occupational);

c)clinical groups, such as patients and healthy controls;

d)intervention groups, usually drawn from the preceding categories, which receive a dietary supplement or other programmes;

e)cohorts in epidemiological studies of diet and health (Riboli and Kaaks, 1997).

Data drawn from studies of groups are used not only for identification of nutritional problems and planning of nutrition interventions to counteract them; they can also be employed in research that seeks to identify nutrient intakes desirable for good health. The results of such studies may feed back into food and nutrition policy in the form of food supplement programmes for children, food stamps for low-income groups, dietary advice to pregnant women, preventive diets for reducing heart disease rates, etc.

National and international levels

National statistics for agricultural production, adjusted for exports, imports, non-food use and gross wastage, are multiplied by nutrient composition data and divided by the total population to produce estimates of gross nutrient availability per capita. These data permit an assessment of the gross adequacy or inadequacy of the national food supply and indicate shortfalls or excesses. Food monitoring systems (e.g. Bilde and Leth, 1990) can follow the consumption of desirable and undesirable substances over a period of years.

Data from individual nations can be assembled to give cross-national or worldwide pictures of food and nutrient availability; such data are used in formulating food and nutrition policy, in setting goals for agricultural production, in formulating guidelines for consumption and particular policies such as food fortification or food supplementation (Buss, 1981).

Internationally, this information has implications for trade and for the development of assistance policies. In research, comparisons of nutrient intakes of different countries, together with other epidemiological data, enable further elucidation of the role of dietary constituents in health and disease. At present, long-term changes in the food supply can only be monitored adequately by the use of up-to-date food composition tables and databases. For example, the fat and iron content of meat have been altered in Western countries by changes in methods of husbandry and butchering. Comparison of today’s cuts with those of ten years ago can be made by reference to past food composition tables (Vanderveen and Pennington, 1983).

Subnational and community levels

Similar calculations can be made to provide estimates of the distribution of nutrients within a country. These findings can indicate actual or potential nutritional problems. Such studies are often critically important for developing countries that have diverse geographical regions. Periodic surveys, as part of a full system of nutritional surveillance, can monitor nutritional change and the effectiveness of food and nutrition policies.

Planning, advising or prescribing food and nutrient intakes (nutritional synthesis)

The physiological requirements or recommended intakes of most nutrients have been estimated (e.g. FAO/WHO/UNU, 1985), and it is the job of the nutritionist to translate these requirements or recommendations into desirable food intakes, at varying levels of cost. Again, this task can be performed at several levels, as outlined below.

Prescription of therapeutic diets

A therapeutic diet must be nutritionally balanced and adequate while at the same time controlling the intake of one or more specified nutrients. The prescription of therapeutic diets, therefore, requires professional training and a detailed understanding of the composition of foods. Table 1.2 lists types of disorder that require therapeutic diets, together with the dietary components that must be controlled. Unfortunately, most available food composition tables and databases do not hold information on all of the components listed in Table 1.2, and primary data sources may have to be consulted to obtain the required information.

Planning of institutional diets

Food composition data are used to translate recommended nutrient intakes into cost-limited foods and menus. Large sectors of the population (e.g. military establishments, workplace cafeterias, hospitals, prisons, schools, day-care centres and hotels) are provided with meals in this way.

National food and nutrition policy

A national food and nutrition policy will often define goals for the intake of certain nutrients. These goals must be translated into food production targets for the agriculture sector or into food consumption targets for the market or the public health sector (e.g. through increased subsidy or promotion of certain foods).

Nutritional regulation of the food supply

Food regulators use nutritional data on primary foods or “traditional” food products as a reference point for desirable nutrient levels for processed and newly introduced foods. For example, consumers should be able to rely on a traditional dairy product having certain levels of calcium and riboflavin; new processing techniques should not significantly alter the essential nutritional quality of the well-recognized product. Similarly, a manufactured or fabricated substitute should provide the same nutritional value as the food it is intended to replace (Vanderveen and Pennington, 1983).

A food composition database can also provide a preliminary check on label information or claims. For example, a food may be advertised as high in nutrient X, and information on the composition of its listed ingredients will indicate whether that food product could be high in nutrient X without fortification (for which special regulations may exist). Further, data on “new” cultivars being evaluated for widespread commercial introduction can be compared with data for traditional cultivars.

Some countries permit the nutrition data used in labelling certain composite foods to be calculated from nutrient data for ingredients taken from food composition tables and databases. In such cases, it must be ensured that nutrient values from the food composition tables and databases are comparable with those of the food regulations concerning food labelling.

Planning of nutrition intervention programmes

Nutrition interventions, such as food aid programmes, supplementation schemes and disease prevention programmes, require the use of food composition data in order to translate specific nutrient needs into food requirements. Note that such programmes may require confirmation by direct analysis, particularly at the research level.

Limitations of food composition databases

The limitations of food composition tables or databases are often not sufficiently understood by many users. Foods, being biological materials, exhibit variations in composition; therefore a database cannot accurately predict the composition of any given single sample of a food. Hence, although food composition tables and databases can be used to devise a diet, meal or supplement, the levels of nutrients are essentially estimates. For metabolic studies a direct analysis is usually necessary to obtain the required accuracy in the measured intake of the nutrients being studied.

Further, food composition databases and tables are limited in their usefulness for regulatory as well as scientific purposes. They cannot predict accurately the nutrient levels in any food; this is especially true for labile nutrients (e.g. vitamin C and folates) or constituents added or removed during food preparation (fat, moisture). Furthermore, the composition of a given food may change with time (e.g. a manufacturer’s formulation may change) invalidating the use of the values in the database. Predictive accuracy is also constrained by the ways in which data are maintained in a database (as averages, for example).

Food composition databases frequently cannot be used as literature sources for comparison with values obtained for the food elsewhere. Values from one country should be compared with values obtained in other countries by reference to the original literature. Food composition databases can be used more confidently when the values are known to be based on original analytical values. Any imputations, calculations, weightings or averaging must be clearly documented and, most important, food items must be adequately described to enable comparisons to be made.

It seems that, despite major efforts during the past 20 years on harmonizing food descriptions, nutrient terminology, analytical methods, calculation and compilation methods, values from existing food composition tables and databases are not readily comparable across countries. In addition, users may not always be aware of the difference in nutrient values between raw and cooked foods and might erroneously use the values for raw foods in place of those for cooked ones. This is often the case in countries using food composition tables that contain mainly raw foods.

Finally, there has been an increase in the consumption of manufactured foods and mineral and vitamin supplements, accounting for up to 60 percent of the total food intake, but these are rarely listed in food composition tables and databases (Charrondiere et al., 2002). As a result, it can be assumed that nutrient intake estimations are increasingly unrepresentative of the actual nutrient intake.

Users

The users of food composition tables and databases vary greatly: economists, agricultural planners, nutritionists, dietitians, food service managers, food and agricultural scientists, manufacturers, food technologists, home economists, teachers, epidemiologists, physicians, dentists, public health scientists, non-specialist consumers and journalists. Access to different types of computerized tables and databases is required to suit these differing needs; this is now achievable due to the availability of computers.

Chapter 2

Initiation and organization of a food composition programme

Over the last decade food composition activities have increasingly been undertaken by a variety of agencies, programmes, projects and people, for an ever-growing number of reasons. Many national, regional and international agencies acknowledge the importance of food composition data and the need to interchange information that is unambiguous and useful to all those who need it (Rand and Young, 1983; Rand et al., 1987; West, 1985; Lupien, 1994).

The creation of a food composition database calls for an integrated approach to the generation, acquisition, processing, dissemination and use of food composition data.

International level

INFOODS, the International Network of Food Data Systems, was established in 1983 by the United Nations University (UNU), with an organizational framework and international management structure that included a global secretariat and regional data centres. Its mandate is “to improve data on the nutrient composition of foods from all parts of the world, with the goal of ensuring that eventually adequate and reliable data can be obtained and interpreted properly worldwide” (INFOODS, 2003). In the mid-1990s, FAO joined UNU in the INFOODS effort. The main activities of INFOODS at the international level include development of technical food composition standards, assistance to regional data centres and individual countries in developing their food composition activities, and publication of the Journal of Food Composition and Analysis (Elsevier, 2003).

Most countries in the world participate in international fora and are signatories to international agreements that directly and indirectly relate to food composition. The World Declaration and Plan of Action for Nutrition adopted at the International Conference on Nutrition (FAO/WHO, 1992), the Rome Declaration on World Food Security and the World Food Summit Plan of Action (FAO, 1996), and the World Trade Organization’s Agreements on Sanitary and Phytosanitary Measures and Technical Barriers to Trade (WTO, 1998a, b) are examples of such agreements.

Regional level

Currently, there are 17 regional data centres in operation (see Appendix 1). Regional food composition tables have been prepared, both electronically and in printed form (Dignan et al., 1994; de Pablo, 1999; Puwastien et al., 2000), and many regions undertake regular food composition coordination activities and have established technical task forces that involve individual countries in the region.

National level

Most countries now undertake activities relating to the production of food composition data. A national food composition programme is usually the result of the combination and coordination of activities, within a defined administrative framework, related to food composition data generation, compilation, dissemination and use. Many countries have established a steering committee to facilitate such a framework. A steering, or advisory, committee is ideally composed of individuals directly involved in food composition work, that is, the data users, generators, compilers and disseminators. The involvement of data users – agriculturalists, analysts, health professionals, dietitians, nutritionists, food industry personnel and consumer groups – is crucial to the effectiveness of a steering committee.

Often a single organization has overall responsibility for the management of a national food composition programme, yet it is rare that a single organization accomplishes all the activities itself. Regardless of their affiliations, laboratory-based data generators must interact closely with the data compilers, and compilers must interact closely with data users. Data compilers therefore serve the central function and usually also act as data disseminators (i.e. they publish the data, electronically and/or as printed tables). In most countries there also exist other agencies whose activities are directly or indirectly related to food composition data, and who operate in concert with the national programme. National food composition programmes also operate in conjunction with their regional data centres and with ongoing international activities.

The organizational framework of a national programme will depend on the policies and procedures already being followed in the country or region where the programme is being established. Indeed, the national food and nutrition policy of the country concerned may already favour the establishment or updating of a food composition database (e.g. Langsford, 1979); any new programme should generally aim to fit into the framework of the existing national policy.

Many countries will already have experience in the production of food composition data and their use in tables. In developing a database programme, the aim should be to build on this experience. Existing data on foods with known, relatively stable composition can be used in the new database, provided that these data are evaluated and meet the criteria for inclusion.

Programme initiation

A decision to embark on the production or revision of a food composition database may be made by government, or within a research institute or department, by professional groups of users (e.g. dietitians, epidemiologists) or, occasionally, an individual researcher.

The advocacy for newly establishing or revitalizing a database programme can effectively be presented in different ways:

a)a carefully researched document, submitted to a government department or committee by professional or scientific societies or by influential individual scientists;

b)published articles in local scientific or medical journals;

c)a conference or session at a conference, culminating in official resolutions addressed to a government committee, department or other authority;

d)production by users or analysts of an unofficial set of food composition tables or a computerized database;

e)establishment of a formal or informal committee, with representatives from all interested parties, to start up a programme.

Any submitted document should emphasize the potential benefits of such a programme, especially in terms of community health and welfare, national esteem and economic benefits accruing through reduced health costs and advantage to the food industry, agriculture and trade. The availability and usefulness of any existing data and resources should be stressed. In addition, cost estimates that take into account the costs of administration, analyses, data management and data dissemination will be required.

Objectives of a food composition database programme

Any group or individual with responsibility for a database programme should pursue the following objectives:

1.produce a system that meets the multiple needs of users in different sectors;

2.work in the most cost-effective manner possible, within a specified time;

3.maintain full and regular consultation with all interested parties to ensure acceptability of the final product;

4.provide for continuing revision or updating of the data system and for periodic revision of any derived database or tables, according to a specified timetable;

5.publicize the programme widely to ensure that the database and its outputs and updates are widely disseminated and adopted into use;

6.provide for continuous access of all users to the database and related products.

Definition of users’ requirements

A food composition database should be defined by the uses for which it is intended. Because such a database is essentially a tool for nutritional work in the widest sense, it must be designed with all immediate and proposed uses clearly defined, and potential users must play a major role in its design.

Three aspects are fundamentally important:

a)the selection of foods to be included (see Chapter 3);

b)the selection of nutrients for which values are required (Chapter 4);

c)the modes of expression to be used (Chapter 9).

When a governmental committee decided to revise the database presented in The composition of foods (Paul and Southgate, 1978), a steering panel was set up to define the requirements of users. The panel consisted of users (government departments, dietitians and research nutritionists) and compilers, as well as the person in charge of the analytical work and those responsible for the design of the computerized database. The steering panel consulted major users of the existing tables (dietitians, researchers, food industry) by questionnaire (Paul and Southgate, 1970) and in personal discussions, and invited comments by advertisements in the scientific and food press. The compilers collated this information and used it to plan the revision.

A user questionnaire was also used in the early stages of the Pacific Island Food Composition Programme (Bailey, 1991). Other methods for obtaining suggestions from users are to hold a public meeting (Greenfield and Wills, 1981) or national conference (Food and Nutrition Research Institute/National Research Council of the Philippines, 1985), or to solicit submissions from scientific societies (Bernstein and Woodhill, 1981).

Users’ contributions to the programme should be continuous, to ensure that the database is both relevant and practical. It may therefore be useful for professional associations of users (or a consortium of them) to form a committee that would continue to supply information and monitor the programme. Including a session or workshop on the subject at an annual national or regional nutrition conference (e.g. the Sociedad Latinoamericano de Nutrición conference), or holding food composition conferences of the type held annually in the United States (USDA, 2003b), may be useful as a forum for this purpose.

This overall strategy in the design of a database programme and definition of users’ requirements is illustrated in Figure 2.1.

Stages of the programme

The stages of an ideal food composition database programme are set out in Figure 2.2. Funding must be obtained and procedures established for communication between all relevant parties. All existing food database programmes and facilities in the country should ideally be coordinated, because much of the analytical work can be done cooperatively by government, research institutes, or industry laboratories working in food research or related fields. Facilitation of this collaboration should be an early, important priority.

Obviously, a budget will have to be drawn up; Box 2.1 lists the various items that need to be provided for.

Reviewing, collecting and compiling existing information

Usually, information on the composition of locally available foods already exists, even in countries that have no formal national tables of food composition. The first stage is therefore to evaluate this information, both published and unpublished, for its suitability as data sources (see Chapter 10 for the principles guiding this evaluation). Consideration of user requirements reveals what new information is required, and proposals for new sampling and analytical programmes are made. In most countries it is necessary at this stage to define priorities; this will require further input from the users of the data system.

Sampling and analytical programmes

Sampling and analysis should be considered together, both to ensure data quality (Chapters 5, 6, 7 and 8) and also because the resources required for sampling and analysis need to be estimated together.

In developing the sampling plan and protocols (Chapter 5), a considerable range of inputs is essential, and the compilers need to consult widely. If, as happens in many countries, part of the programme is assigned to a contractor, the compiler must ensure that the contractor is aware of user requirements and the quality standards that have been set for data entering the system.

Sampling and analytical programmes are most conveniently focused on specific foods or groups of foods. This focus on specific foods is also useful in defining the experience required of groups invited to tender contracts. This stage is shown schematically in Figure 2.3. The proposed time scale for the work will determine resource requirements, and logistical factors need to be considered carefully. Once these factors have been assessed it is possible to estimate the costs of the different sections of the programme and submit a budget for approval.

Analysts must plan carefully to ensure that a balance is kept between personnel, laboratory space, equipment, running costs, and so on. Analysts preparing budgets or submitting contract proposals should highlight funds needed for meeting any specific requirements for their laboratories, as it is unlikely that any laboratory will already be ideally suited to carry out the work. Budgetary considerations will vary from country to country. Where labour is expensive, investment in automated equipment may be most advisable. Where labour is inexpensive, more staff can be employed. Wet chemical methods may be more appropriate if it is difficult to service and obtain parts for instruments.

Tasks in addition to chemical analyses include the regional collection of foods, determination and preparation of edible portions of foods, estimation of serving sizes and consideration of cooking methods (see Chapter 3). Groups with the appropriate technical facilities can carry out this work separately from the analytical programme, if necessary.

Supervision of the analytical programme

In principle, the concept of data quality is built into the analytical procedures (Chapters 7 and 8), and the users’ steering group will ensure that the analysts are aware of the detailed requirements of users. Nevertheless, it is useful to review analytical programmes regularly to reinforce the overall objective of the analyses – the construction of a food composition database for many different types of user.

Conversely, analysts should keep the users’ steering group informed of both the limitations of, and improvements in, analytical methodology, in order to ensure that the group works with realistic expectations.

Arrangements must be made for regular reports from the analytical laboratories. Requirements for reports must be carefully specified so that all analytical data are provided. For example, a protein value alone should not be accepted if the method used was nitrogen (N) determination. In this case, the N value and the factor used or suggested by the laboratory should be provided along with the calculated protein value. Units and rounding criteria must also be specified for reports. Policies must be established regarding the publication of laboratory results before their release in the food composition database. It is generally desirable for the work to be published independently so that the scrutiny of referees will strengthen its scientific validity.

Evaluation of analytical reports

Data provided by the analytical laboratories are subjected to initial evaluation (Chapter 9), ideally in discussion between compilers and analysts, to ensure consistency. Difficulties that may have arisen during the execution of the work can also be discussed at this time. Inevitably, problems will have required those involved in sampling or analysis to depart from the formal protocols. It is vital that the compilers be fully aware of such changes.

Compilation of the reference database

Once sufficient information has been accumulated, it is desirable to initiate reviews by the users’ steering group and by external specialists in the relevant commodity or food. The users’ review provides an assessment of whether the objectives defined by the users are being met; furthermore, it provides a means of managing the progress of the programme.

The external review serves as a conventional peer review and ensures that the data being acquired are compatible with specialized knowledge (which may not be nutritionally oriented) regarding the commodity or foods. Where proprietary products are involved it is desirable to submit the data to the manufacturer for comment. This step will identify inconsistencies with the manufacturers’ quality-control data and will indicate whether the food samples analysed were representative of normal production.

Compilation of a user database

The compilers should work closely with the users’ steering group. A review by users of sections of the database as they are prepared is highly desirable. These reviews enable users to alert the compilers to problems regarding format, user-friendliness and adequacy of data, and enable the compilers to alert users to problems of inadequate data or to indications that further analytical work is needed. As the database nears completion, pilot trials of its operation become desirable. These trials can be organized through the users’ steering group.

Operation of the database

Maintenance

Once the database starts to be used, a series of operational studies is desirable. Although studies designed specifically to test the database are valuable (see Chapter 10), the real tests come with regular use, and provision should be made to collect and collate information on difficulties or inconsistencies encountered by users. Errors must be centrally recorded so that the database can be corrected. It is especially important that the database maintenance be seen as a continuous operation.

Updating

It is also desirable to establish a permanent users’ group, familiar with the programme’s original criteria, which will periodically consider extension and revision of the database.

Continuous or periodic revision is essential for several reasons. The level of a food’s consumption can change, particularly with the appearance of “new” foods (e.g. instant noodles). The nutritional quality of a traditional food may also change (e.g. changes in animal husbandry and butchering affect the fat content and micronutrient quality of meats). New methods for preparing convenience foods may have striking effects on a food’s nutrient composition (e.g. extruded potato-based snacks, depleted of vitamin C) or on its nutritional consequences for sensitive individuals (e.g. the swing towards fructose syrups and sweeteners). Moreover, in addition to changes in foods themselves, advances in analytical methodology may indicate a need to re-analyse foods for a particular nutrient. These trends necessitate continuous nutritional monitoring of the food supply (Paul, 1977) and indicate that a database should be revised from time to time or on a continuous basis. The advent of computer database systems simplifies, in principle, the continuous updating of a database and periodic production of derived databases or tables.

Copyright and other conventions

In view of the fact that copyright and intellectual property legislation varies from country to country (Ricketson, 1995), database compilers will need to familiarize themselves with the national and interna-tional provisions and abide by them. Such provisions may include the need to seek permission to use the data, the format of acknowledgement required and the payment of a royalty. Further, normal scientific conventions should be followed regarding the acknowledgement of all data sources so that users can refer directly to the original source.

The organization responsible for the food composition programme, with the endorsement of the national steering committee, will generally publish the food composition data in various printed and electronic forms, and may charge users for the material cost of the publications. The USDA National Nutrient Database for Standard Reference (USDA, 2003a) is an example of a database that is freely available in the public domain. At the same time, provision should be made for licensing the data for commercial users (Greenfield, 1991b), such as diet analysis software developers, who may then on-sell their product with the data.

Overview of programme structure and organizational requirements

The schematic outline of the programme in Figure 2.4 shows the organizational elements of a food composition database programme and some of the responsibilities of each component. The whole programme requires communication back to the higher level and, indeed, constant interaction as proposals are made, priorities established, work designed and executed, and the final product reviewed. The compilers form the executive members of the programme, ensuring that objectives defined by the users’ steering group are met and that quality is maintained.

In practice, the compilers may be several individuals, each responsible for a single area (e.g. literature review, supervision of analytical programmes or data on certain nutrients, commodities or foods). If resources permit this division of labour, which enables specialized knowledge to develop, it is essential to have a good line management so that the senior compiler has a clear overview of the work as a whole.

Continued interaction with the relevant regional data centre is usually helpful in ensuring that standards are maintained and that data are compatible.

Chapter 3

Selection of foods

Most users of food composition databases would like them to be “comprehensive”. The objective of the food composition programme is to ensure that the database includes a range of food items that covers as completely as possible the foods eaten by the population for which the database is being prepared. However, the ideal of a truly “comprehensive database” is, in fact, an impossible objective, primarily because of the very large number of foods forming the human diet, especially if one includes all possible variations in the range of cooked mixed dishes. The continuous development of new food products by the food industry and new plant varieties and animal husbandry techniques by the agricultural industry means that analysts and compilers are aiming at a constantly moving target. The volume of analytical work required for comprehensive coverage and the resource implications of this work also make it impracticable. Therefore, those involved in the food composition programme – through a national steering committee or other consultative means – have to develop a strategy for establishing priorities in selecting food items for inclusion.

The approach described below is suitable for use in preparing a database de novo. In practice, however, this is very rare because most countries or regions have some existing information available in the form of food composition tables or a computerized database. However, the strategy suggested is equally valid for use in the revision or extension of existing information.

Establishing priorities

A range of different sources of information needs to be considered when establishing priorities. These are summarized in Figure 3.1 on page 34.

Food consumption statistics

The ideal is, first, food consumption statistics. Foods that are most commonly consumed in terms of both frequency and amounts consumed, provide a list of “core foods”. In identifying these foods it is necessary to look beyond the statistics for the total population to the consumption patterns of specific subgroups, particularly infants and those with specific dietary requirements. Within the population, ethnic groups with distinctive dietary patterns also need to be considered, as do different socio-economic and regional groups. Data at the commodity level are available from the FAO Statistical Databases (FAO, 2003), and data from household or individual surveys are often available from government ministries of statistics, health or agriculture.

Nutrient contributions

The food consumption statistics should then be used to estimate the nutrient contributions of the different foods (Chug-Ahuja et al., 1993; Schubert, Holden and Wolf, 1987).

The USDA has developed a procedure using food consumption data and nutrient values for developing the Key Foods list (Haytowitz et al., 1996). Key foods have been defined as those foods that contribute up to 80 percent of any one nutrient. When total nutrient contributions from the key foods are aggregated, they should account for approximately 90 percent of the nutrient content of the diet for the nutrients examined. This method utilizes existing nutrient profiles and nationally representative data collected from food consumption surveys. More samples are collected and prepared for foods that provide important amounts of nutrients of public health significance to the diet, and not every sample is analysed for all the nutrients currently in the database (Haytowitz, Pehrsson and Holden, 2000). This key-foods approach forms the core of the current USDA nutrient analyses contracts (Haytowitz, Pehrsson and Holden, 2002), and many other countries are adopting this method (Galeazzi et al., 2002).

Nutrients of public health significance in the country

The contribution to energy intakes should be the first to be examined; this establishes the foods that may be considered as the staples in the diet. Other nutrients should be examined in a sequence related to their public health significance. In some countries, protein would be considered next; in other countries, the preferred focus would be the nutrients that are not evenly distributed in foods, for example vitamin A (retinol), vitamin C, iron and calcium. Where iodine deficiency is a public health issue, sources of iodine will need to be included. Vitamin A deficiencies would indicate the need to consider foods that are rich in provitamin carotenoids in addition to sources of retinol. The numbers of additional foods will progressively be reduced using this sequential key-foods type of approach.

Trade and economic factors

The importance of food trade needs to be considered when preparing a list of foods. In food-exporting countries, the list may also need to include the foods most important to the export economy, particularly processed foods in view of the fact that nutrition labelling is required for these by many importing countries.

Preparing a list of foods

Food consumption statistics may be very limited for many populations and in establishing priorities alternative strategies may be needed. One useful approach is to prepare a list of foods consumed and make subjective estimates of their importance. The list needs to be compiled using a number of sources, e.g. government departments, university researchers. As food consumption patterns are largely determined by socio-economic factors, it is important to involve those sectors of the community in preparing the list.

Food production and retailing statistics may also be useful sources of information to assist in constructing the list. The Food Balance Sheets and Food Supply databases published by FAO, which are available for most countries, also provide data on national domestic availability of foods and their per capita contributions to the energy, protein and fat supplies (FAO, 2003).

Use of food groups

It is often convenient to structure a food composition database using food groups. This ensures that the diet as a whole is considered and that the focus is not distorted by emphasizing one food group at the expense of the diet as a whole.

There is no internationally standardized approach to food groupings. At the 16th International Congress of Nutrition, the INFOODS presentation reported on the issue of food groupings (Burlingame, 1998).

Most food composition databases have between 10 and 25 food groups. Even though the concept of food grouping seems to be internationally agreed upon, the actual classification of food has been shown to be highly culturally dependent and most national databases have unique examples.

The Pacific Islands food composition tables (Dignan et al., 1994), for example, have coconut products as a group because of the economic and cultural importance of this food and the diversity of products. Other countries divide coconut products between several different food categories such as fats and oils for coconut oil; nuts and seeds for coconut flesh; beverages for coconut water. The Central America and Panama (INCAP) database has three groups that are unique: bananas, maize and cornbreads (FAO/LATINFOODS, 2002). The ASEAN food composition database has edible insects as a group (Puwastien et al., 2000).

Researchers and nutritionists in international organizations often report population nutrient intakes by food group rather than by individual foods, suggesting the importance of standardization for international data comparison. The food groups used in the past by FAO (1982), and currently in the UK (Food Standards Agency, 2002), and Pacific Islands food composition tables (Dignan et al., 1994) are shown in Table 3.1.

Identifying priorities for revision of an existing database

The procedure when revising an existing database is very similar to that of compiling a new one, but it will be necessary to consider also which foods may need updated values.

Changes in food consumption patterns should be taken into account, and the values for food items for which there is evidence, even presumptive evidence, that the food has changed in composition since the last database was prepared should be reviewed. Changes in food production – both primary in agriculture, and secondary in food processing, marketing and storage – will also need to be considered. Consultation with the food industry and, where possible, with research groups specializing in the study of specific commodities, often provides useful information on changes that have taken place.

Selection of foods within food groups

Figure 3.1 (page 34) illustrates the stages in the establishment of priorities and the selection of foods for inclusion in the database. At the level of specific foods in each group the strategy requires knowledge of the marketing and consumption of foods. This information will also be required in drawing up the sampling protocols, which is discussed in Chapter 5.

Information will be required from departments of agriculture, commodity boards, trade associations and research groups involved in the study of specific foods. Retail trade journals and consultations with food manufacturers can also provide information on the relative market shares of different brands of the same product. The inclusion of proprietary or brand-name foods should be restricted to stable, well-established lines if frequent revision or updating of the database is not possible. It may be possible to include brand-name foods where these products are unique, or combine foods such as cheeses (e.g. hard cheeses, blue-vein cheeses) or biscuits (e.g. sweet, savoury, filled) into generic compositional types.

Once a clear idea of the relative importance of various foods has been reached and a provisional list of candidate foods for inclusion drawn up, existing compositional data on these foods should be examined following the principles set out in Chapter 10. This process will review the quality of the data and their applicability to the food currently consumed and will establish whether or not sampling protocols need to be developed to provide the necessary data for their inclusion.

It is often useful to group the foods at this point into subgroups as outlined in Table 3.2. These may be arranged according to the type or use of the foods. Subgroupings of foods with similar matrix and nutrient characteristics often provide a convenient basis for developing common sampling and analytical approaches.

Presentation of foods in the database

The different levels of use of compositional databases require compositional data to be given for foods in the raw state, in the processed state, and as prepared for consumption. Where resources are limited, priority should be given to providing data for the most important foods in their raw state and the most common forms in which they are consumed.

Where foods are commonly consumed in more than one form (e.g. peeled and unpeeled; boiled, fried or roasted), values should ideally be given for all these forms where resources permit. A pragmatic approach may need to be adopted to conserve resources by preparing one form of the food in one way and another type in another way and then extrapolating the composition for the different methods of preparation. For example, different cuts of bacon may be analysed in their raw state and one cut analysed after frying and another after grilling, with the observed changes being extrapolated to all cuts.

The human diet typically includes a wide range of prepared foods with often complex recipes, and it is rarely possible to analyse all the different types of dish. In such cases, it may be decided to calculate the composition of the dishes from the recipes, taking into account the changes in weight on cooking and nutrient retention factors.

The most common cooking methods and the major nutritional changes associated with each are listed in Table 3.3. The table indicates the information required to calculate the composition of the cooked foods from the raw food or ingredients. In some instances calculation is not really suitable and complete analysis should be undertaken if the food is sufficiently important in the diet.

The food preparation may be carried out in a laboratory but it is essential that local cooking methods be reproduced as closely as possible, if examples of the cooked food cannot be collected (e.g. Greenfield and Kosulwat, 1991). Some traditional methods are difficult to replicate in a laboratory, e.g. the Pacific Island earth oven (Kumar et al., 2001) and great care is needed in obtaining values using these methods. In such cases, local knowledge of food cultures, and possibly the advice of anthropologists, is essential to guide the process.

Preparation of edible material

Most databases use analytical values obtained by analysis of the edible material. During the selection of food for inclusion in a database, it is therefore necessary to identify the edible matter to be analysed. This will often be influenced profoundly by the cultural norms of the population for whom the database is being prepared. The inedible portion, or refuse, should also be measured and recorded in the database, since many users, particularly those in food service management, will be calculating nutrient content on the basis of foods as purchased. Table 3.4 provides examples of edible and inedible portions of some foods.

Food nomenclature

Accurate use of any database requires that the food items are correctly identified; thus compilers need to consider carefully how foods are named in the database. Several authors have discussed the issue of food nomenclature (Arab, Wittler and Schettler, 1987; McCann et al., 1988; Truswell et al., 1991).

Consumers in different parts of a country often give foods different names and the same names are occasionally used for different foods. Provision for a thesaurus of alternative names should therefore be made early in the database compilation process. The names of foods should, as far as possible, be those used by the intended users. Foods covered by legislation with regard to labelling and/or composition should be named in the legally approved way.

Use of faceted descriptor system

The name of a food is frequently insufficient for its unequivocal identification, especially when a national database is used internationally. Food descriptors are therefore needed to identify the foods more clearly and identify the type of preparation used. The use of a systematic series of facets (i.e. properties or attributes) is recommended. A faceted descriptor system permits better searching of large databases, where the same word can represent very different things (e.g. “green” can be a kind of pepper, or a state of maturity), and, when standardized, also facilitates data interchange. Various attempts have been made internationally to standardize systems for naming and describing foods (Truswell et al., 1991; Ireland and Møller, 2000), but international agreement has not yet been reached. The most usual facets are listed in Table 3.5, although any facet that aids identification may be used. Information relating to these facets must be compiled during the collection of samples and their analysis; this has important implications for record-keeping during sampling, which will be discussed in Chapter 5.

Resource implications

The priorities for inclusion of foods in a database need to be considered alongside the priorities for inclusion of nutrients and other constituents because the combined requirements will have implications for the total sampling and analytical resources needed. If a large number of nutrients are to be included this may limit the number of foods that can be analysed using the usually finite resources available, and vice versa. Figure 3.2 illustrates the selection of foods for analysis.

The first essential step is to evaluate any existing information. This may show that complete information, which is still valid for the current food supply, is already available. It may also indicate that where a food is imported it may be possible to use data from the country of origin.

However, the information may be limited, or deemed inadequate, and may need to be supplemented by additional analyses – for example, when a constituent has not been measured before, or where the method of analysis used previously is no longer considered reliable. In such cases sampling and analytical protocols will need to be devised.

Where no information is available and the food is judged important, sampling and analytical protocols will clearly need to be devised.

Finally, all the available data will be scrutinized to ensure that they are of compatible quality. This step also has resource implications, as highly trained personnel will be needed to undertake this important last step.

Chapter 4

Selection of nutrients and other components

The aim of food composition databases should be to include all nutrients or other bioactive food components that are known or believed to be important in human nutrition. This ideal can rarely be achieved, especially where resources are scarce, and therefore decisions must be made on priorities. Some measure of selectivity is both desirable and practicable, particularly in respect of analytical work, which constitutes the major demand on resources.

The following considerations, in addition to the availability of resources, will govern the selection of nutrients and other food components:

a)the basic need for information;

b)health problems in the country concerned;

c)the state of current thinking in the nutritional and toxicological sciences;

d)the availability of existing data;

e)the existence of adequate analytical methods;

f)the feasibility of analytical work;

g)national and international nutrition-labelling regulations.

The stages in this process are outlined schematically in Figure 4.1.

The basic need for information

In all countries, information on water, protein, fats, carbohydrates and energy will be required as a minimum base.

Health problems in the country concerned

In countries where deficiency diseases are a pressing problem, information on key vitamins (e.g. vitamin A) and minerals (e.g. iron) will be required. In industrialized countries, however, where problems such as cardiovascular disease, diabetes mellitus, hypertension and cancer are predominant, data on energy, fat, fatty acids, cholesterol, individual carbohydrates and sodium may be seen as top priority. All countries with long dark winters, or where sunlight is prevented from reaching the skin for cultural or other reasons (e.g. purdah, institutionalization), food levels of vitamin D will be required. This range of constituents will be required worldwide if a complete epidemiological evaluation of degenerative diseases is to be made and guidelines for preventive dietary practices are to be established (Rand and Young, 1983). In a country where toxicological problems have been identified, relevant data on food toxins (e.g. goitrogens) or contaminants (e.g. mycotoxins [Van Egmond, 1984; Van Egmond and Speijers, 1999], heavy metals), may need to receive high priority.

The state of nutritional and toxicological sciences

Food components to be included should also reflect the general state of nutritional and toxicological thinking. A comprehensive database should include all nutrients for which recommended intakes have been established nationally and, where appropriate, internationally.

In addition, those involved with the preparation of databases should try to anticipate needs for data. Interest in “new” or “rediscovered” components of food can escalate rapidly (Southgate, 1985); thus those who are responsible for database programmes must be aware of current developments and the interests of nutritional and clinical scientists. There is now, for example, significant interest in values for the glycemic indices of foods (Brand-Miller et al., 1999). These give a measure of the rate at which carbohydrates are digested (see Chapters 6 and 7) and some tables have been produced (Foster-Powell and Miller, 1995). Caution may be necessary in interpreting responses to questionnaires, however. For example, when Paul and Southgate (1970) reviewed the requests of some users of the United Kingdom food composition tables, they discounted advice to exclude nutritionally unavailable carbohydrates, because they were aware of the growing interest in dietary fibre.

Although these guidelines are primarily concerned with the provision of nutritional information, there is growing recognition that a wider range of constituents play an important role in the relationship between diet and health (Ames, 1983). These include naturally occurring biologically active constituents such as a range of phytochemicals including phytates, oxalates, flavonoids, glucosinolates and phytosterols. Some of these components, such as goitrogens (Gaitan, 1990; Speijers and Van Egmond, 1999) alter the nutritional values of foods, through interactions in the food or gut, or during metabolism. There is also interest in including information on food additives and contaminants in databases (Louekari, 1990; Burlingame, 2001). The amounts of additives in foods are highly brand-sensitive and often subject to variation with time, so it is particularly important for these data to be date-marked. The distribution of contaminants is often more complex than the distribution of the naturally occurring constituents within foods and representative values may be difficult to establish. Furthermore, sampling procedures for contaminants are often designed to identify maximum likely exposure in a population, and it may be misleading to list contaminant values in the same record as nutrients. For these reasons, these guidelines make only limited reference to contaminants, although their importance is recognized (Young, 1984).

Availability of existing data

A great deal of information is available for certain nutrients or non-nutrient components that have been the focus of research or have been measured for regulatory purposes. These data should be employed, provided that they meet the programme’s quality criteria. Where resources are limited and preclude inclusion of all components in the user database, it would still be useful to store all available data at the archival levels of the data system.

Existence of adequate analytical methods

The availability of reliable analytical methods is an essential determinant of components for inclusion (Stewart, 1980) (see Chapters 6 and 7). It will not be cost-effective to analyse foods for a particular nutrient, however high its priority, if methods are untried or yield conflicting values. When methods are in doubt, it may be appropriate for methodological studies to be implemented as part of the database programme.

The emergence of a reliable new or improved method for measuring a nutrient may create the need for analysis (or reanalysis) of foods that are important in the food supply or that are known or suspected to be good sources of the nutrient concerned.

Feasibility of analytical work

The commissioning of analyses for each nutrient must be governed by practical factors: the cost and time required, and the availability of equipment, trained personnel, chemicals, etc. These are major considerations, especially in some developing countries. Costs must always be weighed against the nutritional or clinical requirements for particular nutrients. Where resources are limited it may be useful to search out other laboratories, such as governmental regulatory laboratories or those working on soil chemistry, for collaboration. Borrowing or calculating values would be the final option.

National and international nutrition-labelling regulations

Nutrition labelling has emerged in recent years as one of the more important and demanding areas involving food composition. The key international body concerned is the Codex Alimentarius Commission (FAO/WHO, 2003), operated jointly by FAO and WHO. Complete food labelling text, with a section on nutrition labelling, is available in print and electronic form (FAO/WHO, 2001). Compliance with Codex Alimentarius is voluntary, and many countries have their own unique nutrition-labelling regulations (FDA, 2001; EC, 1990; FSANZ, 2001). It is useful for food composition programmes to include all the nutrients required in their national nutrition labelling as well as those required in the labelling regulations in countries within their region. For food exporting countries, the nutrients required in the regulations of major trading partners are also important for inclusion in the food composition database.

Coverage at different stages of data management

As noted earlier, ideally, a food composition database system should include values for as many nutrients and other components as possible, with technical provision for adding more information as it becomes available. However, because a comprehensive database system is a national reference resource, it is useful to list the values for individual forms of nutrients separately, where separate analytical values are available or can be obtained, particularly in a reference database. The factors used for converting the different forms of a nutrient to a single value to give an indication of its biological value may change as the state of nutritional science advances. If only the calculated (derived) value is recorded in the database management system, it will not be possible to recalculate the putative total biological activity; thus, it is desirable that the measured values appear in addition to calculated values. In any event, all conversion factors used should be listed in numeric data fields as equivalent to components, or in the documentation sections of the database.

Component data can be expressed on many different bases. For example, amino acids can be expressed as mg per g nitrogen (N) (or as g per 16 gN) and fatty acids as percentages of the total fatty acids, and this is the preferred format for entering such data, if this is the way in which they were obtained from the analytical laboratory. However, at the user level, it is often more useful to present all the data for a particular food as g per 100 g edible portion (or per 100 ml for some beverages, along with density values). User databases (or, more usually, printed tables) will vary in complexity and coverage; hence specific decisions must be made on each component for the different data outputs. Thus, data may be presented as “total” or “available” values for nutrients, for which several forms exist, calculated using appropriate factors and a documented algorithm.

Analogously, in simplified printed tables it may be desirable to regroup some components, such as fatty acids and cholesterol, into separate sections. This will almost certainly be the case when printing costs are a constraint.

In the case of special-purpose tables, many formats are possible. In tables for nonspecialists, values may be grouped (e.g. fat <1 g, 1–5 g, 5–10 g, etc.), or foods may be listed according to their ranking as sources of nutrients (excellent, good, fair, poor) depending on the proportion of the recommended daily allowance present in an average serving.

Suggested coverage of nutrients for different levels of data management is given in Table 4.1, and Table 4.2 provides examples of data dissemination formats. Comments on some of these components follow, and further details can be found in Chapters 6 and 7.

Water

It is essential to give values for water content in published tables and papers on food composition and at all levels of data management, including the comprehensive user database. Variations in water content are important determinants of the levels of other components, and data on water content make it possible to compare nutrient values (e.g. for different foods or different analyses of the same food) on a similar moisture basis. This information is also essential when data from different sources are being compared or combined. Analyses for some nutrients are conveniently performed on the dry matter (DM) sample. Therefore laboratory data may be reported per 100 g DM, and recorded in the reference database in this way. However, each DM value must be related to the analysed water content of the same sample, so that nutrient values can be recalculated to their appropriate fresh-weight basis. In simplified printed tables it may be unnecessary to list water content, but it should only be omitted when space is a critical constraint.

Protein

Values for protein are required at all levels of the data system. Conventionally, they are based on total nitrogen values using a nitrogen conversion factor (FAO/WHO, 1973), with all factors being recorded at the food level in the database. Values can also be based on the total nitrogen minus the non-protein nitrogen multiplied by a specific factor related to the amino acid composition of the food, or as the sum of amino acids (see Chapters 6 and 7). New amino acid data used in conjunction with the ratio of total amino acid residues to amino acid nitrogen seem to suggest that the nitrogen conversion factor should be lowered. Sosulski and Imafidon (1990) suggest a global conversion factor of 5.7 and Salo-Väänänen and Koivistoinen (1996) of 5.33, both with individual factors for different foods and food groups. At this time no new international agreement on conversion factors had yet been reached.

Total fat

Values for total lipids vary considerably with analytical method (see Chapters 6 and 7) and may be of limited nutritional significance; nevertheless, they are widely used and should be included at all levels of the database.

Fat (-acylglycerols). Inclusion of this item is desirable in the reference database, primarily for use in the calculation of food energy value, and also because of the interest in triacylglycerols from animal and vegetable sources. The widespread and increasing use of mono- and acylglycerols in manufactured foods is an additional reason for its inclusion.

Phospholipids. Values for the different classes of these substances should be included at the reference database level because of their wide use as emulsifying agents, and because of their physiological properties.

Sterols. Although cholesterol was once considered the most important sterol from a nutritional viewpoint, the significance of the other sterols (e.g. sitosterol) is now recognized; they should be included at the user database level.

Fatty acids. Data for individual fatty acid stereoisomers should be included in the reference database. At this level, the most convenient mode for expressing fatty acid values is as g fatty acid per 100 g total fatty acids. In user databases, however, expression as g fatty acid per 100 g of food is more useful. In simplified user databases the fatty acids may be grouped into total saturated, total mono-unsaturated and total polyunsaturated acids, or the ratio between the groups may be cited together with the total fat value. Another grouping of major interest is as n-9, n-6 and n-3 families of unsaturated fatty acids (Gurr, Harwood and Frayn, 2002).

Carbohydrates

Values for available (glycemic) and unavailable (non-glycemic) carbohydrates derived by analysis are desirable throughout the database system. The earlier practice of including carbohydrate calculated “by difference” has proven to be scientifically unsound and should be phased out as soon as possible (FAO/WHO, 1998).

Available carbohydrates (glycemic). These include all the sugars (glucose, fructose, sucrose, lactose and maltose) known to be glucogenic in humans and the polysaccharides (starch and partially hydrolysed starches, and glycogen) hydrolysed by the endogenous secretions of the human digestive tract (Table 4.3).

Unavailable carbohydrates (non-glycemic). These include all the polysaccharides that are not hydrolysed by the endogenous secretions of the human digestive tract: components of the plant cell wall (cellulose, non-cellulosic polysaccharides, pectic substances and hemicelluloses) and a range of polysaccharides used as food ingredients or food additives. These together are the non-starch polysaccharides (NSPs), which are often used as a definition of dietary fibre. There are several other definitions of dietary fibre, each identified by a different methodology, and each measuring different amounts of the non-glycemic carbohydrates, and other non-carbohydrate material (e.g. lignin).

Oligosaccharides. There is growing recognition of the potential nutritional importance of this group and an associated need to start assembling values for these components. Oligo-saccharides include tri-, tetra- and pentasaccharides of the raffinose series, analogous malto-derivatives and a range of fructose polymers, including those at the lower end of the polysaccharides. Individual oligosaccharides need to be recorded separately because they are metabolized differently.

Polyols (sugar alcohols). These comprise a group of polyhydric alcohols structurally related to the sugars where the reducing group has been reduced to a hydroxyl compound. Very small amounts of them occur naturally in foods, but they are widely used as food additives for their humectant properties or as a replacement for sugars in reduced-energy products, low cariogenic sweets and foods for diabetics. Under the labelling regulations of some countries, polyols are included in the carbohydrate declaration, but in a nutritional database it is preferable to list them separately under their specific trivial names. Table 4.3 indicates the more important polyols used in foods.

Organic acids

These are important in relatively few foodstuffs, and their inclusion in a user database should be selective. Values should be given for fruits, fruit products (including juices), a few vegetables (particularly those preserved in acetic acid), and other manufactured products, such as vinegar, salad dressings that have organic acids listed as major ingredients, soft drinks and yoghurt. In these cases, organic acids should be included in energy calculations.

Alcohol

Alcohol (ethyl alcohol) may be a significant energy contributor; levels must be determined and used in energy calculations for alcoholic beverages, and for confectionery and desserts containing alcohol.

Inorganic constituents

Total ash. Values for ash are frequently given in data sources and the values should be entered into the database system primarily because they can be used in internal checks on the sum of all the proximate components, the calculation of total or available carbohydrate by difference and the mineral content. Because the values are not of nutritional significance, they need not appear in simplified tables.

Individual inorganic constituents. All the essential inorganic elements should be included. Current instrumental techniques provide information on a wide range of minor trace constituents with little extra cost, and it is desirable to include a comprehensive list. The forms in which some trace elements occur are important in relation to their bioavailability and should therefore be recorded when this information is available.

Vitamins

Many vitamins occur in several active forms called vitamers; if it is technically possible, the vitamers should be analysed separately and the values held separately in the database system, in some cases at the user database level. In simplified tables, it will usually be enough to provide a value for the total activity of the vitamin in question. It is, however, essential to document the algorithms used to calculate these estimates of total activity.

Non-nutrient constituents

Contaminants. Contaminants include mycotoxins, heavy metals and residues of pesticides, herbicides and animal growth promoters. The distribution of contaminants in foods is such that the concept of representative values for contaminants differs from that for nutrients. It may be misleading to list contaminant values in the same record as nutrients. Listing in archival and/or reference auxiliary data records is preferred.

Bioactive substances. There has been a growing interest in the range of dietary phytochemicals in recent years, particularly in view of their possible protective action against cardiovascular diseases and certain cancers. These include isothiocyanates, polyphenols, flavonoids, isoflavones, lignans, saponins and coumestrol (AICR, 1996; Pennington, 2002). Consequently, there is a parallel interest in the inclusion of phytochemicals in food composition databases (Ziegler, 2001). The collection of data from data sources is useful, although it may not be possible to find complete data sets.

Antinutrients and toxicants. Some constituents have undesirable physiological effects, for example, goitrogens, haemagglutinins, antivitamin factors, trypsin inhibitors, oxalic acid and phytic acid. Data for these components should be included for the relevant foods. Other important natural toxicants include solanine, cyanides, glucosinolates, lathyrogens, mimosine and nitrosamines. Ideally, data for these natural components should be incorporated in the reference database.

Additives. Many additives are measured, in whole or in part, during the course of nutrient analyses. Salts, for example, are included in analyses for various cations and anions; protein additives are determined in nitrogen analysis; and some emulsifiers and thickeners are included in analyses for nitrogen, starch and unavailable carbohydrates. Clearly, specific analyses are preferable. However, the need for data on additives and other non-nutrient components of foods may relate to priorities regarding food safety and not necessarily to nutritional priorities.

Miscellaneous. Where data exist for other compounds of interest, such as caffeine, theophylline, theobromine, tannins and other bioactive compounds (carnosine, carnitine, creatinine), they should be listed in the database at least up to the reference level.

Chapter 5

Sampling

The quality of sampling and analytical data is a major determinant of database quality. Sampling foods for inclusion in a compositional database is one of the more demanding and difficult aspects of database preparation and often requires the compilers to make intuitive judgements and compromises. This chapter reviews the objectives of sampling and discusses the various aspects for consideration in making these judgements.

Where the necessary information on the composition of a food is not available (as is often the case in developing countries) or is inadequate (e.g. it is no longer applicable to the current food supply or the analytical values need to be measured using more recent methods), then sampling and analytical protocols need to be devised.

Ideally these should be developed in conjunction with each other because the requirements of the analysts will determine the amounts of foods necessary for the analyses and how the foods should be stored and, if necessary, preserved.

Objectives in sampling

Users of compositional databases require representative values for the composition of the foods consumed by the population for whom the database is being prepared.

The primary objectives in sampling, therefore, are to collect food samples that are representative and then to ensure that changes in composition do not take place between collection and analysis.

All foods are biological materials and exhibit natural variations in composition. A secondary objective may be to document this variability as it relates to factors such as season, geography, cultivar and husbandry. Such variations are to be expected and should not be confused with variations associated with the analytical conditions. The combined protocols – that is, for sampling and analysis – should also ensure that the representative attributes are maintained in the portions taken for analysis.

Some basic terms

In the context of the following account, the term sampling is used to describe the activities involved in the selection and collection of items of food defined in terms of number, weight and nature of the material to be analysed. Much of the formal terminology developed for use in sampling was designed for use in the commercial sector for the purposes of surveillance and determination of contamination (Horwitz, 1990). Some of these terms have little relevance for nutrient database work and therefore are not discussed further. Table 5.1 outlines the steps involved in the sampling process and provides definitions of the terms that will be used later in this book. Figure 5.1 illustrates the different stages in sampling and analysis, indicating where sampling errors may arise as distinct from analytical errors.

Because of the variability and heterogeneity of foods, all sampling is associated with some degree of error when the results are extrapolated back to the composition of the whole population of a food. Sampling can merely provide data that define the probability that the values will apply to any one isolated unit of the food.

The approach to sampling

The selection of a representative sample and the combined protocols for sampling and analysis must be based on a clear understanding of the nature of the foods and the population of food being studied (i.e. all the individual units of the food). A database will be used for a considerable period of time and the values derived from the combined protocols will be used as if they were representative, in both space and time, over the lifetime of the database (and often for much longer). The design of the protocols therefore represents a monumental task and one in which it may be necessary to accept compromises. It is essential that such compromises are based on knowledge of the food in question.

Sources of food

The principal sources of food samples are summarized in Table 5.2. These groupings correspond to the levels at which databases are used.

Bulk commodities

Compositional data obtained from analyses of bulk commodities have wide-ranging uses. They are commonly used in commerce or for surveillance of imports for contamination with agrochemicals or the misuse of growth stimulants. These data also provide the basis for calculating the nutrient values in food disappearance statistics and sometimes in household and industrial recipes. Standard sampling procedures have been defined for many commodities and these should be followed: International Organization for Standardization (ISO, 2003); Official Methods of the Association of Analytical Communities (AOAC International, 2002, 2003); Codex Alimentarius (FAO, 1994; FAO/WHO, 2003). Care should be taken to ensure that samples are truly representative of the bulk commodity. Several samples may need to be taken from separate sacks, cases, packages or carcasses, and at several points in a silo or container. Random sampling is preferable to the collection of readily accessible units. Collectors should take packages from several randomly identified cases or packages, for example. This level of sampling presents logistical problems that are best overcome by taking samples during the loading or unloading of a consignment. Special probes or triers are required (Horwitz et al., 1978) for sampling finely particulate foods (e.g. sugar, grain), fluids (e.g. milk) or solids (e.g. cheese).

Nutrient analyses at this level are often limited to major components, but generally involve many analysed samples (sometimes in the hundreds), and therefore result in very high-quality values.

Wholesale foods

Sampling of wholesale foods generally follows the principal approaches used with bulk commodities. Randomization of sampling is essential.

Retail foods

These foods constitute the majority of foods included in food composition databases in industrialized countries. For primary products such as meats, fruits or vegetables, the major concern of the sampling protocol is to ensure that the complete range of sales outlets is represented. The primary sample should be made up proportionately of the volumes of food passing through the different outlets. The potential for regional variation also needs to be covered in the design of the sampling protocols.

In non-industrialized countries where food distribution systems may be less developed, regional considerations assume greater importance and variations in composition from one rural market to another may be substantial. Regional stratification (see below) of the sampling may be considered a more useful approach in view of the regional variation in the composition of produce. In many cases presenting data that are representative of a very diverse population may not be acceptable.

Proprietary foods constitute an important range of foods in many countries and their composition should be included in the database. Where a database is prepared by government personnel there is often reluctance to include brand names. In practice, for many proprietary foods, the brand name is essential for identification. In some countries, the range of branded items of a food is very numerous, and covering all the different brands increases the analytical workload. Compositional data supplied by the manufacturer may be acceptable provided that they meet the criteria set for analytical quality, and that the manufacturers can assure the compilers that the samples analysed were representative of products as sold retail. Problems can arise using this approach because many proprietary foods are reformulated at frequent intervals and database values rapidly become out of date. Many compilers prefer to restrict this type of database entry to foods that are stable and well-established. In some cases, pooling the different brands according to market share is considered appropriate.

When collecting samples, care must be taken to ensure that the full range of retail outlets is properly represented. When available, retail sales statistics can be useful. In many cases proprietary products are produced under such strict quality control that limited sampling is satisfactory.

Field or garden produce

These sources of food are often ignored in industrialized countries, but in many countries food produced by the family constitutes an important component of the diet and should therefore be considered by database compilers. These foods tend to be much more variable – the composition of plant foods is especially dependent on the soils and fertilizer treatments. Such factors therefore need to be taken into account in the design of sampling protocols. Most field or garden produce is eaten seasonally as fresh and then preserved according to traditional methods that can differ substantially from commercial practice.

Uncultivated and wild foods

Many communities, especially those living a “hunter-gatherer” or semi-nomadic style of life, consume substantial quantities of wild plant and animal foods. Such foods account for a significant proportion of daily consumption, and their inclusion in a database can be very useful for those studying the nutrition of such groups. Collecting samples of these foods can pose particular problems. They may be difficult to identify properly and also tend to be variable in composition and maturity (Brand-Miller, James and Maggiore, 1993). Frequently random sampling is virtually impossible and “convenience” sampling, as the opportunity arises, is the only option. Provided that this approach is documented in the database, it is acceptable. Documentation will alert users to the limitations of the data and minimize the possibility of them being used inappropriately.

Foods as consumed

Many dietary intake studies, especially epidemiological investigations, require the measurement of food and nutrient consumption at the individual level, i.e. foods as directly consumed. These foods – “on the plate”, as they are often called – comprise cooked foods of all kinds, including complex mixed dishes. The latter are often prepared using a variety of recipes and cooking methods, which poses difficulties in selecting representative samples. Simulation of the cooking procedures in the laboratory or dedicated kitchens is often used to prepare samples for analysis. This approach is generally satisfactory, although in the domestic context being simulated, food preparation is not always carried out in a controlled fashion and decisions on when cooking is complete are a matter of individual preference and judgement. Nevertheless, laboratory-based sample preparation allows for detailed documentation of all the relevant conditions (cooking temperature, duration, end-point internal temperature, etc.). Collection of cooked dishes from a randomly selected range of households would provide more representativeness, and is sometimes, therefore, the preferred approach (Greenfield, 1990b). However, this approach also presents its own logistical problems.

Samples of institutionally prepared foods from, for example, hospitals, industrial and public canteens and educational establishments, are more easily obtained. Samples from fast food establishments and of “take-away” foods are also easier to collect. The difficulties in sampling, the enormous range of possible variation among cooked foods and financial constraints have frequently led compilers to use calculations from recipes to estimate the composition of cooked dishes.

Major sources of variability in nutrient composition

Foods are inherently variable in composition, and the approach to sampling and the design of the sampling and analytical protocols need to take account of this factor.

Geographical samples

In a single country there may be a wide diversity of soil and climatic conditions, resulting in significant variance in food composition. Variations in food marketing and food preparation within different parts of a country – or among countries in the case of a multicountry database – may also produce notable variance. For these reasons, geographically-specific data may be presented in the database as a supplement to nationwide and/or regionwide averages. In other countries, the variations may be of similar magnitude to those due to other causes, in which case the national sample could be weighted according to the proportions of the population living in the regions or the proportions of the total consumption of the foods.

Seasonal samples

Seasonal variations in nutrient composition need to be accommodated in the combined protocols. Plant foods are especially prone to variation, particularly in their water, carbohydrate and vitamin content. Fish also show seasonal variations, especially in fat content, and milk and milk products exhibit variations in vitamin content primarily due to seasonal differences in feeding patterns. The collection of samples needs to be organized, in terms of timing and frequency, to reflect these variations. In some cases, seasonal data need to be given separately in the database. The analytical measurements of the seasonal samples can often be restricted to those nutrients showing variation.

Physiological state and maturity

The states of maturity of plants and animal foods cause variation in composition: in the concentrations of sugars, organic acids and vitamins in many plants, and of fats and some minerals in animal foods. Some of these variations are a consequence of seasonal effects.

The storage of plant foods also often affects water and vitamin contents and levels of some organic nutrients due to residual plant metabolism in storage.

Cultivar and breed

These may be a significant source of variation for some nutrients and the combined protocols will need to provide for this variation. It is desirable to document this cultivar or breed variation within the database. Some research organizations sample specifically to capture cultivar and breed differences. The significance of the differences attributable to cultivar or breed can only be ascertained by controlling for other factors that can influence variation, and by sampling and analysing individually, not in composite, a large number of samples.

Methods of sampling

The main sampling methods used for nutrient composition databases are summarized in Table 5.3.

Random sampling

Random samples are collected in such a way as to ensure that every item in the population of the food being sampled has an equal chance of being collected and incorporated into the sample to be analysed. This is difficult to achieve in practice because it is difficult to visualize the entire population of, say, all the cabbages in a country let alone ensure that each one has an equal chance of being selected. It is more usual to set up a stratification (see below) of the food population.

Stratified sampling

In this method the population of food is classified into strata, taking into account the most important causes of variation.

Stratification by geographical area may be useful even where there are no known significant regional variations (Smits et al., 1998). Stratification according to the distribution of the consuming population, among rural and urban sources, or by type of retail outlet, are other useful examples (Torelm, 1997). The sampling of branded foods can be stratified according to manufacturing plant. Where different brands of the same food are not expected to show significant variation, the sample can be weighted according to market share.

Where this information is not available, extrapolating from similar foods or an intuitive assessment will be required.

Selective sampling

Selective sampling is widely used in experimental studies of plant and animal husbandry and in home economics. The resultant data are valuable guides for the design of sampling protocols but since they are not generally representative of the foods available, they require careful documentation when included in the database.

Where, however, it is clear that the methods of husbandry and the storage of the foods are comparable with current practice for the production of food the data may be useful.

This method is often legitimately used in the analysis of contamination, where the objective may be to identify maximal exposure to contaminants. The distribution of contaminants in foods is frequently highly skewed. Random sampling will therefore often include samples in which the concentration of the contaminant is below the level of detection. This is the primary reason why data on the levels of contaminants are often held separately from representative nutrient data in the database.

Samples of foods prepared in a laboratory can be regarded as selective samples. Laboratory preparation may be the only practicable way to obtain data on the composition of certain foods and therefore the derived data may be useful in databases. Generally, however, samples collected from cooks working in domestic or industrial kitchens are to be preferred as they can be regarded as more representative of foods generally available for consumption.

Convenience sampling

The collection of samples from conveniently accessible points is a very common, and possibly misleading, practice in compositional studies. This method may be acceptable as a preliminary exercise to obtain estimates of variation in composition, but in general data obtained using this method should be regarded as low quality.

Convenience sampling may be the only option in the case of wild or uncultivated foods; provided the sources of the samples are fully documented the values can be used in a database.

Limits of all sampling methods

In all methods the compositional data obtained can only be an estimate of the composition of the food and are subject to limitations imposed by the variation in the composition of foods.

Designing combined sampling and analytical protocols

The objective is to prepare well-documented protocols that provide the basis for those involved in collecting and handling the samples, from their collection in the field through to the laboratory. This process serves to ensure that the data generated meet the objectives of the compilers and the requirements of the database users.

Responsibility for preparing the combined protocols

In some countries the database compilers control the sampling and analytical work and are responsible, in collaboration with the analysts, for preparing the written combined protocols. In most countries, however, the sampling and analytical work will be carried out under contract(s); here the compilers’ input may be restricted to establishing the broad outlines of the work required. These initial specifications should set out the principles of the database requirements with regard to representativeness and the analytical data quality standards that the reports from the contractors must meet.

Detailed combined protocols are then prepared by the contractors in consultation with the compilers. The sampling may be contracted to local sampling groups (e.g. where the database covers a large country or region); again, it is essential that the subcontractors are fully conversant with the sampling objectives.

Where the analytical work is subcontracted, either for all or selected nutrients, the subcontractors must be aware of the preferred analytical methods and have in place the proper data quality assurance schemes. Where the subcontractors wish to use other methods with which they may be more familiar or experienced, they should provide evidence that these are compatible with the preferred methods.

It is of paramount importance that units and modes of presentation of the results are predefined and written into the contracts. For example, laboratories may use ppm (parts per million, mg/kg) or ppb (parts per billion, microgram/kg) to express the results of trace metal analysis, and others use IU (International Units) for some vitamins. Fatty acids should always be reported as units of mass (mg/100 g) and may additionally be reported as a percentage of total fatty acids. It should also be predetermined whether results should be reported on a dry weight basis or wet weight basis. In either case, water content values must be reported.

Choice of sampling method

Some form of stratified sampling will generally be the method of choice. Even where there is no evidence of regional differences in composition, a stratification based on collecting samples on a regional basis of the population of the food consumed will be included in the sampling. For pragmatic reasons it may be necessary to restrict the extent of sampling and most compilations devote the most extensive sampling to the most important “core foods” or “key foods” and those foods that are major sources of particular nutrients, (Chug-Ahuja et al., 1993; Schubert, et al., 1987; Haytowitz, Pehrsson and Holden, 2002; Pennington and Hernandez, 2002; Perry et al., 2000) where, for example, there are public health concerns. Foods that are relatively minor components of the diet are usually less emphasized in the protocols. Many proprietary or branded foods, which are produced in a few factories, can clearly be sampled more simply than, say, meat products which are often “core foods” and which can show great variability, necessitating much more detailed and extensive protocols. Vegetables and fruits, which show seasonal variations in composition, will need to have a seasonal stratification. Each group of foods must be considered on a case by case basis. The logistics of the analytical work often make it desirable to sample foods on a food group basis because sample handling and the actual methods used will be common across the group.

During the course of describing the sampling process a number of stages are met, each of which uses the terms “sample”. Table 5.4 sets out a summary of the stages and some suggested definitions which may be used to make it clear which type of sample is meant at the different points in sampling and analysis.

Size and number of samples

Size. The total amount of food required for the different analyses forms the basis for deciding the size of individual samples. In practice, because foods are heterogeneous, taking small portions at the primary sampling stage can lead to error. For many foods the individual items for collection are readily identifiable; in other cases they will need to be defined. In practice, 100–500 g represents a convenient guide to the size of a primary sample, with preference being given to the upper end of this range. Some food items, for example certain cuts of meat, are much larger than this and cannot easily be reduced to a smaller but still representative unit; for the purpose of the primary sample these should be used in their entirety.

Number. In order to calculate the number of samples needed, information is first required on the variability of the composition of the food (Proctor and Muellenet, 1998). This also assumes that the concentration of the nutrient is uniformly distributed in the food, which is a reasonable assumption for many nutrients but often not true for trace elements.

In practice, the required information is often incomplete and one has to proceed intuitively. Furthermore, many nutrients, especially vitamins, show greater variability than, say, protein, so the number of samples required formally will be greater.

An example of how the calculations are performed is provided in Appendix 2.

Most sampling schemes adopt a standard of at least ten units and the United States requires data for nutrition labelling to be based on 12 units. However, strictly speaking the number depends on the variability of the nutrients being measured and thus different numbers of food samples are required for certain nutrients.

Preparing the protocols

The protocols are written documents that describe the sampling process: the identity of the food, the size and weight of units to be collected, the stratification to be used and the distribution of sampling sites. Tables 5.5a–5.5d give the information that is required for preparation of the sampling protocol, commencing with the description of the primary food sample (Greenfield, 1989; McCann et al., 1988).

Table 5.5a deals with the identification of the food. The record of the collection is recorded in Table 5.5b, a detailed description of the food collected in Table 5.5c, and the handling in the laboratory in Table 5.5d.

The volume of information recommended in this documentation may seem excessive, but experience suggests that information from different stages is very critical when assessing the quality of sampling and subsequent analyses. Moreover, if the details are not recorded at the appropriate time they cannot be recovered retrospectively.

Identification

Table 5.5a sets out the information required. The first section constitutes a label that should be securely and permanently attached to the sample. The laboratory may subsequently add an acquisition number. Most of the information required is self-evident.

Record of collection

Table 5.5b sets out the information to be recorded during sample collection. The items recorded correspond to the sampling plan as set out in the combined protocols. This indicates the designed stratification and the method for achieving randomization within the strata. The use of random number tables is one useful approach. The protocol must also specify the procedure to be followed if the defined sample item is not available for collection. This may be the nomination of a replacement item or the need to choose an alternative sampling point.

Most of the items are self-evident. A record of the purchase price can be useful for auditing purposes and for household budget studies. A photographic record, with a measurement scale and colour standard (e.g. Pantone sheet), if available, is recommended to facilitate the identification of the sample (Burlingame et al., 1995b). If photographic records are not practicable, a simple line drawing may suffice (McCrae and Paul, 1996).

The combined protocol identifies the arrangements for transporting primary samples from the collection sites to the laboratory. The logistical aspects of handling what may be large amounts of food require careful consideration; the storage procedures, including choice of containers and modes of transport, should be specified in consultation with the analysts. These and all other aspects of the combined protocols need to be rehearsed or at least taken through a “paper exercise” with the participation of all those involved. Secure storage in inert containers, which can be heat-sealed using simple equipment, is preferable. Ideally, the samples should be cooled with crushed ice or solid CO_2. If this is not possible, they should be transported to the laboratory with minimum delay. In some cases, the limitations of the sampling and transport arrangements may preclude the analysis of nutrients that are likely to be changed by metabolism (see Table 5.6 on page 80).

Where the distance to the laboratory is short, road or rail transport may be suitable but, where longer distances are involved, air transport may be the only alternative. (This will involve liaison with the airlines to ensure that the storage conditions are compatible with airline safety regulations.) In other cases considerable ingenuity may be required to suit local conditions.

The personal security of the samplers should also be considered, as they often carry relatively large amounts of money to pay for the samples that they are collecting; indeed, the large amounts of food they carry may also be a target for theft. Payment for samples can often be arranged by credit, thereby eliminating one of these concerns.

Description of samples collected

Most of the information suggested in Table 5.5c may be added once the samples have arrived at the laboratory, but the details concerning local use and preparation method may need to be added during sampling.

Labels and lists of ingredients should be retained because they provide key information that may prove useful in explaining analytical discrepancies (e.g. foods where supplementary ingredients have not been added and the labelling is incorrect, differences in formulation of branded foods given the same names).

Record of handling in laboratory

Table 5.5d provides a record of the early preparation of samples in the laboratory leading up to the preparation of the analytical samples. The laboratory may wish to add its own laboratory acquisition number. Laboratory record-keeping constitutes the first stage of a laboratory quality assurance programme, which will be discussed in detail in Chapters 6, 7 and 8. For this reason it is essential to preserve the linkage between the sample ID number and any laboratory acquisition number.

The primary samples will need to be unpacked and the sample compared with the information recorded in Tables 5.5a, 5.5b and 5.5c.

The protocol will specify whether the primary samples are to be analysed individually or combined in some way. Individual analysis of primary samples provides valuable information on the extent of variations in nutrient content, thus helping to define the confidence limits that can be ascribed to the mean values recorded in most databases. Individual analyses require substantial resources, however, and for many databases composite samples are analysed instead. The composite samples may comprise a simple combination of equal weights of all primary samples, or weighted amounts of primary samples from different strata or sampling points according to information on food consumption or production.

Throughout this handling stage, the principal objectives of the sampling process must remain foremost in the minds of everyone involved, namely to ensure the representativeness of the sample and to protect it from changes in composition and contamination. Table 5.6 summarizes the major effects of sample storage and preparation, the nutrients affected and the precautions to be observed.

The samples should be thawed carefully and handled as quickly as possible. Once again rehearsal of these procedures should always be carried out.

In separating the edible and inedible matter the cultural norms of the population consuming the food need to be considered. Complete documentation is essential for later use in the database.

When cutting, mincing or grinding food samples, protective measures must be taken to exclude the possibility of contamination. The procedures should be tested in advance (Wills, Balmer and Greenfield, 1980). The use of plastic or Teflon® coated tools may be necessary. Metal implements should not be used where iron and trace elements are to be analysed; some trace elements may be introduced by the use of stainless steel.

The physical characteristics of the sample are among the important factors to consider in preparing the samples. Lichon and James (1990) have reviewed and evaluated a range of 12 homogenization methods. One should also carry out pilot studies to check on the homogeneity produced by the chosen procedure and that fractionation of the samples has not occurred. Each food will need to be considered case by case.

Storage of the analytical samples

The logistics of sampling preparation usually mean that it is more convenient to store the analytical samples prior to analysis. At least three sample replicates should be stored. Storage in a frozen state is usually the minimum acceptable with preference given to –40 or even –70 °C, which is current common practice. Storage at –20 or –30 °C is acceptable for fat analyses. The container must be closely sealed with the minimum of headspace. When the samples are taken from storage any sublimed water above the sample must be carefully reincorporated in the mass.

Where freeze-drying is possible, storage of the freeze-dried samples in frozen or chilled conditions is satisfactory. Air-dried samples should be stored in such a way as to prevent uptake of water or contamination with insects or mites.

Preparation of analytical portions

In producing values for a compositional database a range of analytical procedures will be performed, requiring a number of analytical portions – often over a considerable time period (unless a large number of analytical staff are available). The procedures for taking the portions and their size will usually be defined by the nature of the analytical method to be used. It is imperative that all portions taken are representative and the methods used follow procedures defined by an established quality control programme.

Where analytical portions are repeatedly taken from the stored analytical samples the risks of contamination or taking an unrepresentative portion increase. It is therefore desirable to store a number of identical analytical samples and to minimize the number of staff involved in taking portions from them.

It is impossible to specify the sampling procedures for all methods and nutrients, but some typical procedures are given as examples in Appendixes 3 and 4.

Resource implications

The combined protocols provide a detailed basis for estimating the resources required for the sampling and analytical work. It may be necessary to revise the protocol, either by reducing the number of samples or being selective about the range of analyses to be carried out. This will require a re-examination of the processes used to establish the priorities described in Chapters 3 and 4. Combinations of analyses or extrapolation from related samples may be necessary.

Many compilers adopt the strategy of using a simplified sampling protocol for foods that are minor components of the diet and restricting the complete sampling protocols for core foods, foods that are major sources of nutrients and foods that are of greater importance in terms of public health.

Training

It is essential that all those involved in the sampling process are familiar with the objectives of the work and are clear about their roles. This can be done by rehearsal of the procedures if only as a “paper exercise”. This process will identify aspects that are unclear or impracticable and require modification.

Table 5.7 summarizes the major sources of error in sampling. These highlight the central importance of documentation, staff training and supervision of the various stages. The sampling stages form the first and critical phases of a fully developed quality assurance programme (see Chapters 6, 7 and 8). Unless the samples are collected and handled correctly the analytical work – however well-executed – will be wasted because the values obtained will not relate to representative samples. It is however a truism that “one cannot inspect-in quality [by supervision], it must be built in”. This depends on adequate staff training so that individuals fully understand their roles in the overall process.

Chapter 6

Choice of analytical methods and their evaluation

Reliable data on the nutrient composition of foods can only be obtained by the careful performance of appropriate, accurate analytical methods in the hands of trained analysts. The choice of the appropriate methods carried out under quality assurance schemes is the second crucial element in ensuring the quality of the values in a food composition database.

For many nutrients, several alternative analytical methods are available that, it is often assumed, give comparable results. In fact, methods vary in their suitability for a given analysis and different food matrices. Before the relative merits of particular methods are discussed in Chapter 7 it is necessary to consider the principles involved in method selection. In doing so it is recognized that the analysts’ choices may be limited by the resources available; this makes it all the more important to understand the principles involved in method evaluation, particularly the need to define the limitations of any given method.

The evaluation of methods is not the purview of the analysts alone. The technical and scientific advisers to the database programme should be thoroughly conversant with the underlying principles of analytical methodology and the various methods themselves, sharing the responsibility with the analyst for choosing a method.

Compilers should also endeavour to be knowledgeable about the analytical methods used. They are responsible for scrutinizing methods when assessing non-commissioned data or published analyses to assess their suitability for inclusion in the database and to devise the specification for contracts for the preparation of sampling and analytical protocols.

It is also desirable that the professional users of a database should have some understanding of the analytical methods used, and that specialist users should be conversant with the methods used for the nutrient(s) relating to their special interests.

At present there are a number of methodological limitations in the production of data for certain nutrients. Based on a review of methods, Stewart prepared a table summarizing the position in 1980 and 1981, which was later extended by Beecher and Vanderslice (1984). In the table the nutrients were grouped according to the availability of valid methods to measure them. The expanded interest in nutrient composition in legislation and for use in epidemiological research has resulted in further work on method evaluation and development. In the United States, the Association of Analytical Communities (AOAC International) carried out a review of methods for use in nutrition legislation (Sullivan and Carpenter, 1993), and major reviews of micronutrient methods were undertaken by the Food Standards Agency in the United Kingdom (2002).

Studies for the development of standard reference materials (SRMs) undertaken in the United States by the National Institute of Standards and Technology (NIST) and in Europe by the Community Bureau of Reference (BCR) have also contributed to method development.

Stewart’s original assessments have been updated in Table 6.1, which presents a revised version based on a review undertaken to assess the compatibility of methods (Deharveng et al., 1999). In the table, “good” methods have been extensively evaluated in collaborative trials, “adequate” methods have been subjected to more limited study, and methods categorized as “not adequate for certain foods” have not been studied on a wide range of food matrices. It is important to note that these assessments hold true only when the analyses are carried out by trained analysts and that they do not include any consideration of speed or costs.

The table does not include the wide range of biologically active constituents that are now considered as candidates for inclusion in food composition databases. The methodologies for most of these constituents have not yet been widely studied in collaborative trials.

Choice of methods for nutrients

The primary objective of food composition databases is to provide their users with compositional information on nutrients; therefore the primary factor in the choice of methods is the appropriateness of the analysis in terms of providing the information required by the users. The measurements must provide values that can be used to assess the nutritional value of foods. This means that the database users’ requirements may differ from those concerned with the regulation of food composition or the quality control of food in production. Thus, while the measurement of crude protein (total nitrogen multiplied by a factor) is adequate for many purposes, amino acid data would provide a better assessment of the nutritional value of a food. A value for total lipids may be adequate in relation to food quality control, whereas a nutritionist would require assessments of triacylglycerols, sterols and phospholipids separately and detailed fatty acid data. Similarly, while total carbohydrate values may be adequate for food quality control, a nutritionist would require specific values for the different carbohydrates (FAO/WHO, 1998). As a consequence, more biochemically orientated methods are often required when obtaining values for food composition databases.

In some countries, the choice of method may be prescribed by national legislation. In other countries, the regulations often permit the use of methods that give comparable, i.e. similar, values to those obtained by the official methods.

Other considerations will also influence the choice of method. The use of some of the most advanced methods may require substantial capital investment to provide the necessary instrumentation. Considerable resources are also required in the form of trained staff to operate and maintain the instrumentation. The development of such instrumental methods represents a preference for investing in capital rather than in recurrent staff costs and for reducing the cost per analysis by speeding up analysis.

It is incorrect to give the impression that nutrient analyses cannot be performed without such sophisticated instrumentation; for many nutrients classical manual methods are available that give equally sound values. These methods are labour-intensive rather than capital-intensive.

It is true that analyses of certain nutrients, fatty acids for example, do require instrumentation; where this is lacking a laboratory would need to seek collaborative arrangements to acquire the data.

Laboratories in developing countries may lack funds for capital outlay (especially as foreign currency) and lack the resources for the specialized maintenance and supplies necessary for high-technology instrumentation. On the other hand, local funds may be available for technical staff with the necessary background for carrying out non-instrumental methods that provide valid data. A comprehensive range of compatible methods has therefore been covered in Chapter 7.

Laboratories should focus their attention on evaluating and improving the quality and performance of the methods currently employed rather than attempting to institute a wide range of methods using new, untried, methods or losing confidence because of their lack of sophisticated equipment. In many cases, implementing a data quality assurance system and training staff are often better ways to produce good-quality compositional data.

The formal training of food analysts, where it is carried out, usually focuses on the highly accurate detection of compounds appropriate for food regulations. These compounds are often contaminants, which are present at low levels, and the choice of method generally emphasizes levels of detection, sensitivity and precision. In nutrient analysis for a food composition database, the requirements for accuracy and precision may be orientated more towards the recommended intake of a nutrient and the relative importance of the food being analysed in the diet (Stewart, 1980). Analysts may, for example, spend considerable effort measuring vitamins in foods at levels that are nutritionally insignificant.

This difference in emphasis underlines the need for all individuals involved in producing data to be familiar with the objectives of the work, from sampling through to analysis. Sampling protocols should specify the levels of accuracy that are expected. It is also important to maintain a regular dialogue between compilers and the sampling and analytical teams throughout the duration of the work.

While the appropriateness of the method may be a primary factor in method selection, it is also necessary to take into account the analytical attributes of the method.

Criteria for choice of methods

It is useful to consider a number of points suggested by Egan (1974):

1.Preference should be given to methods for which reliability (see below) has been established by collaborative studies involving several laboratories.

2.Preference should be given to methods that have been recommended or adopted by international organizations.

3.Preference should be given to methods of analysis that are applicable to a wide range of food types and matrices rather than those that can only be used for specific foods.

The analytical method selected also needs to have adequate performance characteristics. Büttner et al. (1975) summarize these as reliability criteria (specificity, accuracy, precision and sensitivity) and practicability criteria (speed, costs, technical skill requirements, dependability and laboratory safety).

Thus “reliability” represents a summation of the more conventional measures of method performance. Many analysts would also consider another attribute as falling within this summation: “robustness” or “ruggedness”. This attribute is described below.

Attributes of methods

(adapted from Horwitz et al. [1978], with permission)

Reliability

This is a qualitative term expressing a degree of satisfaction with the performance of a method in relation to applicability, specificity, accuracy, precision, detectability and sensitivity, as defined below, and is a composite concept (Egan, 1977). It represents a summation of the measurable attributes of performance. The analyte and the purposes for which the analyses are being made determine the relative importance of the different attributes. Clearly, the analysis of a major constituent such as protein, fat or carbohydrate in foods does not require the same low limit of detection as that needed for the measurement of a carcinogenic contaminant. Conversely, the measurement of a constituent at low levels in foods (e.g. most trace elements, selenium, chromium or vitamins such as vitamin D, vitamin B_12 and folates) cannot be expected to deliver the same high accuracy or precision as found with the major constituents.

Horwitz, Kamps and Boyer (1980) found from a study of the results of a large number of collaborative studies undertaken under the auspices of the AOAC that there was a strong empirical relationship between the concentration of an analyte and the observed precision obtained by experienced analysts. The relationship they found was:

CV = 2(1 – 0.5 logC)

where CV is the coefficient of variation and C the concentration g/g.

Many workers use this relationship when assessing the performance of methods for nutrients present at low concentrations.

Applicability

This is also a qualitative term. A method is applicable within the context in which it will be used, for example the analysis of a specific food matrix. Applicability relates to the freedom from interference from other constituents in the food or from the physical attributes of the food matrix that would make extraction of the analyte incomplete. Applicability is also determined by the usable range of the method. Methods that are applicable at high concentrations may not be applicable at low concentrations. Equally, a method may be applicable to one matrix (e.g. meat) but be inappropriate for another (e.g. a cereal product).

All unfamiliar methods or methods described for a specific food must be checked carefully when used for a matrix that is different from those for which it has been used previously.

Specificity

Specificity is the ability of a method to respond exclusively to the substance for which the method is being used. Many methods are “semi-specific”, relying on the absence of interfering substances in the food being examined. Sometimes a method with poor specificity is acceptable when the purpose of the analysis is to measure all similar substances within a group (e.g. total fat, ash).

Accuracy

Accuracy is defined as the closeness of the value obtained by the method in relation to the “true value” for the concentration of the constituent. It is often expressed as percentage accuracy. Inaccuracy is, as a corollary, the difference between the measured value and the “true value”.

The concept of a “true value” is, of course, hypothetical because the “true value” for a nutrient in a food is not known. All analytical values are therefore estimates of that value.

Büttner et al. (1975) take the view that there exists a true value for all constituents in a sample of food. This is fundamental to the analysts’ art; it is not true that the value for a defined analytical sample of a food is the “true value” for all samples of that food. The sampling error and the analytical errors for any specific method determine the confidence limits for all determined values.

The accuracy of a method is usually determined by reference to standard amounts of the analyte and preferably by the analysis of standard reference materials (SRMs) or certified reference materials (CRMs) that have been analysed, often using several compatible methods, by a group of skilled analysts to provide certified values together with the confidence limits of that value.

Precision

Precision is a measure of the closeness of replicated analyses of a nutrient in a sample of food. It is a quantitative measurement of “scatter” or analytical variability. Strictly speaking, it is imprecision that is measured by carrying out replicate analyses on the same sample (which must be homogeneous and stable). The measurements may be made by one analyst within one laboratory when the assessment is designated “repeatability” (that is, within-laboratory precision) or by several analysts in different laboratories when it is designated “reproducibility” (that is, between-laboratory precision). Comparisons can also be made among different analysts in one laboratory (called “concordance”), and by one analyst on different occasions.

In each case the standard deviation (SD) of the analytical values is calculated (which means that there must be a sufficient number of replications). The SD is customarily divided by the mean value to give a relative standard deviation (RSD), or multiplied by 100 to give the coefficient of variation (CV). In analytical literature, the RSD is used for reproducibility and rsd for repeatability.

It is important to recognize the distinction between accuracy (see the definition above) and precision. One can have very high precision (a low RSD) and poor accuracy and, conversely, have high accuracy with poor precision where the confidence limits of the value obtained will be wide. The ideal is to combine high precision (low RSD) with high accuracy (as judged by the value obtained with an SRM).

Detectability

Detectability is defined as the minimum concentration of analyte that can be detected. This is rarely an issue in nutritional studies, as very low concentrations of nutrients, even some trace elements or vitamins, are not usually nutritionally significant. These are customarily recorded as “trace” in many printed food composition tables. However, it is useful to know whether or not a nutrient is present, and at what level one can confidently record zero in a database. The detectability limit of a method is the concentration at which the measurement is significantly different from the blank. Since blank values also show some variability, the limit can be defined as greater than +2SD (of the blank measurements) above the blank level. The detection limit is below the concentration at which measured values can be made; that is, it is outside the usable range of the method.

Sensitivity

Sensitivity in analytical terms is the slope of the response–concentration curve or line (Figure 6.1). If the slope is steep the method has a high sensitivity; conversely, if the slope is shallow the method has a low sensitivity. When a narrow range of concentration is of interest, a high sensitivity is often desirable; for a wide range of concentrations, a low sensitivity may be preferable. In most nutritional composition studies, trace element analysis requires high sensitivity. In practice, this can often be achieved by increasing the response signal strength by electronic amplification or through chemical concentration of the element.

High sensitivity is usually required for the analysis of contaminants. While contaminants are not usually included in food composition databases, they may become more important in the future, especially those with antinutritional or toxicological properties.

Robustness (ruggedness)

This is a qualitative attribute and refers to the capacity of a method to perform adequately in the face of fluctuations in the analytical protocol. Such fluctuations could include the timing of stages, changes in temperature, or the precise concentrations of reagents. It also includes variations in the skill, training and experience of the analysts carrying out the method. Ideally, during the initial development of a method its authors should have explored and documented the capacity of the method to withstand these types of fluctuation and to perform under a variety of conditions. Methods are available for examining such variations (Youden and Steiner, 1975).

Authors of analytical methods should identify the stages in their methods that require strict attention and control, and document these in the published description of the method.

Summary of attributes

Figure 6.1 provides a diagrammatic summary of the attributes. In the figure the response (height, area, weight, volume, time, optical density or another type of measurement) is shown as primarily a linear function up to a certain level that defines the usable range of the method. Where only a single analyte elicits the response, the method is specific; this specificity may be inherent in the method or may be achieved by chemical separation from interfering substances. This, therefore, is a property of the chemistry of the analyte and of potential interfering substances. The sensitivity of the method is indicated by the slope of the response line. The confidence envelope indicates the precision of the method and the difference between the response line and the hypothetical true line represents the measure of accuracy. The confidence envelope can be calculated at any level, but 95 and 99 percent are commonly used. In the former case, only 1 in 20 measurements can be expected to fall outside the envelope and in the latter only 1 in 100. The white area represents the region of uncertainty where the relative standard deviation is so large that no certainty can be assigned to a value.

Validating analytical methods

Even well-established methods need to be evaluated by the analysts themselves, using their own staff, reagents and equipment (Wills, Balmer and Greenfield, 1980). An evaluation of the attributes of the method should be established under the conditions prevailing in the laboratory and the performance characteristics that are relevant to the purpose of the analyses should be quantified.

Reviewing the method as a whole

In the first stage of the evaluation, the analysts should familiarize themselves with the method as described in the formal protocol for the method concerned. This begins with a “paper exercise” to ensure that the principle of the method is understood and that the various stages are clear in the analysts’ minds. The list of reagents required should be checked against the procedures. Occasionally, a common reagent will be omitted from the reagent list because the authors assume that all laboratories will have it to hand. Standardization of some reagents may be needed before the method is started. At the same time, the analysts should check the equipment required and any specifications listed for the equipment.

Finally, the analysts should go through each stage, familiarizing themselves fully with its purpose. At this point it is suggested that an assessment of the criticality of each stage is made, as recommended in the ANALOP approach (Southgate, 1995); this exercise will determine the possibility for error or uncertainty that might occur if the conditions described are not followed precisely.

Timing may or may not be critical. For example “leaving overnight” may imply a specific time period, say from 18.00 to 09.00 the following day (i.e. 15 hours), or merely that when this point is reached the method can be left until the following day – an indeterminate time period. Timing may represent a minimum time period; alternatively, “heat for 10 minutes in a boiling water-bath”, for example, may mean “exactly 10 minutes” or “while the analyst takes coffee”. Understanding the critical timed stages is especially important when a method is carried out for the first time and until it becomes “routine”.

Analogously, the concentrations of certain reagents are also critical, especially when the reagent must be used in excess for a reaction to be fully completed.

Using the published description of a method as one would follow a recipe in cooking can be fraught with disaster. The analyst must understand the logic of a method. Running through a method as a trial and discounting the results is useful for checking the stages, especially with regard to timing. Less-experienced staff may take time to adjust themselves to a procedure for which the published account of the method suggests that there are many critical operations (e.g. as in the non-starch polysaccharide method [Englyst, Quigley and Hudson, 1994], where the mixing stages are critical). Once this assessment is completed, the analyst will be in a better position to evaluate the various performance attributes.

Applicability

The application of an unfamiliar method to a food matrix other than that for which it was developed or used previously requires careful consideration. It will be necessary to decide, often intuitively, how the matrix will behave in an extraction phase and whether there is any likelihood of interfering substances being present. The chemistry of the analyte and the expected range of the nutrient in the “new” food will therefore need to be considered.

Such matters cannot always be decided intuitively, however, and the method must be tested on the food material. The use of different analytical portions will provide evidence of interference or indicate possible problems with extraction or inadequate concentrations of reagents.

The recovery of standard amounts of the analyte added to the sample can establish whether extraction is complete. Recovery tests are not completely adequate because the added analyte may be more easily extractable than the intrinsic nutrient. Poor recoveries indicate problems; good recoveries may be regarded as encouraging but not conclusive.

Comparisons with values reported in the literature for the matrix may be helpful, as may collaborative studies with another laboratory.

Specificity

Assessing this attribute requires knowledge of the chemistry of the analyte and the food matrix. A value may be required for a group of substances, such as total fat (lipid solvent soluble) or sugars, in which case a semi-specific method may be adequate. Values for triacylglycerols or individual sugars, however, require a much more specific method. Certain vitamin values must include all the active forms; for example, vitamin A (retinol) values should include other active retinoids. Here again, specificity is critical.

Accuracy

This is a difficult attribute to measure because its true value is unknown. The first stage is to analyse standard amounts of the pure analyte. Recovery studies of standards added to the foods are useful, especially if a series of different amounts is used and then a comparison made of the sensitivity of the method for pure standards and the added standards. Recovery studies, as mentioned above, do not provide unequivocal proof of the accuracy of a method because they assume that the added nutrient may be extracted with the same efficiency as the intrinsic nutrient (Wolf, 1982).

Analysis of authentic samples

Analysis of authentic samples that have already been analysed by another laboratory is a useful guide for analysts using a method for the first time. This procedure forms what might be regarded as a simple type of collaborative study.

Analysis of standard reference materials

Standard reference materials are unique materials with a range of food matrices (limited at present but increasing in numbers) that have been produced by a national or regional organization such as the National Institute of Standards and Technology (NIST, 2003a) in the United States or the Community Bureau of Reference (BCR) for the European Union (BCR, 1990; Wagstaffe, 1985, 1990). The samples have been very carefully homogenized and rigorously tested for homogeneity and stability under different storage conditions for different lengths of time (Wolf, 1993). They are then analysed using well-defined analytical methods. Where possible, a number of different compatible methods based on different principles are used. The values generated are then certified with defined confidence limits for the values. The range of nutrients for which SRMs or CRMs are available is limited (but increasing). Coverage is good for many constituents, including some trace elements, some fats, fatty acids, total nitrogen and cholesterol.

SRMs (or CRMs) are expensive to produce and therefore too costly to use routinely (say, with every batch of analyses – which would be the ideal). Each laboratory (or a group of local laboratories) should therefore consider preparing in-house reference materials using similar approaches to that used to produce SRMs (Southgate, 1995).

The homogenized material is stored in a large number of individual containers and used routinely in the application of the method and occasionally alongside the SRM. Recording the values obtained over time on a control chart will help identify any trends towards high or low values. A control chart usually has a central line indicating the control limits for a statistical measure (SD for example) for a series of analyses (American Society for Quality Control, 1973). The laboratory results are plotted on the vertical axis against time (days, hours, etc.) on the horizontal axis. The horizontal scale should provide for at least three months of data and the chart should be checked regularly for evidence of runs above or below the central line or any evidence of lack of randomness (Mandel and Nanni, 1978; Taylor, 1987). Theoretically, the values should be randomly distributed about the central line. When they fall consistently above (or below) the line, they represent possible indicators of systematic bias in the method, which should be investigated.

The preferred materials for in-house reference materials are non-segregating powders such as non-fat milk powders, gelatine, flours, powder mixes for parenteral feeds (Ekstrom et al., 1984) and food matrices common to the local food supply, e.g. soybean meal and fishmeal for ASEANFOODS (Puwastien, 2000). Torelm et al. (1990) describe the production of a fresh reference material based on a canned meat.

One alternative is to carry out analyses using standard samples on a routine basis using a control chart to alert laboratory personnel to problems requiring remedial action.

Precision

The original published description of a method usually gives some indication of the level of precision achieved in collaborative studies, thus providing a “standard of achievement”. Each laboratory, once its personnel are familiar with the method, should evaluate its own levels of precision.

The first step is for each analyst to assess their repeatability by analysing several replicates (preferably at least ten) of the same material and calculating the relative standard deviation. Second, all the analysts within the laboratory should analyse several replicates (preferably ten) of the same material to assess concordance within the laboratory. When setting up a method for the first time, it is useful to test repeatability and concordance using standards. Using blind concentrations of standards prepared by colleagues gives further confidence when using an unfamiliar method.

Finally, participation in a collaborative trial to assess the reproducibility of the method and to evaluate the laboratory repeatability with other analysts is a valuable approach that can be useful as part of the development of analytical skills.

Formal schemes exist for the collaborative analysis of some nutrients; samples for analysis are provided on a regular basis by NIST (2003a) in the United States and by the National Accreditation of Measurement and Sampling (NAMAS) in the United Kingdom (UKAS, 2003). In addition, Wageningen University in the Netherlands is the base for the International Plant-analytical Exchange (IPE, 2003), which provides a basis for developing analytical proficiency, especially for trace elements.

Difficulties may be encountered with regard to the entry of food materials into certain countries and most schemes are quite expensive, which may be a prohibiting factor where resources are limited. In such cases, the organization of local collaborative studies should be considered.

Collaborative studies

There are three major types of collaborative study. The first type, sometimes known as a “round robin”, or “ring test”, provides comparative assessments of laboratory performance. Homogeneous samples of food, often with their identities concealed, are distributed centrally, together with guidance on the preparation of standards and the calculation of results. The results are then collected centrally and analysed statistically. The results are usually provided to the participating laboratories in the form of charts showing the performance of each laboratory against the analyses as a whole. Each laboratory is given a code number and can assess its own performance. Outliers where the values obtained are significantly different from the mean and reproducibility found in the trial are also indicated. This type of collaborative study is of most benefit to laboratories involved in compositional analysis that wish to test and improve their performance.

A second type is that used by the Association of Analytical Communities (Thompson and Wood, 1993; AOAC International, 2003) to establish the performance of a method. In this case the collaborating analysts analyse a series of food samples supplied centrally, using a common analytical protocol. Standards and some reagents, where the specifications are critical (such as enzymes), are also supplied centrally, as are forms for calculating, expressing and recording the results. At least eight, but preferably more, analysts and laboratories are involved in such a study. The results are collected and analysed statistically, usually by an associate referee. The performance characteristics are used in the assessment of the method before it is accepted into the Official Methods manual.

A third type of study is used by the BCR in the European Union, primarily in the development of standard certified materials. Here, a group of laboratories analyses samples provided centrally, initially using their routine methods. Standards may be distributed together with forms describing how the results should be expressed. The results are collected centrally and analysed statistically. The findings are distributed and the analysts subsequently called to a meeting. The object of the meeting is to assess the different methods and identify where laboratories using the same methods found different values. Agreement is then reached on protocols that should be followed in a second round.

The results from the second round of the study will often identify methods that give satisfactory reproducibility and those methods that give similar results, although a third round may be required. These methods are then used in a carefully controlled certification study of food materials intended for potential reference materials. The ideal is to have a number of methods, based on different principles, that are compatible. In some instances the certification can only be given for values obtained by only one method.

It is important that the analysts involved in collaborative studies of this nature see the primary objectives of the studies as raising standards of analytical performance and furthering the development of analytical skills and not as a management tool for checking the performance of analysts.

Checking calculations and analyses

When anomalous results appear in collaborative studies or in routine analyses, for example on the control charts, the first step is to go through the logic and application of the calculations, as these are the most frequent causes of anomalous results. Most collaborative studies define the calculations explicitly to avoid such problems, but they still occur. For this reason the calculation procedures should be set out in a logical fashion within the analytical protocols.

The second stage is to repeat the analyses with a series of freshly prepared standards. Improper dilutions or weighing are frequent causes of error.

In the third stage the analyses are repeated by another, more experienced, analyst. Repeating the analyses using a portion from an earlier stage of the analyses does not constitute a rigorous check; ideally, fresh analytical portions should be used. Neither does simple repetition provide an adequate check because any bias related to the standard or the food matrix may be replicated.

If the results still appear anomalous the analyst should analyse the sample blindly using only its sample code number and, if possible, a colleague should be asked to introduce a “blind” replicate. Southgate (1987) has identified a range of laboratory practices that may lead analysts to believe, erroneously, that they have achieved good repeatability and how these practices can be changed (Table 6.2).

All these operations form part of a data quality assurance scheme and their documentation is vital for database compilers when they come to assess the quality of the analytical data, which is discussed in Chapter 8.

Chapter 7

Review of methods of analysis

This review of analytical methods presents assessments of their applications, limitations and the resources required. The objective of the review is to provide guidance on the selection of compatible methods for the nutrients and some other constituents. The continuous developments in analytical chemistry make it almost impossible to ensure that the review is comprehensive and takes into account all recent developments. The review does not provide detailed analytical protocols; for these the reader needs to consult the relevant specialist texts.

In this review, for each nutrient (or group of nutrients), tables summarize the available methods. Estimates of capital costs have been given in three categories: low, where the method requires basic equipment that would usually be found in a laboratory; medium, where specialized instrumentation is required but normally costing less that US$5 000; high, indicating the need for specialized equipment usually costing more than US$10 000.

The proximate system of analysis

The proximate system for routine analysis of animal feedstuffs was devised in the mid-nineteenth century at the Weende Experiment Station in Germany (Henneberg and Stohmann, 1860, 1864). It was developed to provide a top level, very broad, classification of food components. The system consists of the analytical determinations of water (moisture), ash, crude fat (ether extract), crude protein and crude fibre. Nitrogen-free extract (NFE), more or less representing sugars and starches, is calculated by difference rather than measured by analysis.

Although some of the methods used historically in the proximate system of analysis are not recommended for the preparation of food composition databases (e.g. crude fibre), it is useful to consider the concepts involved as they have dominated views on the composition of foods and food analysis. This system was developed at a time when the chemistry of most food constituents was only partially understood, and the growth of nutritional sciences has shown that for nutritional studies a more detailed and biochemically oriented approach to food analysis is needed. Nevertheless, proximate analysis, including the original methods, still forms the basis for feed analysis, and the analysis of foods for legislative purposes in many countries.

Many people find the concept and term “proximates” useful to represent the gross components that make up foods; the actual analytical methods then become independent. Others believe that the definition of proximates is based on the original methods prescribed by Henneberg and Stohmann, and that method substitution, e.g. dietary fibre instead of crude fibre, negates the use of the term.

Water

Values for water remain an essential constituent in food composition databases because water content is one of the most variable components, especially in plant foods. This variability affects the composition of the food as a whole. The range of methods for water analysis is summarized in Table 7.1.

The methods are based on the direct or indirect measurement of water removed from the food, changes in physical properties that change systematically with water content, or the measurement of the chemical reactivity of water (Egan, Kirk and Sawyer, 1987; AOAC International, 2002; Sullivan and Carpenter, 1993; Southgate, 1999; Bradley, 1998).

For the majority of foods in food composition databases, drying methods are adequate; although slight methodological differences can be observed, these differences are rarely significant. The AOAC Official Methods recommend a lower drying temperature (70 °C) for plant foods to minimize the destruction of carbohydrates. Where this occurs it is usually better to use vacuum drying or freeze-drying.

Vacuum drying is most efficient if a slow leak of dry air is passed through the oven. This approach has the advantage that the analytical portions can be left unattended for long periods. Vacuum drying at 60–70 °C is preferable to drying in an air oven, particularly for foods that are rich in sugars. However, for most foods drying in an air oven is satisfactory for food composition database purposes.

Freeze-drying requires more capital investment but has the advantage that it dries the foods under mild conditions. Freeze-dried material is light, easily transported and can also be ground very easily. The process does, however, usually leave some residual moisture in the freeze-dried material, which must be removed to give values that are comparable with other drying methods.

Drying in a microwave oven is very quick but requires continuous surveillance to avoid charring. Drying with infrared lamps has been very successfully automated (Bradley, 1998). Both of these methods, however, are more suitable for routine quality control.

All the methods mentioned so far are unsuitable for foods with a high content of volatile components because these are driven off with the water. The Dean and Stark method can be used for such foods where a value for the moisture content is required. In this method the water is distilled off as an azeotropic mixture with an immiscible solvent such as toluene, xylene or tetrachloroethylene. The method is an AOAC-approved method for spices and cheese, and has achieved good levels of precision (AOAC International, 2002).

The Karl Fischer method is especially useful for foods with very low moisture content and for hygroscopic foods that are difficult to dry using conventional methods. The levels of accuracy achieved are rarely required for food composition databases.

The physical methods for measuring water content require expensive, highly specialized instrumentation and are most suitable where there is a very high throughput of similar samples.

Near infrared reflectance (NIR) methods, for example, have been widely applied for the analysis of cereal grains. The method requires calibrating with a large number of samples with moisture values measured by conventional methods to develop the analytical equations. Nuclear magnetic resonance (NMR), gas–liquid chromatography (GLC) and gas–solid chromatography (GSC) methods also require detailed calibration and are of greatest value in measuring the distribution of water in foods and identifying the forms of water in meats.

Nitrogen and nitrogenous constituents

Lakin’s (1978) review still provides a comprehensive account of the analysis of nitrogen and nitrogenous constituents, and the methods are discussed briefly by Sullivan (1993) when reviewing the AOAC Official Methods, by Chang (1998) and by Southgate (1999). The range of methods is summarized in Table 7.2.

Total nitrogen

The proximate system, where “protein” is measured as total nitrogen multiplied by a specific factor, continues to dominate food composition studies. Most cited values for “protein” in food composition databases are in fact derived from total nitrogen or total organic nitrogen values. In the majority of cases, total nitrogen is measured using some version of the Kjeldahl (1883) method (which measures total organic nitrogen). In this method the organic matter is digested with hot concentrated sulphuric acid. A “catalyst mixture” is added to the acid to raise its boiling point, usually containing a true catalytic agent (mercury, copper or selenium) together with potassium sulphate. All organic nitrogen is converted to ammonia, which is usually measured by titration or, more rarely, colorimetrically. In the original method, a relatively large analytical portion (1–2 g) was used, but this requires large amounts of acid. Micro-Kjeldahl methods are much more commonly used as they produce a reduced amount of acid fumes and also require less acid and catalyst mixture. Environmental considerations exert considerable pressure to ensure the safe disposal of mercury and, especially, to minimize acid usage.

The micro methods can be automated at several levels (Egan, Kirk and Sawyer, 1987; Chang, 1998). Automation of the distillation and titration stages works well but automation of the digestion has proved quite difficult.

The Dumas method measures the total nitrogen as nitrogen gas after complete combustion of the food. Comparison of the results obtained with those obtained using the Kjeldahl method shows good agreement (King-Brink and Sebranek, 1993). The method has been successfully automated and, although the instrumentation is expensive, a high throughput of samples is possible, with good precision. The equipment uses very small analytical portions, and a finely divided analytical portion is essential.

NIR can also be used to measure nitrogen in some foods, although a large number of calibration samples is required.

Protein

Since the development of the proximate system of analysis, “crude protein” values have been calculated by multiplying the total nitrogen (N) by a certain factor. This factor was originally 6.25, based on the assumption that proteins contained 16 percent of N. It has been known for a considerable time that proteins of plant origin (and gelatin) contain more N and therefore require a lower factor. Jones, Munsey and Walker (1942) measured the nitrogen content of a wide range of isolated proteins and proposed a series of specific factors for different categories of food. These factors have been widely adopted and were used in the FAO/WHO (1973) review of protein requirements. These are listed in Table 7.3. Several authors have criticized the use of these traditional factors for individual foods (e.g. Tkachuk, 1969). Heidelbaugh et al. (1975) evaluated three different methods of calculation (use of the 6.25 factor, use of traditional factors and summation of amino acid data) and found variations of up to 40 percent. Sosulski and Imafidon (1990) produced a mean factor of 5.68 based on the study of the amino acid data and recommended the use of 5.70 as a factor for mixed foods.

In principle, it would be more appropriate to base estimates of protein on amino acid data (Southgate, 1974; Greenfield and Southgate, 1992; Salo-Väänänen and Koivistoinen, 1996) and these were incorporated in the consensus document from the Second International Food Data Base Conference held in Lahti, Finland, in 1995, on the definition of nutrients in food composition databases (Koivistoinen et al., 1996).

If these recommendations are to be adopted, the amino acid data should include values for free amino acids in addition to those for protein amino acids because they are nutritionally equivalent. The calculations require very sound amino acid values (measured on the food) as discussed below, and involve certain assumptions concerning the proportions of aspartic and glutamic acids present as the amides and correction for the water gained during hydrolysis. Clearly, this approach would not be very cost-effective when compared with the current approach.

At the present time it is probably reasonable to retain the current calculation method, recognizing that this gives conventional values for protein and that the values are not for true protein in the biochemical sense. However, it is important to recognize also that this method is not suitable for some foods that are rich in non-amino non-protein nitrogen, for example cartilaginous fish, many shellfish and crustaceans and, most notably, human breastmilk, which contains a substantial concentration of urea.

A number of direct methods for protein analysis have been developed for specific foods based on reactions involving specific functional groups of the amino acids present; these are thus not applicable to the measurement of proteins in general. Such methods include formol titration (Taylor, 1957) and the biuret reaction (Noll, Simmonds and Bushuk, 1974). A widely used group of colorimetric methods is based on reaction with Folin’s reagent, one of the most widely used biochemically in the dairy industry (Lowry et al., 1951; Huang et al., 1976). These methods are most commonly calibrated with bovine serum albumin, which is available at high purity.

Dye-binding methods have been widely applied in the dairy industry (Udy, 1971); dye-binding can be made more sensitive by extracting the dye (McKnight, 1977), and the methods have been included in the AOAC Official Methods. Most of these methods depend on calibration against the Kjeldahl method. Pomeranz, Moore and Lai (1977) have published a comparison of biuret, NIR, dye-binding and alkaline distillation in the measurement of protein in barley and malt. Ribadeau-Dumas and Grappin (1989) have published a review of protein measurements in milk. In general, dye-binding methods have their widest application in the routine quality control of analysis of large numbers of similar types of sample (Van Camp and Huyghebaert, 1996).

Amino acids

Before the development of ion-exchange chromatography (IEC) individual amino acids were measured by colorimetric methods or by microbiological assay. Although these methods yielded acceptable results they have been almost completely superseded by chromatography procedures (Moore and Stein, 1948). These use automated systems that give complete analyses rapidly and with reasonable levels of precision.

The amino acids in the protein must first be released by hydrolysis and this constitutes the most critical stage of the analysis. Acid hydrolysis, usually with 6M HCl in an oxygen-free solution, gives complete release of most amino acids. Tryptophan is completely degraded in acid conditions and threonine, serine and the sulphur amino acids are partially degraded. Alternative hydrolysis conditions must therefore be used to measure tryptophan. Cystine and methionine are usually protected by specific oxidation before hydrolysis. Losses of threonine and serine are time-dependent and it is necessary to carry out serial hydrolyses to estimate the rate of degradation and correct the values accordingly. Conversely, the branched-chain amino acids are slowly released on hydrolysis, and serial hydrolyses are necessary to estimate complete release (Neitz, A., personal communication). Williams (1982) reviewed the development of IEC techniques and discusses the use of high-performance liquid chromatography (HPLC) as an alternative.

The conditions for acid hydrolysis require pure acid and a high ratio of acid to analytical portions of the food. Even so, high-carbohydrate foods often react with the amino acids during hydrolysis, leading to losses that are difficult to quantify (Silvestre, 1997). Vapour phase hydrolysis has been suggested as an approach that minimizes the degradative losses. In this method the dried food (or protein) sample is hydrolysed by condensing acid. 6M HCl corresponds to the constant boiling mixture for the acid (De Geeter and Huyghebaert, 1992).

Sulphur amino acids are usually oxidized with performic acid before hydrolysis. Some chlorination of tyrosine can occur and the addition of phenol to the acid is often used to reduce this. The hydrolysis should be carried out under nitrogen or, preferably, in sealed tubes.

Hydrolysis must be carried out for three different time periods – 24, 38 and 48 hours – to allow correction for slow release and degradation losses. If pure bovine serum albumin is hydrolysed as a standard, this should also be hydrolysed for the same time periods.

Tryptophan is measured after alkaline hydrolysis (KOH, Ba(OH)2 or LiOH) (Landry and Delhave, 1993). It is usual to measure the leucine in the hydrolysate to adjust the values to be consistent with the acid hydrolysis. A number of alternative reagents and pre- and post-column derivatives have been used, but ninhydrin, despite its instability, is probably the most widely employed. Most other reagents vary in their sensitivity. Capillary gas chromatography has also been used, but most of the reagents vary in their rates of reaction with different amino acids.

In calculating the results of amino acid analyses it is important to express the amino acid values as mg amino acid per g nitrogen applied to the column. As a check on the analyses it is also important to calculate the recovery of nitrogen as amino acids and ammonia from the measured amino acids. There will usually be some losses during hydrolysis and the chromatography. If the losses are found to exceed 10 percent, repeating the hydrolysis should be considered.

Since 1990, HPLC methods of derivatized amino acids have replaced IEC for the analysis of protein hydrolysates in most laboratories as they offer reduced analysis time and improved limits of detection of about 1 picomole (pmol) (Cohen and Strydom 1988; Davey and Ersser 1990; Sarwar and Botting, 1993).

HPLC may be used to separate amino acids on ion-exchange columns with postcolumn derivatization with ninhydrin or OPA (o-phthaldialdehyde) (Ashworth, 1987) or by precolumn derivatization followed by separation on reversed-phase octyl- or octadecyl silica (Cohen and Strydom, 1988). For the analysis of amino acids in protein hydrolysates, reversed-phase HPLC with precolumn derivatization with PITC (phenylisothiocyanate) is becoming established as a cheaper alternative to commercial amino acid analyses using IEC. The PITC derivatization method enables the accurate determination of all nutritionally important amino acids except tryptophan in 12 minutes, while a liquid chromatographic method requiring no derivatization enables the determination of tryptophan in about eight minutes (Sarwar and Botting, 1993).

The range of methods is summarized in Table 7.4.

Available lysine

Lysine can become nutritionally unavailable under certain conditions that lead to the ε-amino group reacting with carbohydrate. This reaction reduces the biological value of the protein. Using the Carpenter method (1960) available lysine can be measured by its reaction with 2,4-fluorodinitrobenzene. This method has been the subject of many modifications (Williams, 1982). HPLC separation of ε-DNP lysine is described by Peterson and Warthesen (1979).

Other nitrogenous substances

Several groups of foods, fish and other marine foods, meats, fungi and vegetables contain a range of nitrogenous materials, amines (Steadman, 1999) and nucleic acids. Many of these react with ninhydrin and can be separated by IEC. Methods for nucleic acids were reviewed by Munro and Fleck (1966). They may also be separated by HPLC and detected by their strong ultraviolet (UV) absorption.

Lipid constituents

FAO/WHO (1994) recommended that adequate food composition data on fats should be widely accessible and that standard methods and reference materials should be used for the analysis of fatty acids and preparation of nutrient databases. The report provides good coverage of the compounds and nutritional issues of interest. Christie (2003) is a key reference for lipid analysis.

In the proximate system of analysis, ‘fat’ is measured as the fraction of the food that is soluble in lipid solvents. The extracted material contains a range of different classes of substances. For nutritional purposes the measurement of ‘total fat’ has limited value; nevertheless, it still is widely reported and is retained in many requirements for food labelling and the regulation of food composition.

The range of methods is summarized in Table 7.5.

Total fat

The values obtained for total fat or total material soluble in lipid solvents are very method-dependent. Carpenter, Ngeh-Ngwainbi and Lee (1993), in their review for the AOAC of methods for nutritional labelling, set out the nature of the problems encountered. Gurr (1992) and Gurr, Harwood and Frayn (2002) discuss in detail the methods available for separating the different classes of lipids.

The classical method is based on continuous extraction performed on dried samples of food in a Soxhlet extractor, sometimes preceded by acid hydrolysis. This technique is time-consuming and subjects the extracted lipids to long periods at high temperatures. Its main drawback, however, is that it yields incomplete lipid extractions from many foods, especially baked products or those containing a considerable amount of structural fat. The extractant used is often petroleum spirit (which is less flammable that diethyl ether and less likely to form peroxides), which requires completely dry analytical portions and the removal of mono-and disaccharides. Values obtained using this method require close scrutiny before their inclusion in a database and their continued use is not recommended.

Other solvents, for example, trichloroethylene, are used in a number of automated systems of the ‘Foss-Let’ type; these appear to give more complete extractions (Pettinati and Swift,1977).

The use of mixed polar and non-polar solvents has been shown to extract virtually all the lipids from most foods. In the case of baked (cereal) products, however, incomplete extraction of fat may occur. Chloroform–methanol extraction is well known (Folch, Lees and Stanley, 1957; Bligh and Dyer, 1959); this combines the tissue-penetrating capacity of alcohol with the fat-dissolving power of chloroform. The resultant extracts are complete but may also contain non-lipid materials and require re-extraction to eliminate these. This extraction method is preferred when the extract is to be subsequently measured for fatty acids and sterols (Shepherd, Hubbard and Prosser, 1974). The method is effective for composite foods and is included in the AOAC Official Methods. It has been shown to be useful for foods such as brain and egg that are rich in phospholipids (Hubbard et al., 1977). The measurement of lipids after acid (Weibull and Schmid methods) or alkaline (Röse-Gottlieb method) treatment also provides good extraction from many foods. These techniques are recognized as regulated methods by the AOAC and the European Union. Alkaline methods are almost exclusively used for dairy foods and are the approved method for such foods. The extracts from acid and alkaline treatments are not suitable for fatty acid analysis because some oxidation and losses due to (acid) hydrolysis of fats may occur. The AOAC has adopted methods for determining total fat (also saturated, unsaturated and monounsaturated fats) in foods using acid hydrolysis and capillary gas chromatography (Ngeh-Ngwainbi, Lin and Chandler, 1997; House, 1997) to comply with the Nutrition Labeling and Education Act (NLEA) definition of fat as the sum of fatty acids expressed as triacylglycerols.

Lipid classes show strong carbonyl absorption bands in the infrared region. NIR has been used for legumes (Hunt et al., 1977a) and various other foodstuffs (Cronin and McKenzie, 1990). The effective use of the method depends on extensive calibration against comparable matrices using another approved method; for this reason the technique is most commonly applied in routine analyses of large numbers of very similar samples, for foods such as cereals and dairy products.

Triacylglycerols

Although it is probable that the composition of triacylglycerols (triglycerides) has nutritional significance, few databases contain compositional information. Methods for separating the individual components have not been extensively developed (Gurr, Harwood and Frayn, 2002). Thin-layer chromatography in combination with chromatography has been used. Total values can be found by separating the free fatty acids from the total lipid and can be used to give a “by difference” value. HPLC techniques have been proposed for the complete fraction of triacylglycerols (Patton, Fasulo and Robbins, 1990a, b; Gonzalez et al., 2001).

Fatty acids

Separation by GLC of the methyl esters of the fatty acids prepared by transmethylation of the lipid extracts from foods is the method of choice. The development of column packing materials, capillary techniques and detector amplification systems has extended to application of the method for the separation of isotopic forms and longer-chain fatty acids. The technique published by the International Union of Pure and Applied Chemistry (IUPAC) (Paquot and Hautfenne, 1987) forms the basic procedure.

The exact method chosen will depend on the food to be analysed and the fatty acids of particular interest. Many users will be particularly interested in n-3 and n-6 fatty acids, trans acids and levels of long-chain fatty acids such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Automation of sample injection and the computerization of the chromatographs have added to the costs of the analytical apparatus but greatly improve accuracy, precision and analytical throughput. The American Oil Chemists’ Society (1998) methods are: Method No. Ce 1-62 (packed column method for methyl esters of C9–C24 acids, and animal fats), Method No. Ce 1b-89 (capillary method for marine oils and for ethyl or methyl esters of C14–C24 acids (percentage relative values and mg/g levels of EPA and DHA), Method No. Ce 1c-89 (capillary method for fatty acids, trans isomers and cis, cis methylene-interrupted isomers in vegetable oils), Method No. Ce 1e-91 (capillary method for C4–C24 fatty acids), and Method No. Ce 1f-96 (capillary method for cis- and trans fatty acids in hydrogenated and refined oils and fats).

Infrared detectors are useful in the measurement of trans fatty acids (AOAC International, 2002). The major difficulty is the assignment of unequivocal identity to isomers. This requires good standards or combining the GLC separation with mass spectrometry (Beare-Rogers and Dieffenbacher, 1990), which may make it impractical for some developing countries.

Infrared absorption is currently the preferred method for the measurement of trans fatty acids in hydrogenated fish oils. GLC measurement of trans fatty acids in partially hydrogenated vegetable oils using a flame ionization detector (FID) often underestimates the trans fatty acid content, even on very long, highly polar, capillary columns (Aro et al., 1998).

Food composition laboratories lacking GLC instrumentation do not usually undertake fatty acid measurements but may seek cooperation with a laboratory with the necessary capital resources. The samples may be transferred to the laboratory as fat (which requires cold storage during transit and the addition of an antioxidant) or methyl esters (which also need to be protected against oxidation). It is important to verify these arrangements with the analysing laboratory to avoid interference by the antioxidants during chromatography.

The unsaturation of a fat can be estimated by iodine value determination (IUPAC, 1979; AOAC International, 2002); this remains a useful technique when full fatty acid analyses are not undertaken.

Sterols

In the past, nutritional analyses emphasized the measurement of cholesterol, but there is an increasing focus on the measurement of other sterols, especially phytosterols.

Cholesterol. The older techniques, using gravimetric and colorimetric methods, are now regarded as obsolete and are no longer used. The preferred methods are chromatographic, with widespread use of GLC of a range of derivatives separated on low-polarity columns (Punwar, 1975; Hubbard et al., 1977). One problem with sterol analysis in general is that the greater proportion of other lipids in most foods limits the application of the methods to the lipid extract directly.

Saponification is required before the preparation of derivatives. The use of trimethysilyl (TMS) derivatives met the standards required by the AOAC (Carpenter, Ngeh-Ngwainbi and Lee, 1993) for use with mixed foods. The procedures are somewhat complex and simplified methods have been proposed that require shorter sample preparation times (Thompson and Merola, 1993).

Improvements in the development of capillary GLC have provided the basis for developing procedures that do not require derivatization and that meet the appropriate standards (Jekel, Vaessen and Schothorst, 1998).

Other sterols. The method described above can also be used for the separation and measurement of the range of phytosterols found in the diet (Jonker et al., 1985), as can derivatization with TMS (Phillips, Tarrogo-Trani and Stewart, 1999).

Phospholipids

A comprehensive review of phospholipids published in 1973 (Ansell, Hawthorne and Dawkins.) summarized the analytical procedures available. Subsequently, HPLC techniques were developed (Hammond, 1982; Patton, Fasulo and Robbins, 1990a, b) and are now the methods of choice. Gunstone, Harwood and Padley (1994) provide an overview of methods for measuring the range of phospholipids.

Carbohydrates

The range of carbohydrates found in the human diet (see Table 4.3) illustrates the nature of the task facing the analyst who wishes to follow the recommendations published by FAO/WHO (1998) for measuring the carbohydrates in foods separately. Not all types of carbohydrates are, of course, present in all types of foods.

The distinctive metabolic and physiological properties of the different carbohydrates emphasize the fact that for nutritional purposes it is inadequate to consider the carbohydrates as a single component of foods.

The calculation of “carbohydrate by difference” using the Weende proximate system of analysis described at the beginning of the chapter was a reflection of the state of knowledge of carbohydrate chemistry at the time. Moreover, the system was designed for animal feedstuffs, especially for ruminants, and most of the carbohydrates (except lignin-cellulose of which crude fibre was an approximate measure) would therefore be digested in the rumen.

For nutritional purposes carbohydrates can be considered as falling into three groups based on the degree of polymerization:

- sugars (mono- and disaccharides);

- oligosaccharides (polymers of three to nine monosaccharide or uronic acid units);

- polysaccharides (polymers containing more that nine units), which fall into two broad categories: α-glucans (starches, starch hydrolysis products and glycogen) and a much more diverse group of non-α-glucans (non-starch polysaccharides [NSPs], which are the major constituents of dietary fibre).

These broad chemical groupings do not correspond precisely with physiological properties or with analytical fractions. The analyst faced with the analysis of carbohydrates, particularly NSPs, is “bound to make a compromise between the ideal of separating the many components and measuring them or a scheme that is entirely empirically based” (Southgate, 1969). In many cases, a food contains a limited range of carbohydrates and simpler procedures can be used for its analysis (Southgate, 1991).

The range of methods is summarized in Tables 7.6 to 7.8.

Sugars

A range of methods can be used for the analysis of the free sugars in foods; the choice depends primarily on the qualitative composition of the free sugars present in the food. Where a single carbohydrate species is present virtually any procedure can be used, but most foods contain a mixture of three or more components and separation of the components is required to produce accurate results. Specific enzymatic methods are available for the analysis of certain common mixtures without separation.

The methods for free sugars (and uronic acids) provide the end-analytical methods for most of the higher carbohydrate polymers after hydrolysis and separation of the components.

The evolution of the methods closely parallels the development of analytical techniques coupled with the pressures of the demands for analytical results. Thus the physical techniques were initially developed for the analysis of sucrose solutions in the sugar-refining industry. The reducing sugar methods were also developed for this industry and the methods were refined and their protocols codified under the auspices of the International Commission for Unified Methods of Sugar Analysis (ICUMSA, 1982). These methods still give satisfactory results providing the protocols are followed closely.

Colorimetric techniques were developed later, with the advent of improved methods for assessing optical density (although early measurement involved the visual matching of solutions). The range of chromogenic reagents for different monosaccharide classes and uronic acids mostly involve reactions in concentrated acids although colorimetric methods are based on reductiometric methods and a few on other reactions (Hudson et al., 1976). The methods are not especially robust, but on simple sugar mixtures with proper quality control they give sound values. The methods are not truly specific and this limits their use for the analysis of mixtures (Hudson and Bailey, 1980).

Specific enzyme methods have been developed, the most notable being the glucose-oxidase method, which has a colorimetric end-point. A series of coupled reactions with NADPH–NADP using specific enzymes permits the analysis of mixtures of glucose/fructose and glucose/fructose/sucrose and maltose/galactose (Southgate, 1991).

Chromatography, initially on paper or silica plates, provided good separations and semi-quantitative methods, but ion-exchange techniques were difficult to develop.

Gas chromatographic analysis depended on the preparation of suitable volatile derivatives. Initially trimethyl-silylation provided suitable derivatives for the analysis of sugar mixtures, although the chromatograms were very complex. The most widely used and powerful method for the analysis of mixtures involves the reduction of the monosaccharides to the alditols and acetylation.

HPLC columns are now available that give good separation of sugar mixtures without the need for the preparation of derivatives. The first detectors used refractive indices to measure the eluted peaks, but these are relatively insensitive and have been superseded by the pulsed amperometric detector, which has improved sensitivity.

Polyols (sugar alcohols)

Polyols are not widely found in foods. Some can be measured by specific enzyme methods although HPLC methods are more commonly used.

Oligosaccharides

These are widely distributed, especially in vegetables, and the malto- series is found particularly in foods that have partial starch hydrolysates and glucose-syrup preparations as ingredients. The malto-oligosaccharides are hydrolysed by brush-border enzymes and are “glycemic carbohydrates” that need to be measured separately.

Fructo-oligosaccharides are increasingly used as ingredients and should be measured after hydrolysis with specific fructan hydrolases. The remaining galacto-oligosaccharides should also be measured after specific enzymatic hydrolysis. GLC and, particularly HPLC separation techniques also offer powerful methods for the analysis of these oligosaccharides (Quigley, Hudson an Englyst, 1997).

Polysaccharides

These are best considered, for nutritional purposes, under two headings – starch and non-starch polysaccharides (NSPs).

Starch. This category includes all the α-glucans, starches, partially hydrolysed starches and glycogen. The latter is a minor component of most animal products; it is found in significant concentrations in fresh liver and horse flesh and as traces in lean muscle.

Polarimetric methods are limited to some cereals, but with proper calibration and standardization can give satisfactory results (Fraser, Brendon-Bravo and Holmes, 1956; Southgate, 1991).

Dilute acid hydrolysis can be used for highly refined foods with low concentrations of NSPs, and virtually any monosaccharide method can be used to measure the glucose produced.

The use of a glucose-specific method such as glucose-oxidase extends the range of foods for which this method is useful (Dean, 1978; Southgate, 1991).

Enzymatic hydrolysis with specific amylolytic enzymes, followed by precipitation of the residual NSPs with ethanol, and measurement of the glucose produced, is the most satisfactory and widely applicable method. The choice of enzymes and the conditions for hydrolysis are critically important. If values for total starch are required, any enzymatically resistant starch must be treated with alkali or dimethyl sulphoxide (DMSO) before hydrolysis (Southgate, 1991).

Resistant starch. Although enzymatically resistant starch was first observed analytically, the current view is that it should be defined as resistant physiologically, that is, resistant to hydrolysis in the human gastrointestinal tract (Gudmand-Hoyer, 1991). Englyst, Kingman and Cummings (1992) have distinguished three types of resistance, due to physical enclosure of starch, starch granule structure, and retrogradation. The latter type is more common in processed foods. The most common approach is to measure starch before and after treatment with DMSO.

Rate of digestion. Englyst and his coworkers (1999) have proposed that the rate of digestion of starch is the major determinant of variations in the glycemic responses to food and proposed that the starch can be considered to fall within three classes: rapidly digestible starch, slowly digestible starch and resistant starch. While the rate can be distinguished in vivo, simulation analytically is quite difficult. Collaborative studies have shown that reasonable precision can be obtained (Champ, 1992).

Glycemic index. There has been great interest in including glycemic index (GI) values in food composition databases and a set of tables of GI values has been published (Foster-Powell and Miller, 1995). The GI values (strictly speaking, a ranking of the carbohydrates in foods) are based on their glycemic effect compared with that of a standard food. The GI is defined as “the incremental area under the blood glucose response curve expressed as a percentage of the response to the same amount of carbohydrates from a standard food taken by the same subject” (FAO/WHO, 1998). The standard food is usually white bread or glucose. FAO/WHO (1998) have published a protocol using six or more subjects and define carbohydrate as “glycemic (available) carbohydrate”. A working definition used by the main Australian laboratory measuring GI defines carbohydrate as “total carbohydrate by difference minus the sum of dietary fibre plus resistant starch (if known) or the sum of starch plus sugars, including polyols and other slowly absorbable sugar derivatives” (Brand-Miller and Holt, personal communication).

In Australia, the use of a GI symbol on food labels is permitted and a Web site is available for consultation (http://www.glycemicindex.com). The GI of meals can be calculated but not of cooked recipe foods because the GI of a food is affected by cooking and processing.

Estimates of the different rates of digestion of starch in foods show some correlation with glycemic indices measured in vivo. These require a number of human subjects to have blood glucose levels measured at intervals for three hours after consumption of a fixed amount (50 g) of glycemic carbohydrates. The area under the curve is compared with the area under the curve for a 50 g glucose load or, better, 50 g of glycemic carbohydrates from white bread. White bread is preferred because glucose loads can be emptied slowly from the stomach because of osmotic effects. An interlaboratory study (Wolever et al., 2003) showed that within-subject variation in glycemic response needs to be reduced to improve precision of the method.

An in vitro method for rapidly available glucose published by Englyst et al. (1999) showed a high correlation with glycemic response.

Non-starch polysaccharides. Methods for NSP analysis involve treatment of the sample to remove free sugars and starch by enzymatic hydrolysis. The unchanged NSPs are recovered by precipitation with ethanol (80 percent v/v), then washed and dried. The NSPs are hydrolysed using one of two methods: sequentially with dilute acid, which hydrolyses most of the non-cellulosic polysaccharides (NCPs), and with 12M H_2SO_4, which hydrolyses the cellulose; or the NSPs are hydrolysed completely using 12M acid (see “Measurement of NSPs” below for further details).

The monosaccharides are analysed by GLC after derivatization (as the alditol acetates [Englyst, Wiggins and Cummings, 1982]) or by HPLC, or as a total colorimetrically (Englyst, Quigley and Hudson, 1994). The methods are not very robust (Southgate, 1995), although collaborative trials have shown that when careful attention is given to the protocol the methods give reasonable precision.

Choice of method for carbohydrates

There is no single method that meets the recommendations of the FAO/WHO (1998) review. Ideally, when planning to measure the carbohydrates in foods one should aim to measure the different carbohydrate species in a food sequentially using one analytical portion; this approach avoids the possibility of double measurement of an overlapping fraction.

The basic principles of such an approach are set out schematically in Figure 7.1.

Extraction of free sugars, polyols and oligosaccharides. This could be done with an aqueous extraction, but this procedure will extract proteins with the result that subsequent analysis is more complex. The removal of fat is desirable for technical reasons, as this facilitates a more complete extraction of sugars. Extraction with aqueous alcohol is the most common approach: 80 percent v/v aqueous ethanol is most commonly used, but 85 percent v/v methanol is also useful, as is isopropanol. The extractions are usually made with a boiling solvent; care should therefore be taken to protect the analysts from solvent fumes. If the extract is likely to be acid it is important to neutralize the acid to avoid hydrolysis of di- and higher saccharides.

The aqueous alcohols will also extract some lower polysaccharides – short-chain polysaccharides as defined by Englyst and Hudson (1996). These should preferably be measured after selective enzymatic hydrolysis. Modern enzyme technologies have produced a wide range of very specific enzymes with high activity; many companies specialize in this area, for example, Boehringer Mannheim, Germany; Megazyme, Ireland; Nova, Denmark; and Sigma, the United States. Several of these companies prepare enzyme method “kits”. The rate of development of enzyme technology is such that it is expected that selective enzymatic hydrolysis will become increasingly important analytically because of the specificity offered (McCleary and Prosky, 2001).

Starch hydrolysis. The next stage is to remove starch using selective enzymatic hydrolysis. A number of enzymes can be used for this purpose. A mixture of amylase and pullulanase has been used to give complete hydrolysis to glucose but many glucoamylases give virtually complete hydrolysis to glucose. The conditions for enzymatic hydrolysis are critical, both to ensure complete and rapid starch hydrolysis and to minimize hydrolysis of NSPs, especially β-glucans. Unhydrolysed NSPs are recovered by precipitation with ethanol to 80 percent v/v.

Measurement of NSPs. The precipitated NSPs are washed and dried gently and then hydrolysed. This may be done in boiling 1M H_2SO_4 followed by hydrolysis in 12M acid at ambient temperature. This produces, first, a hydrolysate containing the monosaccharides from the NCPs and, second, the monosaccharides from a cellulosic fraction. Alternatively, the NSPs may be hydrolysed in 12M acid followed by the dilute acid, which produces a hydrolysate containing the monosaccharides from the NSPs as a whole. Uronic acids are not hydrolysed completely by these methods, and colorimetric analysis is widely used (Englyst, Quigley and Hudson, 1994). Specific enzymatic hydrolysis of the uronic acid containing polymers is now possible (Quigley and Englyst, 1994).

Dietary fibre

Dietary fibre should be considered as part of the carbohydrates in foods. The major problem in the choice of method lies in the definition of dietary fibre and its interpretation in an analytical context. The term was first used in 1953, by Hipsley, to describe the sum of the hemicelluloses, cellulose and lignin in food, in other words the components of plant cell walls in foods. Trowell, in 1972, took up the term for “the indigestible components of the plant cell wall in foods”. Both these terms were too vague to use as a basis for an analytical method and in 1976 Trowell et al. (1976) proposed that it be defined as “the sum of the plant polysaccharides and lignin that are not digested by the enzymes of the gastrointestinal tract”. This was closely analogous to the “unavailable carbohydrates” as defined by McCance and Lawrence (1929) and measurable by the procedures proposed by Southgate (1969).

In this method the aim was to measure the carbohydrates specifically using colorimetric techniques. Englyst developed this approach using the more specific GLC methods, which gave values for the non-starch polysaccharides and incorporated a stage to convert resistant starch to non-enzymatically resistant starch. The procedure was developed in a series of collaborative studies and the most recent protocols are described by Englyst, Quigley and Hudson (1994) and Southgate (1995). This method measures only the NSPs and does not include lignin.

In other parts of Europe, especially Sweden and Switzerland, and in the United States, the focus was directed at the “indigestibility of the polysaccharides and lignin”. A gravimetric method was developed where the residue after starch removal is weighed to give a measure of total dietary fibre (TDF); this has evolved into the Official AOAC Method No. 982.29 (Prosky et al., 1992). The method requires correction of the residue for undigested protein and for mineral contamination; total nitrogen and ash in the residue are measured and deducted to give the TDF values. These include lignin, resistant starch and all other indigestible carbohydrates (Guillon et al., 1998). A modification has been introduced to include the measurement of indigestible oligosaccharides.

The Englyst NSP and the AOAC TDF procedures are not very robust, especially where low levels are present (Southgate, 1995). The NSP method uses analytical portions of 100–200 mg and the preparation and homogeneity of these portions is absolutely critical. The mixing procedures also require close attention during the execution of the method.

The AOAC gravimetric procedure requires great skill when measuring low levels but gives good precision with high-fibre foods such as bran and wholemeal products. The residue also includes heat-induced artefacts.

In many countries, the choice of method for nutrition labelling will be defined by legislation. Nutritionally specific measurement of the different carbohydrate fractions is the preferred approach. The measurement of soluble and insoluble fractions is highly method-dependent and the FAO/WHO (1998) review concluded that there was no physiological justification for recording separate values based on solubility.

It is important to recognize that the hypothesis concerning the protective effects of dietary fibre was based on differences between diets (Burkitt and Trowell, 1975), i.e. it was a statement about the protective effects of diets that were rich in foods containing plant cell walls in a relatively unprocessed state. These diets are rich in many other components in addition to dietary fibre.

Alcohol

The classical method for measuring the alcoholic content of beverages is distillation of the de-gassed beverage and measurement of the specific gravity of the distillate. While this is still a valid and precise approach, measurement by GLC (which is simpler and quicker) or, alternatively, a specific enzyme procedure using alcohol dehydrogenase (Bergmeyer, 1974) are preferred methods as the distillation methods can be interfered with by other volatile constituents.

Organic acids

A variety of specific enzyme methods for different organic acids (Bergmeyer, 1974) remain valid, but these approaches have been superseded by HPLC methods (Wills et al., 1983). In a food product that contains acetic acid, simple acid-base titration can be used (Sadler and Murphy, 1998).

Inorganic constituents

The majority of methods for inorganic constituents require the organic matrix of the foods to be removed, or extraction and concentration, before they can be applied. Destruction of the food matrix removes a large number of potential sources of interference and provides the inorganic material in a concentrated form. In classical food analysis the organic matrix was incinerated (usually in a muffle furnace at a controlled temperature) and the resultant inorganic residue was weighed to give a value for ash in the proximate system of analysis. The organic matrix can also be destroyed by being heated in concentrated acids. This procedure minimizes losses during the oxidation and avoids any reaction between the inorganic constituents and the vessel used for dry incinerations.

Once the organic matrix has been removed the inorganic constituents can be measured using a variety of techniques. These include classical gravimetric or volumetric methods, polarimetry, ion-selective electrodes, colorimetric procedures (which may or may not be highly specific) and instrumental methods (which offer an increase in speed of analysis, automation and good precision). Many of the instrumental methods can be used for analysis of a number of constituents. In using these methods it is important to ensure that interference from other constituents is eliminated and it is essential to use standard reference (or in-house reference) materials with a similar matrix and apply other quality control measures. This approach is of fundamental importance in the measurement of trace inorganic constituents.

Total ash

Nutritionally, there is little value in recording ash values other than to provide an approximate estimate of the total inorganic material and to check for replication in the destruction of the matrix. A value for total ash is, of course, essential when it is necessary to calculate carbohydrate “by difference”.

In dry ashing, the food is incinerated in a crucible, usually made of silica, although porcelain (can be used but less suitable) or platinum (very expensive but the least reactive) can be used. The food matrix must be destroyed by heating gently at first to char the sample and then at 500 °C in a muffle furnace (Wills, Balmer and Greenfield, 1980) to prevent foaming of lipids (and sugars) until a white (or light grey) residue is produced. Heating above 500 °C can result in the loss of alkali metals. The general procedure is described by Osborne and Voogt (1978) and in the AOAC Official Methods (see Sullivan and Carpenter, 1993).

In the case of “wet ashing” acid digestion, the food sample is heated with acid – usually a mixture of nitric and sulphuric acids. Perchloric acid is often included in the digesting acid mixture although this introduces the risk of explosion and the procedure must be carried out in a fume hood designed for the use of perchloric acid. Wet ashing offers the advantage that no reactions with the crucible can occur that can lead to the formation of insoluble silicates. Digestion can be carried out in a Kjeldahl flask but this requires a larger quantity of acid. Particularly for trace element analysis, digestion is best carried out in a sealed container. Tubes designed for this purpose are available from most laboratory suppliers. They are made from resistant glass and have a cap with a plastic insert to provide an inert gas-tight seal. The analytical portion and the acid are placed in the tube, which is then capped and may be heated in a conventional or microwave oven. The tube is then allowed to cool completely before the gases are released with care.

For trace element analyses, the acids used must be of the highest analytical quality; blanks should be run as a matter of course and digestion of the reference materials should be included.

The most widely used instruments are atomic absorption spectrophotometers, which are suitable for the analysis of most cations of nutritional interest. The more simple flame photometers can be used for the analysis of Na and K.

Plasma emission instruments such as inductively coupled plasma spectrometers are available that permit the analysis of a wide range of elements and have the capacity to handle a large number of samples and analytes (McKinstry, Indyl and Kim, 1999). They do, however, require high initial capital expenditure and routine maintenance. Ihnat (1982;1984) provides a detailed review of the application of these methods to foods. Sullivan (1993) discusses the use of these techniques in the AOAC’s Methods of analysis for nutrition labeling (Sullivan and Carpenter, 1993).

Preparation of analytical portion. The residues from dry ashing are usually dissolved in dilute acid and made to volume before analysis. The solutions from wet ashing usually need dilution to a suitable volume before analysis.

Tables 7.9 and 7.10 show methods of analysis for cations and anions, respectively, in foods.

Cations

Sodium and potassium. Flame photometry and atomic absorption spectrophotometry (AAS) are the preferred techniques. Mutual interference can occur and interference from phosphorus has been observed. These can usually be overcome by the application of appropriate standards.

Calcium. Flame photometry and AAS techniques have similar sensitivities. Interference from phosphorus can occur but this can be suppressed by the addition of lanthanum salts or by the use of N_2O flames. Compleximetric titrimetric methods have been used and classical gravimetric methods can be used with foods rich in calcium.

Magnesium. AAS is the method of choice as this offers greater sensitivity than other procedures, with the exception of activation analysis.

Iron. This can be measured by AAS or inductively coupled plasma spectroscopy (ICP) instrumentally. There are, however, sound colorimetric methods available.

Zinc. While colorimetric methods are available, AAS or ICP are the better techniques to use.

Selenium. Hydride generation AAS has been widely used and is probably the method of choice at the present time (Foster and Sumar, 1996; Murphy and Cashman, 2001). Cathodic stripping voltammetry has also been proposed as a method (Inam and Somer, 2000).

Copper and other trace elements. These can be measured satisfactorily by AAS but may require the use of special conditions. ICP, when available, is a satisfactory technique (Coni et al., 1994). Colorimetric methods for copper are quite sound (Sullivan and Carpenter, 1993).

Anions

Phosphorus. This can be measured by ICP but a well-established colorimetric method is the preferred method when applied to wet-digested samples (Fiske and Subbarow, 1925). If dry-ashed samples are used, the pyrophosphates formed during ashing must be hydrolysed.

Chloride. A range of methods can be used. Ion-specific electrode analysis represents the simplest approach, but the classical reaction by titration is also satisfactory (Cotlove, Trantham and Bowman, 1958). Procedures using automated conductimetry also seem to perform well (Silva et al., 1999).

Iodine. This is regarded as one of the most difficult inorganic elements to measure. Dry ashing followed by titration or GLC has been used by the AOAC (Sullivan and Carpenter, 1993). Ion-specific electrodes offer some potential.

Fluorine. Polarographic methods have been developed that produce a very good sensitivity (Guanghan et al., 1999). Methods using ion-selective electrodes also seem to perform well (Kjellevold-Malde, Bjorvatn and Julshamn, 2001).

Sulphur. Sulphur may be measured via conversion to barium sulphate (Paul and Southgate, 1978) or by x-ray fluorescence (Isherwood and King, 1976).

Nitrate and nitrite. Methods include colorimetry (AOAC, 1980), HPLC (Wooton et al., 1985) and capillary ion electrophoresis. Ion-specific electrodes can also be used (Marshall and Trenerry, 1996).

Vitamins

“Vitamin” is a physiological term rather than a chemical term, expressing a certain physiological activity that is related to the chemical substances responsible for this activity. Vitamin activity may be due to a group of chemical compounds, usually related structurally to one another (vitamers).

The analysis of vitamins presents a number of challenges to the analyst and considerable analytical activity has been, and still is, directed at achieving the ideal analytical method for providing chemical values that predict the physiological vitamin activity for human beings in the current context. The ideal method would measure the different vitamers separately so that a value could be calculated for the total vitamin activity (Brubacher, Müller-Mulot and Southgate, 1985). This ideal is rarely possible, in part because of the presence of interfering substances without vitamin activity.

The discussion of methods for individual vitamins will emphasize the handling and preparation of samples for analysis; these are crucial factors because of the lability of some vitamins. Many vitamins are sensitive to light and some can be oxidized very rapidly. Heating can increase the rate of oxidation and may also lead to isomerization to inactive forms; unnecessary heating should therefore be avoided.

A number of detailed reviews on the analysis of vitamins in foods are available (Bates, 2000; Eitenmiller and Landen 1998; Machlin, 1984; Christie and Wiggins, 1978; Van Niekirk, 1982). Brubacher, Müller-Mulot and Southgate (1985) was the result of a collaborative European project which tried to establish a handbook of tested methods. A review of the AOAC Official Methods of vitamins is given by Sullivan and Carpenter (1993). Table 7.11 summarizes the methods for lipid(fat)-soluble vitamins and Table 7.12 summarizes those for the water-soluble vitamins.

Lipid-soluble vitamins

These are the vitamins A, D, E and K, and the carotenoids with provitamin A activity. As nutrition interest is now also focused on the non-provitamin A carotenoids, it is also desirable to cover more of these carotenoids.

Vitamin A. Vitamin A is a generic term that includes retinol, its esters and some isomers. The international standard for vitamin A is all-trans-retinol, for which the international reference IU was defined as 0.3 μg (= 0.344 μg retinol acetate) of this form of retinol. Other retinoids show some activity, including cis-isomers of retinol, retinaldehyde, retinyl ester, dehydroretinol and dehydroretinaldehyde. The structures for these substances are given in Figure 7.2. The activity of the vitamers is broadly similar and by convention they are given equal vitamin A activity as all-trans-retinol.

The older procedures relied on the colorimetric Carr–Price reaction of separation on ion-exchange columns. This reaction is highly prone to interference and the method of choice is now separation by HPLC with spectrophotometric measurement. Vitamin A is very sensitive to light and all preparations of analytical portions must be carried out in subdued lighting, preferably gold lighting. The food samples are saponified in alcoholic potassium hydroxide with the addition of an antioxidant, ascorbic acid, butylated hydroxytoluene (BHT) or pyrogallol. The vitamins are extracted into a suitable organic solvent. The extract is evaporated with additional BHT at a controlled temperature. Both normal-phase and reversed-phase HPLC can be used for the separation. In normal-phase separations measurement is usually by fluorescence; in reversed-phase separations UV detection and measurement is preferred. Standards should be followed throughout the entire sample preparation and analysis and must be controlled regularly for purity (Brubacher, Müller-Mulot and Southgate, 1985).

Nutritional interest originally focused on the carotenoids that demonstrated provitamin A activity, that is, were converted in the body to vitamin A. These are β-carotene, γ-carotene, α-carotene and β-cryptoxanthin (Figure 7.3). During the 1990s it was recognized that many other carotenes are biologically active as antioxidants and therefore this review is concerned with methods that permit the measurement of a wider range of carotenoids. There are some 600 carotenoid isomers (Bauemefeind, 1972), but many of these have restricted occurrence or are present in minor amounts in most common foods. Debate about how to present different carotenes and their relative activity in databases continues.

The classical method was to perform a simple chromatographic separation of the carotenes as a group, and measure spectrophotometrically against a common β-carotene standard (Brubacher, Müller-Mulot and Southgate, 1985). This has been replaced by more detailed separation using ion-exchange columns and HPLC. The conditions applied in saponification are critical and need to be carefully controlled using standard mixtures. If this is done, then comparable values can be obtained (Mangels et al., 1993) with sufficient confidence to construct a database for the provitamin carotenoids (Chug-Ahuja et al., 1993).

HPLC is now the most widely used and preferred method. Scott (1992) and his colleagues (Scott and Hart, 1993; Scott et al., 1996), as part of an EU project to develop a SRM mixture of carotenoids, made an extensive series of studies on the various stages of the saponification extraction and HPLC analyses. Other analysts have also carried out detailed studies of the method (Wills and Rangga, 1996; Taungbodhitham et al., 1998). These studies provide the basis for obtaining sound analytical values for the most important carotenoids. A revised system for evaluating published carotene values taking into account these studies has been proposed and the production of quality codes is now being evaluated.

Vitamin D. Two forms of vitamin D are found in foods, cholecalciferol (D_3) and ergocalciferol (D_2). One IU is equivalent to 0.025 μg of cholecalciferol or ergocalciferol. Vitamin D_3 is the more widely distributed (e.g. in fish oils, many fatty fish tissues, eggs, butter and cream cheese), and D_2 occurs naturally in low concentrations in fish oils and mushrooms, and is the form used in fortification. Some meats contain 25-hydroxy-cholecalciferol in concentrations that contribute to vitamin D activity and need to be considered. Figure 7.4 summarizes the structures of vitamin D. Estimates of the relative activities of cholecalciferol, ergocalciferol and their metabolites vary. The convention appears to be to attribute a factor of five times the activity of cholecalciferol to 25-hydroxycholecalciferol (Chan et al., 1995, 1996). Therefore values for different forms should always be presented separately in analytical reports and reference databases.

Vitamin D in foods is found at a very low concentration, which makes its analysis difficult. The original methods were biological using chicks or young rats (e.g. Method No. 936.14 [AOAC International, 1995]). These methods are difficult to perform and had generally low precision. The major problem with vitamin D analysis is that most food sources contain other lipids that tend to interfere (Ball, 1998).

Gas chromatography is discussed by Koshy (1982), but HPLC is now the preferred technique and several methods have been published (cholecalciferol and 25-hydroxycholecalciferol in egg yolk [Mattila et al., 1993], ergocalciferol and 25-hydroxyergocalciferol in edible mushrooms [Mattila et al., 1994], and cholecalciferol, ergocalciferol and their 25 hydroxy metabolites in milk and meats [Mattila et al., 1995]). Similar methods (unpublished) were used for meats in the United Kingdom food composition tables (Chan et al., 1995, 1996) (V. Grace, UK Food Standards Agency, personal communication). The most useful method available involves a preliminary semi-preparative HPLC stage that eliminates much of the interference from other lipids. The food sample is saponified in alcoholic potassium hydroxide under nitrogen, with an antioxidant, ascorbic acid, hydroquinone, pyrogallol or BHT having been added before the saponification solution. The unsaponified lipids are extracted with a suitable organic solvent. An internal standard of the form of vitamin D not present in the sample is used. The unsaponified lipids are concentrated by rotary evaporation at low temperature. The extract is dissolved in the mobile phase of the semi-preparative HPLC. The conditions are carefully controlled to give a precise collection of the vitamin D.

The analytical separation may be carried out on normal or reversed-phase HPLC with UV detection. Reversed-phase is recommended for the analytical separation after normal-phase for the semi-preparation stage.

25-hydroxycholecalciferol can be measured by HPLC, as mentioned above (MAFF, 1997), but radio-immunoassay is probably the best choice at the present time where the necessary funds and equipment are available (Bates, 2000).

Vitamin E. Vitamin E activity is exhibited naturally by eight substances structurally based on tocopherols and tocotrienols (see Figure 7.5). Each vitamer has a different vitamin activity compared with α-tocopherol, which is seen as the primary structure. The preferred analytical method is therefore one that separates and measures all the different vitamers.

The food samples are saponified using alcoholic potassium hydroxide. The vitamin E vitamers are susceptible to oxidation at higher temperatures in alkaline conditions and should be protected by saponifying under nitrogen with the addition of antioxidants. The saponification conditions are similar to those used for vitamins A and D.

A colorimetric method, the Emmerie–Engel reaction with the reduction of ferric chloride and reaction with α, α'-dipyridine or 4,7-diphenanthroline, is also available. The complexes are rather unstable and give a total tocopherol value. The colorimetric method has been superseded by, first, GLC and, then, HPLC, which is now the preferred method.

Both normal-phase and reversed-phase HPLC can be used, although the normal-phase represents the better approach and separates all the vitamers. Detection uses fluorescence (Piironen et al., 1984, 1987). External standards are used and these need to be checked spectrophotometrically.

Vitamin K. Vitamin K activity is possessed by phylloquinone (K_1), the menaquinones (K_2 group) and menadione (synthetic K_3). The structures are shown in Figure 7.6.

Vitamin K is sensitive to alkali and UV radiation and the appropriate precautions need to be taken during analytical operations. Colorimetric procedures are available, but these lack specificity and have been replaced as the methods of choice. Most analytical attention has been given to the measurement of vitamin K_1. One major problem in the analysis is the presence of lipid, which must be removed by digestion with lipase before extraction with hexane (Indyk and Woollard, 1997). The solvent is evaporated under a stream of nitrogen and the residue dissolved in methanol, which is applied to a reversed-phase HPLC column. The eluate is reduced post-column with zinc and the fluorescence is then measured.

Semi-preparative separations have been used after digestions (Cook et al., 1999) and dual electrode detection systems have also been proposed (Piironen and Koivu, 2000). Most authors comment on the great variability of the values obtained and emphasize the need for proper repeat sampling and replication of analyses (Piironen et al., 1997; Jakob and Elmadfa, 1996).

Water-soluble vitamins

These include vitamin C and a number of vitamins of the B-group. The study of vitamin C has a long history (Carpenter, 1986) and this vitamin is discussed first.

Vitamin C. Two substances show vitamin C activity, L-ascorbic acid and the first product of its oxidation – L-dehydroascorbic acid (Figure 7.7). The D-isomer (erythorbic acid), which is used as an antioxidant food additive, is not active. Ascorbic acid is a powerful reducing agent which is oxidized very quickly, especially at raised temperatures and in alkaline solutions. During the preparation of food samples for analysis it is especially important to minimize the losses due to oxidation (Brubacher, Müller-Mulot and Southgate, 1985).

In most fresh foods the amounts of dehydroascorbic acid are very low and for many purposes the measurement of ascorbic acid alone may be adequate. Thus, the reduction of 2,6-dichlorophenolindophenol is the simplest and most reliable method (AOAC Method Nos 967.21 and 985.33 [Sullivan and Carpenter, 1993]).

The colorimetric method of Roe and Kuether (1943) involving the reaction with 2,4-dinitrophenyl hydrazine measures both ascorbic and dehydroascorbic acid.

The method of Deutsch and Weeks (1965) also measures both active forms fluorimetrically, after oxidation, and is recognized as an Official Method by the AOAC, both as originally described, and in a semi-automated version (Method Nos 984.26 and 967.22 [Sullivan and Carpenter, 1993]). Where the presence of erythorbic acid is not suspected, the fluorimetric method is probably the preferred method. HPLC techniques developed in the 1980s (Finley and Duang, 1981; Rose and Nahrwold, 1981; Keating and Haddad, 1982; Wimalasiri and Wills, 1983) for the separate measurement of ascorbic, dehydroascorbic and erythorbic acids are now widely used and give satisfactory performance (Schüep and Keck, 1990).

B-vitamins. This group includes a number of structurally distinct vitamins that were initially grouped together because they were water-soluble. The initial approach to the measurement of these vitamins, some of which are present at very low concentrations, was selective microbiological methods (Bell, 1974; Ball, 1994), and for some vitamins, total folates and vitamin B_12, microbiological assays remain the only practicable methods. For the remaining B-vitamins, more specific chemical procedures, especially HPLC, have been developed and collaboratively tested.

Thiamin. The structures of the substances showing thiamin activity (B_1) are shown in Figure 7.8. Thiamin is sensitive to heat and alkaline conditions and appropriate precautions must be undertaken during its analysis. Thiamin can be measured microbiologically using Lactobacillus viridescens or L. fermentum, but most analyses are based on its oxidation to thiochrome, which can be measured directly fluorimetrically. This is most conveniently carried out in conjunction with HPLC separation of interfering compounds. Thiamin, riboflavin and vitamin B_6 are present in foods as enzyme cofactors combined with phosphate and must therefore be hydrolysed and treated with phosphatase before analysis. In early descriptions of the methods for these vitamins different conditions were used, but a number of collaborative studies (van den Berg et al., 1996; Ndaw et al., 2000) have shown that a common method for preparing the food samples can be used.

The food sample is hydrolysed with acid and then treated with takadiastase or a phosphatase. Some authors use a ion-exchange pre-column (Bognar, 1981). The extract is then oxidized with potassium ferricyanate to form the thiochrome; it is then analysed using a reversed-phase HPLC column and the thiochrome is measured fluorimetrically. The analyses are controlled using an external standard. A post-column oxidation can also be used. In the large collaborative study reported by van den Berg et al. (1996) variations between the different practices in a range of laboratories did not affect overall performance of the method. Microbiological results also showed good agreement with the results from the HPLC methods.

Riboflavin. The structure of riboflavin (vitamin B_2) is shown in Figure 7.9. It is found in foods as the free riboflavin or riboflavin-5'-phosphate (FMN) and as flavin adenine dinucleotide (FAD). The vitamin is very sensitive to light and UV radiation but relatively stable to heat and atmospheric oxygen. The analytical operations must therefore be carried out under conditions that minimize the exposure to light. The vitamin must be extracted from foods by treatment with acid and a suitable phosphatase enzyme. The riboflavin can be measured directly using fluorimetric methods, although many foods contain interfering substances and separation from these by HPLC is the preferred approach (Wimalasiri and Wills, 1985; Schüep and Steiner, 1988; Arella et al., 1996). Reversed-phase HPLC separation using fluorescence detection is the method most commonly used. In the collaborative study reported by van den Berg et al. (1996) minor variations in local methods did not affect performance. Microbiological assay using Saccharomyces carlsbergensis and S. uvarum tended to give slightly higher results than the HPLC, as observed previously by Hollman et al. (1993).

Niacin. Niacin activity is due to nicotinic acid and nicotinamide (Figure 7.10). Both forms are stable to atmospheric oxygen, light and heat in the dry state and in aqueous solution. A number of bound forms have been found in cereals that are extractable by alkali but these are probably not bioavailable. Tryptophan is also metabolized to niacin and the total niacin activity must include the contribution from tryptophan (Paul, 1969).

Niacin can be measured microbiologically with Lactobacillus plantarum (AOAC Method Nos 960.46, 944.13 and 985.34 [Sullivan and Carpenter, 1993]). Colorimetric methods based on the Konig reaction using oxidation with cyanogen bromide and reaction with p-amino-benzoyl-diethylaminoethanol have also been used (AOAC Method Nos 961.14, 981.16 and 975.41 [Sullivan and Carpenter, 1993]), but the toxic nature of cyanogen bromide makes it difficult to recommend these for routine use.

An HPLC method has been proposed and seems to perform reasonably well (Finglas and Faulks, 1987). After acid hydrolysis the food sample is filtered, treated with alkali, autoclaved and microfiltered before reversed-phase HPLC and fluorescence detection. A simplified extraction protocol has been proposed (Lahély, Bergaentzlé and Hasselmann, 1999) and has been shown to perform well with a range of foods (Rose-Sallin et al., 2001).

Vitamin B_6. There are five compounds showing vitamin B_6 activity whose structures are shown in Figure 7.11: pyridoxamine, pyridoxine, pyridoxal and the corresponding phosphate esters.

Vitamin B_6 activity cannot therefore be measured using a method for a single substance. Microbiological assay using Saccharomyces carlsbergensis provides a measure of total activity (AOAC Method Nos 960.46, 961.15 and 985.32 [Sullivan and Carpenter, 1993]). The assay is carried out after an acid hydrolysis and hydrolysis of the phosphates enzymatically, and the same extraction procedures as for thiamin and riboflavin can be used (van den Berg et al., 1996; Ndaw et al., 2000). The acid hydrolysis also hydrolyses glycosides, which are present in plant foods and which may or may not be bioavailable to humans.

Comparison of HPLC and microbiological assay has indicated that further work is required (van den Berg et al., 1996; Bergaentzlé et al., 1995). Ndaw et al. (2000) used an extraction procedure without the acid hydrolysis stage and the HPLC method of Schüep and Steiner (1988) and the procedure performed well with standard materials.

Vitamin B_12. A group of complex structures possesses vitamin B_12 activity (Figure 7.12). Classically it has been measured microbiologically with Lactobacillus leichmanii.

The levels of vitamin B_12 in foods are very low and it is extracted with hot water or a buffer in the presence of potassium cyanide, which converts the vitamin into the cyano form (AOAC Method Nos 960.46, 952.20 and 986.23 [Sullivan and Carpenter, 1993]).

A number of sensitive methods have been developed for clinical use (Bates, 1997; 2000) using competitive protein binding and a range of radio-immunoassays, but these have not been evaluated in a range of foods.

Folates. The folates comprise a group of compounds related to folic acid (pteroyl-glutamic acid). Folic acid does not occur naturally in foods but is widely used in food fortification or as a supplement. Most of the naturally occurring folates are derivatives of 5,6,7,8-tetrahydrofolic acids and exist in the monoglutamate or polyglutamate forms. Their structures are summarized in Figure 7.13.

The biological activity of the forms differs and the ideal analytical nutritional procedure therefore should involve the measurement of the different vitamers.

Total folate values are best measured by microbiological assay using Lactobacillus rhamnosis (caseii). Most organisms cannot use the polyglutamate forms, and deconjugation with a suitable enzyme (hog kidney, chicken pancreas, human plasma) is a preliminary stage in the analysis. The extraction is carried out in the presence of ascorbic acid to minimize oxidation. The extract is treated with a combination of protease, lipase and amylolytic enzymes, which improve the efficiency of extraction. The different conjugase enzymes give similar performances. At one time it was assumed that the measurement of folate before and after deconjugation would give values for “free” folate and total folates. The organisms respond to varying extents to the glutamate derivatives and the concept is flawed. The conditions for the microbiological assay were studied by Phillips and Wright (1982, 1983), Wright and Phillips (1985) and Shrestha, Arcot and Paterson (2000); these procedures give satisfactory quantitation.

Separation of the different folate vitamers using HPLC techniques is now widely used (Finglas et al., 1999) and some databases give values. Intercomparison studies have shown that values for 5-methyl tetra-hydrofolate showed reasonable agreement, but the agreement with other vitamers was not satisfactory (Vahteristo et al., 1996). Subsequent studies on the standardization of the HPLC methods have shown that while it is possible to measure the 5-methyl form with reasonable confidence, the other vitamers are still not measured properly by existing methods that use fluorimetric detection. A kit is available for folic acid and an evaluation has been published by Arcot, Shrestha and Gusanov (2002).

Pantothenic acid. The structure of pantothenic acid is given in Figure 7.14. Pantothenic acid in the free form is unstable and extremely hygroscopic. It is usually present bound to proteins or in the form of salts. Only the dextro- form is active. The classical method is microbiological using Lactobacillus plantarum as the test organism (Bell, 1974; AOAC Method Nos 960.46 and 945.74 [Sullivan and Carpenter, 1993]). The food is extracted with water and where the food is rich in fats these are best removed before analysis. The aqueous extract is usually autoclaved and the pH adjusted with acid and alkali to around 6.8. The mixture, after incubation overnight, is heat-treated to stop growth and growth is measured turbidometrically.

Biotin. Biotin is found in foods as the free vitamin and bound to protein. Figure 7.15 shows the structure of the vitamin. The classical method is microbiological using Lactobacillus plantarum (Bell, 1974; AOAC Method No. 960.46 [Sullivan and Carpenter, 1993]). An HPLC method has also been described (Lahély et al., 1999). Preliminary extraction with acid followed by papain treatment is required to extract the vitamin from the food. The HPLC method uses a reversed-phase separation, post-column derivatization with avidin-fluorescence 5-isocyanate and fluorescence detection.

Radio-assays using the specific binding protein have also been described (Bates, 2000).

Bioactive food components

Pennington (2002) has published a comprehensive review of food composition databases for bioactive food components, including flavonoids, tannins, allyl sulphides, capsaicin, indoles, lignans, monoterpenes, phenolic acids, plant sterols and probiotics, categorized by food and by compound, and available as an annotated bibliography of over 400 pages on individual components (Pennington, 2001). Given the number and diversity of these components, it is not possible to review the methods for all of them (Speijers and van Egmond, 1999). This section therefore focuses on methods for measuring flavonoids, isoflavonoids, lignans and total antioxidant activity in view of the fact that these have been the subject of much interest in recent years. Methods for plant sterols were reviewed earlier in this chapter.

Flavonoids. A rapid method based on reversed-phase HPLC with UV detection was developed by Hertog, Hollmann and Venema (1992) for the quantitative determination of five major flavonoid aglycones (quercetin, kaempferol, myricetin, luteolin and apigenin) in freeze-dried vegetables and fruits, after acid hydrolysis of the parent glycosides. More recently Merken and Beecher (2000) published a gradient HPLC method with photodiode array detection for 17 prominent monomeric flavonoid aglycones representing all of the five common classes of flavonoids.

Phytoestrogens. The main plant compounds with known or suspected estrogenic activity are lignans, isoflavones, coumestans and resorcyclic acid lactones (Price and Fenwick, 1985). The modes of estrogenic action are discussed by Clarke et al. (1996). The major isoflavonoids are genistein, daidzein, formononetin, biochanin A and glycitein. Genistein, daidzein and glycitein occur in foods as their glycosides, all of which are biologically inactive. The free aglycones are formed by metabolic action of the human gut microflora, although this hydrolysis varies considerably from person to person (Xu et al., 1994). The total bioactivity is represented by the analysis of aglycones; however, this potential activity is represented by analysis of the conjugates and aglycones separately. The most active plant estrogen known is coumestrol (a coumestan); zearalenone is a potent resorcyclic acid lactone formed as a secondary metabolite of fungal species, mainly Fusarium (and is thus regarded as a contaminant). The lignans matairesinol, secoisolariciresinol, pinoresinol and isolariciresinol are potent phytoestrogens and are precursors of the mammalian lignans, enterolactone and enterodiol.

Given the very large number of plant compounds with estrogenic activity and the question of whether to analyse both the conjugates and the free forms or only the aglycones (after hydrolysis), many methods of analysis are in existence and there is little agreement on which method is best. No method is available to separate and quantify all bound and free compounds of interest in this category. Probably the most comprehensive method for the aglycones is the isotope dilution gas-chromatographic–mass spectrometric method of Adlercreutz and coworkers (Mazur et al., 1996), which analyses daidzein, genistein, biochanin A, formononetin, coumestrol, secoisolariciresinol and matairesinol, but not glycitein, as silyl derivatives. The method is expensive and needs access to mass spectrometry (MS). Another comprehensive method for foods that analyses daidzein, genistein, biochanin A, formononetin, coumestrol, secoisolariciresinol and matairesinol, but not glycitein, uses an HPLC-MS method originally developed for plasma and urine (Horn-Ross et al., 2000; Coward et al., 1996; Horn-Ross et al,. 1997; Barnes et al., 1998).

Isoflavones and coumestrol. For the USDA–Iowa State University Isoflavones Database (2002), the reference method adopted was the linear gradient method of Murphy et al. (1997), which separates daidzein, genistein, glycitein and their conjugates in soy-based infant formulas. Hutabarat, Greenfield and Mulholland (2000) have published a rigorously validated isocratic HPLC method for genistein, daidzein, formononetin, biochanin A and coumestrol (but not glycitein), while King and Bignell (2000) have published an HPLC method for daidzin, genistin, glycitin and their aglycones. A collaborative trial published by Klump et al. (2001) led to a recommendation to adopt as first action AOAC Method No. 2001.10 for the determination of isoflavones in soy and selected foods containing soy. This method uses reversed-phase liquid chromatography to separate and measure genistein, glycitein and daidzein and their glucosides, and also produces values for total isoflavones expressed as aglycones.

Lignans. Meagher et al. (1999) measured isolariciresinol, pinoresinol, secoisolariciresinol and matairesinol using HPLC with photodiode array detection, and Liggins, Grimwood and Bingham (2000) have published a GC-MS method for the determination of matairesinol, secoisolariciresinol and shonanin in foods as trimethylsylyl derivatives.

Total antioxidant activity. There is growing interest in ways to represent the total antioxidant activity of foods. A number of methods have been used but no standards exist and at this stage the inclusion of values for total antioxidant activity in foods in databases is not recommended. The topic is fully reviewed by Frankel and Meyer (2000).

Energy

The gross energy content of a food may be determined experimentally with a bomb calorimeter (Brown, Faulks and Livesey, 1993). An adiabatic bomb calorimeter is preferred for precise measurements, but the ballistic bomb calorimeter (Miller and Payne, 1959) gives a precision that is adequate for most nutritional studies. The values obtained using an adiabatic bomb calorimeter are corrected for the heat generated from the oxidation of nitrogen and sulphur in the food. The calorimeters are usually calibrated using benzoic acid as a thermo-chemical standard.

The values obtained are the gross heats of combustion and are not the values used in nutritional sciences and food composition databases; for these purposes, metabolizable energy is used. This is the energy that is available for use in metabolism by the body. Metabolizable energy values are calculated using energy conversion factors (Atwater and Bryant, 1900; Southgate and Durnin, 1970; Merrill and Watt, 1973; Allison and Senti, 1983) for the protein, fat, carbohydrate and alcohol contents. Recently, Livesey (2001) has argued that a better system for calculating the energy values of food would be the net metabolizable energy system (Blaxter, 1989).

Recently, the contributions from dietary fibre, polyols and oligosaccharides have been widely discussed (Livesey, 2001; FAO/WHO, 1998), but most databases do not yet use the energy conversion factors for these components.

In many countries, Le Système International d’Unités (or International System of Units [SI]) (BIPM, 1998, 2003) is used to express the energy values of foods and diets, using the Joule (J) (work): 1 kcal is equivalent to 4.184 kJ (thermochemical equivalent) (Royal Society, 1972). When expressing the energy value of foods, no more than three significant figures should be used. Whichever system of calculation is chosen for energy, it should be clearly indicated.

Chapter 8

Assuring the quality of analytical data

Without a defined quality assurance programme all analytical results must be suspect.

(Harnly and Wolf, 1984)

The current uses of food composition data depend on the reliability of these data; however, achieving reliability and demonstrating that it has been achieved require a systematic and documented approach. There is now an extensive literature on analytical quality control for food analysis. Efforts to improve and standardize analytical quality at the international level have been advanced by organizations such as the International Organization for Standardization (ISO, 2003) and by the application of formalized principles such as good laboratory practices (GLP) (OECD, 1992, 1999) and total quality management (TQM) (Parkany, 1995).

The criteria for data to be entered into food composition databases were discussed in Chapter 1. To summarize, food samples should be representative of foods as consumed, as available for consumption or as produced (e.g. data for raw foods or commodities). The values should accurately represent the food samples analysed (see Table 8.1). It follows, then, that the basic principles of producing good-quality data are attention to:

a)the collection and preparation of the food sample (see the first group of activities in Table 8.1);

b)the choice of the analytical method and its validation within the laboratory carrying out the analyses;

c)the proper execution of the method (which implies the use of quality control procedures);

d)critical review of the values obtained.

Sampling and methods of analysis have been addressed in Chapters 5, 6 and 7; this chapter deals with the latter two topics.

Definitions

Definitions of data quality, quality control and quality assurance used in this text (Table 8.2) are derived from those proposed by the International Organization for Standardization (ISO, 2003) for application to either a product or a service.

In practical terms, “quality assurance” is the sum of all the activities taken to ensure that the information generated by the laboratory is correct (Wilcox et al., 1978). This should be a deliberate process, not left to chance or brought into operation only when inadequacies are identified. A well-designed quality assurance programme (QAP) also provides laboratory workers and their supervisors with objective measures of performance, and an indication of whether or not the laboratory is achieving its goals.

Quality control has a much narrower meaning than quality assurance; it usually refers to procedures that are designed to ensure that data quality remains within defined limits. These include standards of precision and accuracy of the analytical operations, which depend on criteria set by the users of compositional data and the compilers of databases. The quality control standards set by analytical chemists may be unnecessarily strict for most nutritional purposes; however, quality control is still vital to ensure that bias is not introduced.

The aim of quality control, then, is to produce food composition data that meet the required standards and that can be produced efficiently and economically. This achievement requires the integration of several related steps: the proper specification of the quality of data required; production of the data to meet the intent of the specification; evaluation of the data to determine whether or not they meet the specification; and review of data usage to provide for the revision of the specification.

The term quality control is often used in only the narrowest sense (i.e. the monitoring of the performance of analytical methods) (Büttner et al., 1975); it should in fact cover all aspects of the analytical process from food sample collection, handling and treatment of analytical samples, standards preparation, signal measurement and method validation, to data handling and evaluation (Harnly and Wolf, 1984; Garfield, 1984).

Scope and implementation of quality assurance

Quality assurance within the laboratory is implemented in three major modes:

1.Preventive – steps taken prior to the analysis, intended to ensure accuracy in analytical testing (e.g. maintenance and calibration of instruments, testing of reagents, training of personnel).

2.Assessment – procedures undertaken during testing to determine whether the test systems are performing correctly (e.g. the use of standards and blanks, maintenance of calibration charts, etc.).

3.Corrective – action taken to correct the system when error or possible error is detected (e.g. equipment re-calibration, replacement of reagents, etc.) (adapted from Wilcox et al., 1978).

The central feature of a quality assurance programme is the proper documentation of all the activities involved in the production of compositional data, from the design of the sampling protocol to the final production of analytical data.

The activities involved in quality assurance should include:

1.training personnel in the appropriate methods, and provision of proper facilities and equipment;

2.quality control of reagents, glassware and solvents, and of the operation of instruments and other equipment;

3.maintenance of a proper record-keeping system;

4.close attention to all aspects of sampling (Chapter 5);

5.proper use of controls and reference standards;

6.replication of sampling and analysis;

7.careful scrutiny of results, including comparison with those of other laboratories; selection of repeat analyses;

8.preparation and review of reports.

Quality assurance is effected through GLP, which comprise three major areas: management, quality control of sampling and quality control of analytical method performance.

Management

Management is the general function of directing the food analysis laboratory to attain its goals. It not only involves administrative functions, but also determining how the laboratory operates, what it is to accomplish, and whether or not it accomplishes what it sets out to do. The tasks of management in the present context are as follows:

1.Set and explain the laboratory’s objectives to all involved in sampling and analysis.

2.Develop the laboratory’s plan of action and policies. This involves defining measures that are necessary to ensure the quality of the work, establishing them and ensuring that they are implemented.

3.Organize and integrate the personnel, facilities, equipment and materials so that the laboratory can meet its objectives on a day-to-day basis.

4.Evaluate the laboratory’s performance and implement changes or innovations determined to be desirable for correction or improvement purposes.

Effective management is required for three areas of critical importance in laboratory function – the physical environment, personnel and administration.

The physical environment

Many food composition laboratories are less than ideal as physical structures. However, much can be achieved in adverse conditions, especially if the available space is well-organized and attention is paid to safety. Horwitz et al. (1978) list the following special needs of a food analysis laboratory: extremely good ventilation and fume hoods because of the extensive use of solvents and evolution of toxic and corrosive fumes; adequate power for heaters and instruments; high quality and volume of distilled (or deionized) water for reagent preparation and aliquot dilution; freedom from contamination – environmental (lead, asbestos, etc.), laboratory-generated (mercury, fumes, etc.) and housekeeping (dust, insects, rodents, etc.); and a large storage capacity for samples and reagents, including refrigerator and freezer space. Special facilities for the analysis of certain nutrients may be required, such as a “clean” environment for trace minerals and special lighting for light-sensitive nutrients. Few laboratories have such a complete range of specialized facilities, but the above list may be helpful in planning for upgrading of an existing laboratory. Practical advice is also available in a review by Rappoport et al. (1978).

As far as equipment is concerned, many laboratories may not be in a position to pick and choose. The main criterion is that the equipment be able to perform the tasks set. Specialized equipment and/or automation can lead to higher levels of precision and generally improve the quality control of analyses, but are not an essential prerequisite for sound analytical work.

Schedules for regular servicing, testing and replacement of equipment are helpful, and attention should be paid to safety and security; these topics are discussed in detail by Wilcox et al. (1978).

Personnel

Selection and training of staff are critical, as is the opportunity for updating of skills. Ideally, each employee should have a clear job description and a clearly defined path for reporting to a supervisor. A high level of motivation is essential for good-quality work. It is best achieved by setting clear objectives and ensuring that the analysts see their role clearly in the operation as a whole. In all laboratory work, the worker at the bench is the main determinant of analytical quality, and this fact must be understood by the bench worker and by those responsible at all levels. Ideally, each employee should feel that her or his own work counts and that good-quality work is not only a team responsibility but also a team achievement.

Many laboratories conduct food composition work under contract by staff employed on a short-term basis. Maintenance of morale in such staff, though difficult, is an important objective of the programme.

Administration

Administration includes all aspects of paperwork in the laboratory. All the laboratory’s procedures should be included in a quality assurance manual (QAM) that includes instructions on sampling, methods of analysis and quality control procedures. Further, a system must be designed and used for registering all food samples arriving in the laboratory. This register includes all the information required for identifying the food sample (see Chapter 5) and is linked to the recording of the final analytical results. This system accounts for all the samples arriving in the laboratory. The preparation of the manual formalizes procedures and, provided that the laboratory staff are encouraged to contribute comments and suggestions, assists in the development of good laboratory practice.

It is important that the manual is used by the staff whose working procedures it provides. There is a danger that if the QAM is seen as an end in itself and not intended for use and to provide guidance it will not fulfil the objectives behind its preparation.

Staff should be encouraged to keep well-organized laboratory notebooks, and standard data sheets need to be developed for recording of the final analytical values. The separate but related process of setting up recording systems also provides a disciplined approach to the laboratory work and can identify potential problems. It is prudent, however, to test a new system in a pilot study before implementing it and to recognize that modifications may be required over time. A good recording system facilitates searches back through all calculations and measurements to identify and correct any errors that arose in recording.

Quality control of sampling

Sampling is discussed in detail in Chapter 5; it is only necessary to stress here that quality control of sampling is the crucial first step in the entire quality assurance programme, and that the analytical staff should be involved in the design of the sampling plans. Indeed, direct participation in collection of the food samples provides the analyst with insight into the practical problems of sampling. The necessity for defined sample handling procedures in the laboratory must also be regarded as the concern of the analyst.

Quality control of analytical method performance

(from Horwitz et al. [1978], adapted, with permission)

The third major area of implementing laboratory quality assurance is in the quality control of the performance of the analyses. In food composition studies a great deal of attention must be paid to this since all food samples received for analysis should, in principle, be treated as having an unknown composition.

The performance of an analytical method requires validation of the entire system (Horwitz et al., 1978): the laboratory with its environment, equipment and reagents; the analyst, with her or his individual skills, experience and knowledge; and the method, with all of its idiosyncrasies and attributes.

The method is selected by the relative importance of the various attributes, as a result of previous experience or on the basis of reports in the literature. Choice of method is discussed in Chapters 6 and 7. However, it is essential for a laboratory to verify that the method performs properly in actual practice. As discussed in Chapters 6 and 7, each food substrate may present an entirely different set of problems for the analysis of any constituent. The selection or production of an appropriate standard matrix can require considerable skill and ingenuity.

Specifications for the analytical values

First, the quality required of the analytical data must be specified. These specifications will be based on reliability criteria discussed in detail in Chapters 6 and 7 (specificity, accuracy, precision, sensitivity [Büttner et al., 1975]), and will depend on both the component to be analysed and the matrix in which it occurs.

For example, in setting specificity criteria for analyses of vitamin C, it is essential that the method measures only ascorbic acid and dehydroascorbic acid, both of which exhibit vitamin activity. Interferences in most vitamin C methods are reasonably well understood by analysts, and may be controlled or allowed for in the analysis. For other nutrients, methods that measure a wide range of substances may be adequate (Chapter 7). Some components are hard to define analytically, and for these the currently available analytical methods are likely to be superseded.

The level of accuracy to which an analysis is conducted and reported should be set at a certain number of significant figures, dependent on the precision of the method. Three significant figures are (in most cases) the maximum required in a food database, but more (and often spurious) significant figures are generated in many analytical systems. In nutrient analyses, the pursuit of accuracy in order to cite values to four or five significant figures implies a false view analytically (because no method has this degree of accuracy) and is a misdirection of resources.

The precision required should be related not only to the method itself, but also to the expected level of the nutrient. As with accuracy, it may be wasteful to devote resources to improving precision if the level of the nutrient in the food is low in relation to the dietary intake as a whole, or if the food is rarely eaten. It is essential to establish realistic criteria for acceptable kinds of precision; improvement to values that fall within less than 10 percent of the mean may be unnecessary. Stewart (1980) has suggested acceptable precision and accuracy standards for nutrient composition studies.

Wilcox et al. (1978) list the following as the most common causes of error in method performance:

a)improper choice of method of analysis;

b)lack of proficiency or experience on the part of the analyst;

c)errors arising in the performance of the method unrelated to analyst proficiency (e.g. faulty reagents);

d)inadequate attention to calibration of instruments and to the integrity of the reference standards.

Techniques for validating method performance

Verification of method performance – an essential step when a method is introduced into the laboratory – can be carried out by the following techniques (Chapters 6 and 7 discuss the range of procedures used in validating methods when selecting analytical procedures):

1. Standard samples. Ideally, standards would be prepared containing known amounts of the constituent of interest, in the same physicochemical form and in a similar food matrix to the one to be analysed. Clearly this ideal is virtually impossible to achieve, but various substitutes are available for use as standards.

Reference materials (RMs) and standard reference materials (SRMs) support accurate and compatible measurements by certifying and providing samples with well-characterized composition. These materials are used to perform instrument calibrations in situ as part of overall quality assurance programmes, to verify the accuracy of specific measurements and to support the development of new measurement methods. The RMs and SRMs are for use in determining the nutrient contents of mixed diets and individual food matrices. The SRMs are certified for dietary constituents as ash, protein, carbohydrate, fat, energy, cholesterol, selected fatty acids, vitamins, selected minerals and trace elements. In the United States the National Institute of Standards and Technology (NIST, 2003a) provides many SRMs.

In Europe, the Institute for Reference Materials and Measurements (IRMM, 2003) operates as part of the Directorate-General Joint Research Centre of the European Commission. It provides certified reference materials (CRMs) in a variety of food matrices for macrocomponents, major and trace elements, 15 vitamins, five different fibre methods and other food components.

The SRMs and CRMs are as a rule rather expensive; they may be regarded as too costly to use routinely and alternatives must often be used.

For this reason, ASEANFOODS undertook the development of food reference materials with consensus values of various nutrients (Puwastien, Sungpuag and Judprasong, 1999; Puwastien, 2000), in collaboration with expert laboratories within and outside the Asia-Pacific region. Four food reference materials, namely rice flour (AS-FRM1), soybean flour (AS-FRM2), cereal-soy (AS-FRM3) and fish flour-1 (AS-FRM4), with consensus values of main nutrients and minerals, were developed and are now available from the ASEANFOODS Regional Data Centre. These reference materials have been used for laboratory quality control programmes and as test materials for laboratory performance studies in ASEAN (Association of Southeast Asian Nations) and other developing countries.

It may prove impossible to produce reference standards for some nutrients contained within a complex food matrix. A mixture of pure substances can be prepared but cannot simulate the physical properties or the interrelationships of components within such foods. In the absence of an SRM, a laboratory routinely performing certain types of determinations should provide itself with working standard materials (in-house standards); these consist of a large amount of a homogeneous product (with great care taken to achieve homogeneity) dispensed into small, sealed bottles and stored under conditions that prevent deterioration (Southgate, 1995). Portions of this material should be analysed periodically along with each analysis or series of analyses, and the results monitored by the use of control chart techniques. An example of a local standard “fresh” food reference material produced in Sweden, canned meat with certified values for moisture, ash, fat, nitrogen, sodium, sodium chloride and hydroxyproline contents, is described by Torelm et al. (1990). Other in-house standards can be developed and validated against purchased RMs, and this is useful when large purchases of costly RMs are out of reach.

The control chart is “a graphical chart with control limits and plotted values of some statistical measure for a series of samples or subgroups. A central line is commonly shown” (American Society for Quality Control, 1973). The results of a laboratory test are plotted on the vertical axis, versus time (in hours, days, etc.) plotted on the horizontal axis. Each laboratory test should be checked frequently, and the horizontal scale should be wide enough to hold up to three months of data. Since the control chart is a tool providing “real-time” analysis and feedback information, it should cover a sufficient period of time and provide sufficient data to indicate trends, “runs” above and below the central line, or any other manifestation of lack of randomness (Mandel and Nanni, 1978; Taylor, 1987).

Non-segregating powders, such as non-fat dry milk, gelatin and flour, have been proposed for use as in-house standards. Powder mixes for parenteral feeding are used routinely by at least one laboratory that runs a nationwide quality control programme (Ekstrom et al., 1984).

Constituents that occur only in fat create problems because they are not stable indefinitely, even at low-temperature storage, and antioxidants added to stabilize lipid components can interfere in analyses. One solution is to store high-lipid foods under nitrogen. However, in general, the RM should be renewed periodically, with provision for old and new standards to be analysed simultaneously as a further check.

When an SRM or in-house standard is available, it provides the most efficient method for regularly monitoring the performance of the laboratory’s technique. The inclusion of a standard material in a series of determinations is considerably simpler than many of the other techniques described here. Standard samples carried through the regular analytical routines promptly will alert laboratory personnel to any problems, permitting immediate corrective action.

2. Normal (routine) samples. If an analysis is to be carried out on a substrate that is new to the laboratory, the selected method should be applied to a series of routine food samples containing a fairly wide range of the constituent of interest. If such a series is not available, a set should be prepared by the careful blending of known amounts of the constituent with a food sample of known composition. Direct addition of small quantities of a constituent to large weights of a food should not be attempted; low levels should be obtained by serial dilution, preferably starting with a solution of the constituent. The nature of the solvent, and whether or not the solvent is removed, will depend on the nature of the substance and the substrate. If the food sample cannot be fortified, the addition of known amounts of the constituent should be made at the earliest possible step in the method. The most useful type of series for validation is prepared from two samples of the same particle size (in the case of solids), one with a high level of the constituent of interest and the other with a low level. Analytical samples containing intermediate concentrations are prepared by weighing and mixing appropriate amounts of the two food samples.

3. Analytical check sample series. Certain organizations provide, on a continuing basis, food samples designed to check the stability and reliability of the analyses performed in member laboratories. Some of these samples, which may be of particular use to analysts carrying out food composition work, are detailed by Horwitz et al. (1978) and Wolf and Ihnat (1985a).

4. Authentic samples. It is sometimes helpful to analyse sets of samples that can be considered to be authentic representations of the foods concerned and whose composition is fully described in the literature, e.g. cow milk, wheat flour, etc.

5. Food samples previously analysed by a different method. When introducing an unfamiliar or new method, it is helpful to re-analyse food samples that have previously been analysed by another, established method. Such samples should be analysed by replicate determinations, and then re-analysed after accurate dilution with some inert material such as water, oil or sand. If replicates and differences between samples are satisfactory, it is usually safe to proceed.

6. Internal methods of checking reliability. The wide variety of commodities analysed in food composition studies usually precludes the immediate availability of reference standards, previously analysed samples, authentic samples or even normal samples. This provides a singular challenge to the analyst to prove the validity of the values obtained. Replicate determinations are an obvious choice. Reproducible replicates, particularly if the replicate analytical portions are of unequal size, usually indicate that no gross mistakes are being made, although they do not rule out consistent errors. Other internal methods of checking performance involve the preparation of a series of fabricated samples, the method of standard additions and check analyses by different analysts, methods and laboratories. Some of these internal methods are discussed in more detail in Chapters 6 and 7 and below.

Replicate determinations. Both precision and accuracy are assessed by means of replicate assays on portions of the same food sample (which are assumed to be stable and identical regarding the quantity of analyte being investigated). In statistical terminology, the replicate results are considered as random samples from a hypothetical population of replicates; the mean (as well as other measures of location or central tendency) of these samples reflects the performance of the method with respect to accuracy, and the standard deviation (as well as other measures of dispersion) reflects its precision.

Duplicate analyses are normally the minimum required for food composition studies. Agreement between duplicates should fall within the established precision of the method. Where agreement is outside these limits, additional replications are necessary. The mean should then be calculated on the basis of all the results, unless there are very persuasive reasons for excluding certain replicate values. It is not possible to make hard and fast rules for precision; guidelines must be developed for each nutrient, at the levels expected in each food matrix.

Recovery studies. When a constituent is available as a well-characterized material with known purity, it is possible to conduct recovery studies in which a defined amount of the constituent is added to portions of the food being analysed. Analysis of the food alone and with the added constituent can be used to calculate recovery of the added constituent (or “spike”). If a range of additions are made, effects of concentration can be measured. Recovery of an added constituent often gives a misleading indication, however, of the measurement of the constituent naturally occurring within the food matrix. If no materials are available for fortification, it may be necessary to fortify portions of the food sample itself, using the method of standard additions (see below).

In either case – a series of samples with added material or a sample enriched by a series of additions – the calculated concentrations after analysis should be a straight-line function of the added concentrations. To be classified as satisfactory, recoveries of more than 90 percent are necessary.

Wolf (1982), stating that the method of standard additions is “used as a panacea for matrix effects”, cautions that “care must be taken that this technique is not misused. A basic assumption … is that the element added to the sample completely interchanges chemically with the endogenous element and that the two react identically to the matrix. It is often difficult to validate this assumption. Also, the method of standard additions does not correct for spectral interferences, where the matrix introduces spurious signals to the detection system … The method of additions also assumes a linear response curve in the range of the additions.” He concludes, however, that “the method of additions can be useful when the matrix effect has been fully identified as chemical in nature”, and the assumptions regarding interchange with endogenous elements have been validated.

Fortification of the sample itself is also inapplicable (despite apparently satisfactory recoveries) if the analyte is easy to recover when added as a pure material, but in its natural state is physically or chemically combined with other constituents of the sample making it difficult to recover. This problem frequently occurs when protein is present, as it is in most foods. The problem of extraction of the analyte is the most critical in this case.

Clearly, recovery tests have severe limitations as measures of the accuracy of recovery. Poor recovery indicates that the method is not behaving properly, but good recovery does not guarantee satisfactory performance.

Check calculations and analyses. Perhaps the most useful check procedures used in food composition studies are check calculations and analyses.

The first step is for another analyst to perform independently all the calculations of the first analyst. These checks should include all the secondary operations, such as derivation of equations, standardization of solutions, preparation of standard curves, measurement of recorder peaks and calibration of instruments. This practice is one of the most cost-effective operations in laboratory management, because of the high frequency of mathematical errors and simple mistakes.

A second cost-effective practice is preparation of a new standard curve from freshly prepared standard solutions. The new standard curve should correspond fairly well with the original. Improper preparation of standard solutions from incorrect calculations, weighing or aliquoting is a frequent source of error. Because they are unstable, dilute standard solutions should be freshly prepared from more concentrated solutions.

The best kind of check analysis is for a second, preferably more experienced, analyst to repeat the analysis by the same method on a separate portion of the same analytical sample. The analysis cannot be considered a check analysis if it starts beyond the initial stage, for example, with an aliquot of a wet oxidation digestion. Preparation of a new analytical sample from the original food sample is better, because it permits estimation of error introduced during preparation of the analytical sample.

Repetition by the same method is not satisfactory, however, when that method contains an inherent bias, or a bias is consistently introduced by some characteristic of the commodity being analysed. In these cases a check analysis using a method based on a different principle (if available) is desirable. This approach is usually used only when rare or uncommon foods are analysed and are found to contain a nutrient at unusually high or low levels. It will not reveal errors introduced in analytical sample preparation.

Another possibility, which should be used more commonly and not as a last resort, is to send a sample of the food to another laboratory for analysis as an unknown. The order of magnitude of the constituent may be indicated, in order to eliminate the need for exploratory analyses. Analysis by a second laboratory for occasional checking of normal samples (see above) is also a good way to maintain analyst proficiency in both laboratories. Exchange of food or analytical samples is particularly useful when a new laboratory or an unfamiliar method is being set up.

Blind analyses. Ideally, all food samples should be coded, and a series of concealed replicates should be prepared by an analyst who will not make the actual determinations, so that the analysis can be carried out free from bias.

Permitted analytical variations

The variations permitted between replicates by the same analyst and between analysts in the same laboratory should be established for each routine analytical procedure and type of food. In the case of a well-documented method, the results of collaborative studies provide sufficient criteria for acceptability of values. The variations within a laboratory should be smaller, or at least no larger, than the variations between laboratories. In principle there is no reason why they should differ, but in practice variations occur in equipment, reagents and the approaches of the individual analysts.

In studies of a method or in check analyses, replicates should be analysed in separate batches and on different days. Comparison of results obtained under these conditions sometimes reveals systematic errors.

Techniques for detecting and correcting errors in calculations and record-keeping

The correct recording of results can be aided if standard data-recording systems are drawn up for the laboratory. Data sheets may be printed or photocopied and supplied for use by the bench workers. In laboratories where computers are used for data acquisition directly from instruments, a computerized system can be used. All laboratory records must be kept in a systematic and accessible fashion so that an “audit trail”, or search back through the records to identify sources of error, can be instituted when required.

Horwitz et al. (1978) mention the problems experienced by AOAC associate referees conducting interlaboratory studies of new and improved analytical methods. They comment on the number of reports from collaborators who incorrectly calculate results, failing in simple tasks such as correct measurement of recorder peaks and insertion of appropriate values into a proportional equation.

To meet the obvious need for arithmetical accuracy in the performance of calculations, laboratory instruction manuals should describe the logic of the calculations and should provide examples; this clarity will help ensure that data are recorded correctly and inserted appropriately into the correct equations.

When area calculation is done by hand, each chart should be clearly labelled with the identity of each peak, the basis for identification, the peak area, etc. to permit cross-reference to laboratory notebooks. Rubber date-stamps are useful, and for some analyses a specially prepared rubber stamp may provide a convenient guide for entering peak identification, etc. on charts.

To eliminate calculation errors, a second person should ideally review the original raw data – recorder charts, meter readings, weights, volumes and times – and check the calculations. For chromatographic traces or spectral charts, the proper choice of peaks should be reviewed and compared with peaks of standards. This is also important when computing integrators are used if the printout is separated from the chart itself. The printer peak areas must be equated with the peaks, and retention times must be checked.

Charts should also be examined to ensure that instruments functioned properly, that there were no interfering materials, that peaks were all adequately resolved or separated, that appropriate sensitivities were used, and that blanks and controls were properly chosen and used.

When only one or a few samples of a given commodity are examined, little evidence is available from which to judge the reliability of the results; it becomes even more important to use proper checks on procedures at all stages.

A final check on the suitability and reliability of reported results lies in their consistency with previously reported values, with the literature, and with the known attributes of method performance.

Interpretation of the analytical values

Once an analytical result is obtained by a valid method of analysis, properly performed on a homogeneous analytical portion, several steps must be taken to ensure that the results are correctly interpreted in the context of the purpose for which analysis was carried out.

All values, whether expected or unexpected, should be subjected to scrutiny. Although the common practice of comparing new information with previously published values for the same food is useful, it can be a source of bias if the analyses are repeated only for deviant values; there may be a tendency to accept only data that conform to established values. Nonetheless, any samples producing unusually high or low results should be subjected to repeat analyses and specific validation, along with a few foods that yielded the expected values.

If the unexpected values are validated analytically, the collection, handling or preparation of the food sample must be investigated. For example, any high values for minerals may be due to contamination in the laboratory (perhaps by a mill or homogenizer). In these cases, the analysis must be repeated in such a way that contamination does not occur. If all steps in the laboratory are shown to be non-contaminating, then one must consider possible sources of contamination in the environment of the plant or animal from which the food was obtained. If the food sample was collected in the cooked state, one must consider possible sources of contamination during cooking (e.g. iron pot, metal skewer, or an iron plate or roasting grid). If the food sample was prepared and collected in a way that represents the food as it is usually available to the community, then the contamination may be regarded as contributing a real and representative value to the food. However, since contamination arising from the environment or during cooking does not necessarily contribute to the usual composition of the food, attention should be drawn to these unusual values and their nutritional significance in any written reports.

Some simple calculations can be applied as approximate checks on the appropriateness of values. For example, the summed quantities of ash constituents must not exceed the total ash, nor should the sum of the determined constituents exceed 100 percent of the weight of the analytical sample in a complete analysis (summations falling within the range of 97 to 103 percent of analytical sample weight are generally acceptable). When complete analyses are available, common-sense tests such as these can assist in determining the reliability or, more frequently, the unreliability of the reported results.

Final reporting of analytical data

All reports of analytical data, published or unpublished, must list the procedures that were carried out in the laboratory to ensure the quality of the data (e.g. the levels of recoveries, the use of SRMs or other standards).

As a general rule, correction factors should not be applied in calculating the final reported result. Usually, the actual value found and the recovery factors determined in the course of analysis should both be reported. Recovery factors are usually not constant from one run to the next, and their variability is an important performance-related factor used in interpreting the results of the analysis. When the correction factor varies with the type of food, the appropriate factor should be used and then calculated to a “recovery-corrected” basis. As previously indicated, the easiest way to avoid mistakes and ambiguity is to report the actual findings, the recovery factors and the corrected values.

Concluding remarks

A continuous system of quality control is difficult to maintain, but is essential. In a laboratory with a workload consisting of a variety of foods analysed for a variety of constituents, effort must be concentrated on as many applicable quality control procedures as possible. This situation requires use of standard and previously analysed food samples, or of food samples analysed in other laboratories to be used as simultaneous controls, and greater than normal participation in check sample series and collaborative trials. Analysts and laboratories that consistently maintain high-quality performance in check sample series and in collaborative trials would be expected to produce more reliable results in day-to-day routine analysis than laboratories that cannot produce evidence of the adequacy of their performance.

The consequences of failure to maintain a quality assurance programme justify the time and expense of its implementation. Incorrect data may have important consequences for consumers and for food composition data programmes; if the data are rejected by increasingly sophisticated database compilers, the laboratory that produced them loses credibility.

Chapter 9

Conventions and modes of expression of food composition data

A wide range of base quantities, units and modes of expression is required in a food composition database, as determined by the specific uses of the data. Generally, compositional data are expressed as mass quantity, for which the kilogram (kg) is the base unit (BIPM, 2003; NIST, 2003b). For food composition purposes, this is understood to be weight and, by convention, the data are usually reported per 100 g edible portion. However, data may be expressed on other bases such as serving sizes or domestic units, per 100 ml or per kg, or on the basis of energy (e.g. nutrients per 1 000 kJ), protein (amino acids per 100 g protein), nitrogen (amino acids per g N), total lipid (fatty acids per g total fatty acids) and others.

In principle, all specialized user databases can be derived from a main comprehensive reference database. The ways in which data are held and manipulated within any computer data management system, determined by the preferred operating system or data management routine, are not discussed here. However, compilers of a food composition database should be aware of several general issues relating to data capture and data documentation.

Data values

The following suggestions are made for data values.

Analytical values

These should be carefully documented so that the primary source of the data can be traced and the analytical methods used identified.

Missing values

It is virtually impossible to have complete data sets for all nutrients. It is essential that the database identifies missing values and alerts the user whenever food items with missing values are selected for entry or retrieval. This is particularly important in software programs in which calculated nutrient intakes (or calculated nutrient composition of recipes) with missing values need to be flagged for the attention of the user. Missing values must never be assigned a zero value.

Zero values

Zero may be used when it has been shown analytically that a constituent is not present in the food sample. Strictly speaking, the use of “zero” means that any amount present is below the detection or quantification limits of the method of measurement used. Although zero may be used to indicate the amount present is below the nutritionally significant level, it is however preferable to use the designation “trace” in these circumstances. An exception is where there is good reason to believe that none of the constituent is present, for example vitamin B_12 in plant foods. In these cases, analyses may not be required and the source or origin of the values may be referred to as “assumed” or “presumed” zero.

Trace values

“Trace” signifies that the constituent is present, but at a level that cannot be measured adequately. It may also be used when the level is judged to be nutritionally insignificant. It is desirable to define these limits in the database documentation. In many food composition databases trace is expressed as “T” or “tr” and it often represents the only acceptable non-numeric entry in a data value field. Table 9.1 contains some suggestions regarding more formal limits for the various constituents based, albeit intuitively, on the methods in current use.

Imputed values

In certain circumstances an estimated or imputed value, based on a similar food, may be substituted for a missing analytical value (see Chapter 1). Each imputed value should be fully documented for data type and source/origin.

Calculated values

Values derived by calculation are often used for mixed food dishes, recipes and some processed foods. Such foods should be distinguished by a statement to this effect in the description, and a field should be provided with a list of the ingredient food records used in the calculations. All values should be fully documented for data type and source/origin.

Modes of expression

If food composition database systems are to be compatible, the mode of data expression must be formalized (Klensin et al., 1989). In most cases, the basis for this should be long-standing nutritional conventions or international agreement on the preferred usage. For cases in which agreement has not been reached, the guidelines in this chapter suggest the most widely used conventions. Interchange and compatibility of data would be facilitated if the data were also more uniformly expressed in original data sources.

Bases of expression

The basis of expression should be chosen to fit the specific use of the database. The most common basis is g per 100 g of edible portion of food, although expression in terms of portion size or household measures is appropriate for many special-purpose user databases. Expression per kg is less convenient for users and can involve the use of greater numbers of significant figures than can be justified (see below). It is proposed that the 100 g basis be used for food composition data and databases, except for special-purpose databases and certain other items identified below.

Edible portion is itself a value that should be recorded in the database. It refers to the proportion of edible part in the raw food as collected or purchased, expressed on the basis of weight. The proportion of edible matter in cooked food is often expressed on the basis of the raw food.

Liquid foods

Since liquid foods are frequently measured by volume, expression on a 100 g or 100 ml basis could be used. It is desirable to record the density of these foods so that appropriate conversions can be made. Liquids with a high viscosity are usually measured by weight, making this the preferred mode of expression.

Significant figures

The last digit cited in the value should reflect the precision of the analysis and values should not be cited in such a way as to give a false impression of the precision with which a constituent can be measured. Because foods vary in composition, it is also fundamentally incorrect to cite values that imply that the composition is defined to a higher level than its natural variation. Significant digits should not be confused with the number of decimal places in a value. For example, the numbers 123, 12.3, 1.23, 0.123, and 0.0123 all have three significant digits.

Rounding procedures

Values for nutrients may be reported with more significant figures in the data source than are needed in a database. When capturing the data the figures are entered without any rounding. At higher levels of data management it is desirable to retain one more significant digit than is necessary in the user database, as outlined in Table 9.1. Where values are being summed for statistical purposes, the conventional rounding rules are appropriate, with even values ending in the digit 5 being rounded down (e.g. 0.25 becomes 0.2) and uneven numbers rounded up (e.g. 0.55 becomes 0.6) to avoid significant bias (Snedecor, 1956). It should be remembered, however, that digits beyond those indicated in Table 9.1 may have little analytical meaning and are of minimal nutritional significance.

Nomenclature for foods

While nomenclature for foods is of crucial importance (Chapter 3), the topic is too wide to be considered here. Food nomenclature, classification and description systems include Eurocode (Arab, Wittler and Schettler, 1987), LanguaL (McCann et al., 1988; Feinberg, Ireland-Ripert and Favier, 1991) and INFOODS (Truswell et al., 1991). Some authors have evaluated and compared the various systems for their advantages and disadvantages (Burlingame, 1998; Ireland and Møller, 2000). Food classification systems can also be based on the Codex Alimentarius, the FAO Agricultural Statistics Databases, the Harmonized System for Trade and the UN System for Classification of Individual Consumption According to Purpose (COICOP). Descriptions and links for all these nomenclature and classification systems can be found on the INFOODS Web site (INFOODS, 2003).

Nomenclature and conventions for constituents

Nomenclature for nutrients (see Chapters 4, 6 and 7) is in the main formalized; the following guidelines are based on international conventions.

Edible matter refers to the proportion of edible matter in the raw food as collected or purchased, expressed on the basis of weight. The proportion of edible matter in cooked food is often expressed on the basis of the raw food.

Water content (moisture content) values are method-dependent (Chapters 6 and 7), but for the most part the differences are of minor nutritional significance. Freeze-drying is the exception; residual water content from this method can affect the accuracy of all other results expressed on a wet-weight basis.

Nitrogen (total) is usually measured by the Kjeldahl or Dumas methods or a modification of these methods.

Protein is usually a calculated value, derived from the total nitrogen value multiplied by a nitrogen conversion factor. Food-specific factors have been elaborated, based on the nature and composition of the proteins contained in different materials (Jones, 1931). The specific factor for almonds is 5.18, while the specific factor for milk is 6.38. Jones’ factors are still widely used in food composition work (see Table 7.3). In the absence of food-specific factors, the general factor of 6.25 is applied. Some food composition databases use the general factor exclusively for all protein calculations, and in many countries/regions, food-labelling regulations require the use of the general factor (EC, 1990). All other methods for measuring protein are still calibrated against this type of value. It may be useful also to include in a food composition database protein calculated by both specific factors and the factor 6.25. For some applications, e.g. the formulation of diets against dietary requirements, the factor 6.25 is more appropriate because this is the factor used to derive protein requirements (FAO/WHO/UNU, 1985).

It has been proposed on several occasions (Southgate, 1974; Southgate and Greenfield, 1992; Salo-Väänänen and Koivistoinen, 1996) that protein definitions and methods of determination should be redefined. Many believe that the sum of the amino acids is the most appropriate representation of the protein content of foods (Salo-Väänänen and Koivistoinen, 1996). In all cases, the factor and the nitrogen values should be included in the reference database.

Fat (total) refers to the total lipid in a foodstuff, including triacylglycerols. The values are highly dependent on the method used. In the United States, the NLEA (Federal Register, 1990) and FDA (Federal Register, 1993) defined “total fat” as the sum of fatty acids expressed as triglyceride (sic) for nutrition labelling purposes (FDA, 2001).

Total carbohydrate (total “by difference”) is an unsatisfactory expression that should be phased out (FAO/WHO, 1998). It is a derived value, obtained by subtracting the percentages of water, protein, fat and ash from 100 to give the percentage of carbohydrate “by difference”. It includes all the non-carbohydrate material not analysed in the other proximate analyses and the cumulative errors from the other measurements. However, some food composition databases also subtract alcohol values for relevant foods.

Available carbohydrate is defined as the sum of free sugars (glucose, fructose, sucrose, lactose, maltose), starch, dextrins and glycogen. In reference databases it is useful to include the individual carbohydrate components separately in addition to the summated values for total available (glycemic) carbohydrate. In reference databases it is useful to include the individual carbohydrate species separately in addition to the summated values for total available (glycemic) carbohydrate. Values for the individual species are increasingly being given in user databases, in addition to those for total available carbohydrate. Available carbohydrate and its fractions can be expressed as weight (i.e., anhydrous form) or as monosaccharide equivalents (i.e., including the water of hydration). Available carbohydrate can also be calculated “by difference”, by subtracting a dietary fibre value, preferably “total dietary fibre”, from total carbohydrate by difference.

Dietary fibre is the focus of considerable scientific dispute in terms of the methods for its measurement. As the values are method-dependent they therefore need to be identified by the method used. The most widely used method is probably the AOAC total dietary fibre (TDF) method (see Chapter 7), but more specific definitions have also been used, for example the sum of the non-starch polysaccharides and lignin. If the non-starch polysaccharides approach is used, it may be preferable to use this term to identify the values in the database.

Ash (total) refers to the residue after incineration of organic matter. Values are method-dependent, but differences are of little nutritional significance.

Because it is rare to measure proximate or major constituents to an accuracy greater than ±1 percent, three significant figures are a maximum; values should be limited to 0.1 g/100 g, with “trace” defined as less than 0.06 g/100 g.

For inorganic constituents the appropriate elemental names or symbols are used. INFOODS tagnames are equivalent to atomic symbols for elements. Measurement to a precision of ±1 percent is extremely satisfactory, but may not be possible with trace constituents. The limits suggested in Table 9.1 are based on expected analytical limits combined with accepted levels of nutritional significance.

Vitamin is the term used when there are several active forms of an agent with a defined physiological activity, “vitamers” (see Chapter 7). The International Union of Nutritional Sciences (IUNS, 1978) system should be used to record defined chemical species. In the reference database, the values should be listed for each vitamer separately (e.g. the individual carotenoids). Values for total vitamin A activity and total vitamin D activity are calculated values and are therefore best restricted to the user databases, and the factors used in the calculation should be clearly specified. Over time, conversion factors for vitamer activities are likely to change, requiring a recalculation from the individual vitamer data in the reference database. Equivalences given in Chapter 7 should be used for conversion from international units. In general, methods for measuring vitamins are somewhat less precise than those used for inorganic analyses. The limits of expression are shown in Table 9.1. Expression to three significant figures is seen as a reasonable level for citation.

Amino acids are referred to by the approved trivial names, or three-letter symbols that are equivalent to INFOODS tagnames. At the reference level, amino acids are usually expressed as mg per g of nitrogen or as g per 16 g nitrogen (approximately 100 g protein), but at the user database level, expression as mg/100 g of food is useful. As with fatty acids, it is often useful to have both modes of expression available for comparative evaluation at all levels of the database system.

If amino acid values at the reference level are expressed in relation to total nitrogen, non-protein and non-amino-acid nitrogen should be deducted from the total nitrogen in order to express values as mg/100 g of food. Expression to three significant figures is seen as appropriate for amino acids cited as mg.

Fatty acids are listed with the chain length and double bond numbers. Systematic names may be needed to define values for specific isomeric fatty acids. Some of the more important isomers, e.g. trans isomers, should be included in the user database. At the data source and reference database levels, values for individual fatty acids are usually expressed as percentages of total fatty acids since this is the most common form of analytical presentation. At the user database level, values per 100 g of food are required. At all levels of data management both modes of expression are useful for comparative evaluation. A conversion factor derived from the proportion of the total lipid present as fatty acids is required (Paul and Southgate, 1978) for converting percentages of total fatty acids to fatty acids per 100 g of food (Table 9.2). For fatty acids expressed in g per 100 g total fatty acids, precision is best limited to the 0.1 g/100 g level, with trace being set at <0.06 g/100 g total fatty acids.

Other constituents are referred to by the recognized chemical terms, using either trivial

or systematic names depending on common usage.

Energy value refers to a value for metabolizable energy, derived by calculation from energy-yielding constituents using energy conversion factors (see Chapter 7). The energy values of foods in the user database are often derived by application of conversion factors to the values for proximate or energy-supplying constituents. Direct determination of gross energy values (i.e. heats of combustion) may be useful for some purposes; however, these values cannot be compared with values for metabolizable energy as used in nutrition.

It is important not to imply great accuracy in the citation of energy values. The convention is based on the following questionable assumptions:

a)Gross energy (heat of combustion) of different proteins, fats and carbohydrates is constant for all foods.

b)Measurements of apparent digestibility give an accurate indication of available energy.

c)Coefficients of apparent digestibility are constant for all foods.

d)Digestibility does not vary significantly among individuals.

Attempts have been made to derive specific factors for individual foods or food groups, recognizing assumptions a) and c) (Merrill and Watt, 1955), but not b) or d) (Southgate and Durnin, 1970).

Energy values should not be cited to more than three significant digits with a limit of 1 kcal or kJ.

Chapter 10

Quality considerations in the compilation of a food composition database

This chapter describes the stages involved in database compilation, from the collection of data to their entry into the computerized (or published) database. In most database programmes, this is the process in which the programme’s own sampling and analytical procedures for the production of values merge with indirect, literature-based operations.

The compilation process is not merely a clerical task of assembling numerical values into a suitable format. The operation includes the appraisal of all the information entering the database management system. In the process, each item of data is evaluated against a series of criteria. In many cases the compilers must consult individuals with a sound knowledge of foods and nutrients and an understanding of analytical procedures before deciding whether or not to include certain values.

The evaluation of data is an iterative process between the various stages of the database system (Chapter 1). Although the compiler reviews the data at all levels, questions will frequently be raised as the compilation proceeds that require a return to the primary data source. It is therefore essential that the evaluative process be fully documented.

Experiences of compilation within a national food composition programme are described by many authors and published in proceedings of INFOODS Regional Data Centre meetings (e.g. Aalbersberg, 1999) and in proceedings and special journal issues from national and international food data conferences (Greenfield, 1995; Food Chemistry, 1996; Journal of Food Composition and Analysis, 2000, 2001, 2002, 2003a)

Sources of data

Before outlining criteria for data scrutiny, it is necessary to consider the primary sources of the data. They can be considered to fall into four broad categories (Table 10.1), each with its own characteristics that the compiler must take into account. Although in principle all data should be evaluated against the same criteria, it should be recognized that much existing information on the composition of foods does not fully meet the ideal criteria. The four major categories of sources of data are as outlined below.

Primary publications

This category includes compositional data in papers published in scientific journals. In addition to journals on food science and nutrition, those concerned with the analysis of food by-products, studies of soil treatment, animal and plant husbandry, and analytical method development, among others, are included.

While these papers have usually been subject to peer review and refereeing, the work will generally have been evaluated with regard to the primary purpose of the study, and not necessarily the quality of the compositional values as such. Thus, the experimental sections of papers may often contain insufficient detail to permit use of these values without application of the formal criteria discussed below. Nevertheless, these data have a clear unequivocal source and can usually be related directly to specific foods and analytical work.

Secondary publications

This category includes reviews, other published compilations of compositional data (including food composition tables and computerized databases) and material published in books or unrefereed journals. The values given in this category may be more difficult to evaluate against the formal criteria. For example, data from other food composition tables should ideally lead the compiler to the sources of the data, published or unpublished, but frequently the source leads only to another set of tables. When compositional values are published in unrefereed publications, the compiler may have to consult the author or the compilers of the database before the values can be properly evaluated.

Some compositional data are published in their original form in food composition tables, as in, for example, The composition of foods (McCance and Widdowson, 1940, 1946, 1960; Paul and Southgate, 1978), where primary analytical values were published. In the 1960 edition, material taken from the literature was fully referenced. The 1978 edition provided keys to the laboratories supplying analytical values specifically obtained for the edition, the methods used and references for material taken from the literature. In subsequent editions (Holland et al., 1991; Food Standards Agency, 2002) and the supplements (Holland, Unwin and Buss, 1992a, 1192b; Holland et al., 1991; Holland, Welch and Buss, 1992; Holland, Brown and Buss, 1993; Chan, Brown and Buss, 1994; Chan et al., 1995, 1997; MAFF, 1998) to the United Kingdom food composition tables (which constitute the primary United Kingdom nutritional data), the publication of the keys was discontinued for reasons of economy, but the information is still available from the publishers. Many countries continue to publish details of their sample and analytical documentation, abridged or complete, and this is to be encouraged. Whether released in printed publications or not, all data compilation centres should be able to make documentation details available to users as required.

Unpublished reports

This category includes compositional data that have been collected into a document prepared for limited circulation, frequently for internal use in commercial companies, institutions or government departments. The application of formal criteria to these data is often difficult and depends on the nature of the document. These reports often contain original analytical data and, as such, can be valuable sources of compositional values. Alternatively, the data may be used as confirmatory or to provide some indication of variation in a particular constituent. The authors should be consulted, where possible, if there is any doubt or confusion about the values.

Unpublished analytical data

This category includes two broad types of data. First are the analytical data that were not generated specifically for a nutrient database (where, for example, the collection of food samples was not designed to be representative and the analyses were not controlled or supervised by the organization or group responsible for the database). In these cases the compiler must carefully scrutinize sampling and analytical procedures, and must also be confident that appropriate quality control procedures were in use. Direct access to records of food samples and analytical notebooks is especially valuable. Also, a proper evaluation can be made if the compiler can discuss the values with the person responsible for sampling and analysis.

The second type is unpublished values obtained specifically for the database programme. These values should be scrutinized, even though the compiling organization controlled the sampling and analytical procedures albeit through contracts. In a strict sense, these new analytical data merely join the existing population of values, and should be compared with other sources of compositional data. Only when there is good evidence that a food has changed (for example, if a new variety has been introduced or changes in agricultural or secondary production practices have been made), or that improved analytical procedures have been used, can older values be rejected (see sections on “Changes to values” and “Obsolete foods” on pages 185–186). Differences not obviously due to these factors must be investigated, and it is often desirable to repeat the sampling and analysis in confirmation.

Criteria to be applied during data scrutiny

The bases for these criteria have been reviewed in earlier chapters. They are summarized in Table 10.2.

Identity of the food

The compiler must be certain of the identity of the food sampled for analysis. Primary plant foods may need to be identified by both species and variety, while fish and carcass meats may need to be identified by species. Age and maturity will also often be relevant to proper identification. When the food consists of a part of a plant or animal, this must be clearly identified. Proprietary products and cooked dishes are particularly difficult to identify. Foods that cannot be unambiguously identified should be flagged as such in the database. A photo-graph or graphical image may assist in clearer identification in the future (Burlingame et al., 1995b).

Nature of food sample

The food samples must be representative. Thus, scrutiny includes evaluation of the sampling plan used to obtain the food in terms of number/weight of items collected, date and time of collection, geographic location, mode of combination of items, etc. (Chapter 5).

Nature of material analysed

The nature of the material analysed must be clearly established: raw or cooked (with method), how prepared (e.g. with or without peel), edible portion description and weight, refuse description and weight, typical serving description (e.g. one slice for bread) and weight.

Analytical sample preparation and analytical procedures

The preparation of the analytical sample and the analytical procedures are often described together in reports. Their evaluation requires close familiarity with nutrient analyses. First, the protocol for preparation of the analytical sample should be scrutinized to see whether it meets the criteria discussed in Chapter 5. Second, the analytical methods should be evaluated; preference should be given to values obtained by means of validated methods that are compatible with methods in international use (Chapters 6 and 7) and to values whose sources indicate that appropriate quality assurance procedures were in place (Chapter 8).

Mode of expression

The compiler must be able to identify clearly the mode of expression used and, especially, the bases on which analytical values have been expressed. This is particularly important when the published values have been derived from analytical values by the use of conversion factors.

An approach to the formalization of the above criteria is given in Table 10.3.

The compilation process

Assembling data sources

The first stage is the assembling of data sources, including published tables. A rigorous search of the literature is essential. Special care is necessary in designing the search strategies when using computerized searches that are highly dependent on keywords, and some additional manual searching can be useful. Authors’ abstracts should not be relied upon as sources of values; the full papers need to be examined. Literature searches usually start with the abstracting journals, and each bibliographic reference normally leads to several others. Recent papers should be sought by regular consultation of abstracting literature and databases. Journals not covered by an abstracting service must be referred to directly. It is desirable also to establish contact with sources of unpublished data: university, government and private laboratories; research institutes; commodity boards and food manufacturers.

The INFOODS Web site (2003) is especially valuable as a source of advice when seeking information on uncommon, and indeed all, foods. This site gives access to the INFOODS mailing list, which provides regular access to queries and responses, and to notices of meetings.

Discretion may be needed in obtaining and using manufacturers’ data, as they may insist that the information be treated confidentially. Nevertheless, the data may be valuable for confirming information from other sources.

Where data appear in the sources as mean values of several determinations on replicate food samples, where possible the authors should be asked for the individual replicate values.

Archival stage

All the relevant information obtained should be recorded systematically using one of the many computerized database management systems available. The primary requirement is that the system should be very flexible with regard to the number of fields and the facility for interchanging data with other computerized systems. International food composition data interchange formats have been proposed (Klensin, 1992; Schlotke et al., 2000), and development continues as an international effort under INFOODS.

Data from each source should be assessed for general quality and consistency, and entered into the system for easy access. Computer programs should be able to accommodate in specific relational tables all the data and metadata, including source details and notes on methods of analysis, sampling procedures, etc.

Comprehensive compilation at this stage is critical for the quality of the database. This represents the archive or store of all values reported for the composition of foods. It is important to retain historic data in the collection because these provide information that helps in the assessment of whether the composition of a food is changing over time or whether it has a stable composition. The significance of methodological changes can also be assessed from comparisons of data over time. Many users are involved in the analysis of historical records of food intakes and require access to the most relevant compositional data. In the context of this account, an archival database is seen as the computerized store of all available data, both recent and historic.

All the information on food identity, sampling, analysis, quality assurance procedures and modes of expression need to be evident for each record because it will be used in the next stage. Values recorded in the data source will need to be converted to the form in which they will be presented in the reference and user databases.

Bringing together all the values for the foods will identify discrepancies that will require the compilers to return to the original data source and to rescrutinize the data. Very commonly, transcription errors will be found but even when these have been eliminated, discrepancies often remain. These can be due to inconsistencies in the identification of the foods, for example differences in plant varieties. Comparing the values of other foods analysed within the data source with values reported in another data source can provide some idea of the confidence that can be applied to the credibility of the source.

However, even after the strictest scrutiny, differences in the compositional data will persist; these may represent analytical artefacts or reflect the natural variations in composition. In these cases, the ideal, if resources permit, is to set up a sampling and analytical protocol to confirm the values. Failing this, one can only retain the questionable value and assign to it a lower confidence code (Exler, 1982).

Reference stage

The archival stage provides the basis for preparing the reference database. In this, all the acceptable data for each food from the different archival records are combined and presented in a common compatible format with links to the archival records and their metadata.

To do this, the compilers have to review all the available data for each food. Most data sources do not provide coverage of all the constituents required for the database as a whole and typically cover a limited range of components. The compilers must consider whether the different samples of foods are compatible. This requires comparing water and fat contents and considering whether the adjustment of values to a constant base is justified. Each stage in evaluation of the data must be documented so that the logic of any decisions taken or calculations used in the construction of the reference database can be followed later.

This review may require a return to the data sources to check points or confirm that values have been recorded correctly.

It is also necessary to consider which statistical techniques would be appropriate for evaluating the data for the food.

In this, all acceptable data from the archival records are identified and the logic of statistical combinations is recorded together with the average (if this is seen as appropriate), the median, or a selected value based on an assessment of the reliability of the sources (Paul and Southgate, 1978). This last approach may be seen as subjective but if one has an array of values and their number is inadequate for formal statistical combination the compilers must make these judgements if a useful database is to be prepared. At this level, a sensible degree of disaggregation is necessary. For example, a single record for “apple” would not be appropriate when data for individual apple cultivars are available. Final checks for internal consistency are required.

Preparation of the user databases

Dietitians may have requirements for a user database with certain types of food and certain forms of data presentation; agriculture and food industry professionals may require another type of user database. Several different user databases and tables can be prepared from a single, well-constructed reference database. The preparation of user databases requires examination of the food records in the reference database and their combinations (where necessary) and final checks for internal consistency. In many cases, the database for all the foods is provided in the “reference database” for the country or region. In this book we see the “user databases” as those that contain one set of data for each food item, and in which the nutrients and other constituents are given one value per food item. It may be necessary to provide two or more entries for a single food, for example where seasonal differences in composition are sufficient to justify two separate food records. The preparation of user databases should not entail actual data entry. All data to be used in preparing the user databases should have been entered during the archival and/or reference stages.

Scrutiny of values

First, values for each nutrient in each food are subject to a rescrutiny that at least equals that used at earlier stages of the database compilation. The reported values for each nutrient in each food are examined specifically for consistency. The use of objective statistical techniques is preferable where sufficient data are available. Discordant values may be statistical “outliers” that arose at sampling or analytical stages. The tests for outliers (Youden and Steiner, 1975) are designed to eliminate two categories: values that lie outside the measured variability of the values and those in which the measurements themselves show excessive variance. Once outliers have been identified, the mean or median statistics can be recomputed and the variance recalculated without their inclusion.

Outliers should not, however, be deleted from the database per se. They can simply be marked for exclusion from the calculated mean in a user or reference database. Upon returning to the data sources to investigate the values, the compiler may find that the outliers are methodologically distinct and may be preferred, perhaps because they are the product of a more specific procedure or because the analytical sample was better handled (e.g. a preservative was used).

Combination of values from different sources

Because individual data sources rarely include the complete range of nutrients for a given food, it is often necessary to combine values from a variety of sources. In combining these values, it is vital to make certain that the various sources are compatible and that there is internal consistency.

Use of average values

When several values exist for the same food and nutrient, the compiler must review the procedures used in the reference records and reconsider how best to derive a single value for use in the database. When a large number of values are available, the use of an arithmetic mean value, or possibly the median, is the preferred approach.

When only a small number of values are available and the values exhibit a wide variance or range, the situation is much more complex. The variability may be due to the presence of outlying values or due to poor quality or non-representative food samples. In many cases, the compiler must judge which values have a higher level of confidence (i.e. better-documented food samples, choice of most appropriate method, or clear evidence of a quality assurance programme). In the United Kingdom food composition tables (Paul and Southgate, 1978), these were called “selected values”. In such cases the compiler must record the evidence used to select the values, so that the decisions can be re-evaluated independently.

In some instances the compiler may employ a weighting procedure. For example, if a value is required for a food with a seasonal variation in consumption or composition, a value reflecting the year-round composition can be calculated by weighting the values in relation to the consumption pattern. Again, documentation of this weighting is essential.

Calculations from analytical values

The database will include some derived values, calculated from analytical data. These have been discussed in Chapter 7. Some points, however, require further emphasis.

Energy value. The values in all nutritional databases are estimates of “metabolizable energy” calculated using energy conversion factors with the energy-yielding constituents in foods – proteins, fats, carbohydrates, alcohol and sometimes organic acids or other constituents. The factors commonly used are the Atwater factors (Merrill and Watt, 1955; Southgate and Durnin, 1970; Allison and Senti, 1983), in their general or specific versions. These were originally expressed as kcal but now more commonly are given as kJ. The kcal factors were rounded by Atwater (Merrill and Watt, 1955) and therefore direct use of the kJ factors is preferred so that this rounding is not carried out twice. In many databases, energy is a dynamic value rather than a fixed value. This allows the compiler to prepare different energy values for the different user databases. For example, a dietitian may prefer energy values calculated from specific Atwater factors, while for food-labelling purposes, food industry personnel may require energy calculated from general Atwater factors. Furthermore, energy calculation recommendations can change over time, requiring recalculation of all energy values in a database. Recommendations from the FAO/WHO Expert Consultation on Carbohydrates in Human Nutrition (1998) suggested that energy factors for dietary fibre should be used. Dealing with such situations is a straightforward data management task when the energy values exist, with simple algorithms programmed into the system to allow energy calculation to take place as needed. Energy conversion factors should be treated in the same way as other numeric data and should be included in the reference database with their INFOODS tagnames.

Protein. Protein values are conventionally calculated by the application of conversion factors to values for total organic nitrogen. More accurate values are produced, however, if conversion factors are applied to amino acid-nitrogen values (see Chapter 9), or by summation of the amino acids. All data and factors used in calculations should be included in the reference database.

Vitamin equivalents. The recommendations to derive values for vitamin equivalents are described in the conventions on nomenclature (IUNS, 1978).

Vitamin A activity. Derived values are usually used for vitamin A activity, since values for pre-formed vitamin A (retinol and its derivatives) and for the provitamin carotenoids may be combined by algorithm at the user database level. The convention is to express vitamin A activity in μg retinol equivalents that equal the sum of μg retinol, and μg β-carotene divided by the factor 6, plus total μg of other carotenes divided by the factor 12. Other conversion systems allow for the contributions of other carotenoids. Data for retinol, all individual provitamin A carotenoids and all activity conversion factors need to be recorded in the reference database with their INFOODS tagnames. It should be noted that the conventional conversion factors are not supported by recent research (van het Hof et al., 2000) and that new factors have already been adopted in some countries for some purposes (Murphy, 2002).

Recalculation of vitamin A activity with updated factors is straightforward when the original values are given, as with energy, and preference should be given to μg values for the individual carotenoids. The international unit conversions for vitamins A and D are given in Chapter 7. The convention adopted for calculation of vitamin A activity should be included in the database documentation.

Niacin activity. Equivalent values for niacin activity are also widely used where the contribution of tryptophan is included. The convention is to express niacin activity (mg) as mg niacin (or nicotinic acid) plus mg tryptophan divided by 60.

Fatty acids. Calculation of fatty acids per 100 g food from data for fatty acids per 100 g total fatty acids is demonstrated in Appendix 5.

Calculation of composition of composite prepared dishes

In the absence of analytical values from representative samples of prepared composite dishes, estimated compositional values for these dishes can be based on recipes and the composition of each ingredient. The yield, or change in weight during cooking (i.e. weight of raw dish and cooked dish) must be known. Several authors have published guidelines on calculation procedures (Rand et al., 1991; Bognár and Piekarski, 2000) The simplest version of the calculation procedure does not acknowledge fat gained (e.g. from frying oil) or lost during cooking, because the calculation assumes that changes in weight reflect only a loss or gain of water. Estimates of vitamin losses can be made using nutrient retention factors (Bergström, 1994; USDA, 2003c), but these values must be assigned lower confidence levels than analytical values. One version of the calculation has the following stages:

1.From the weights of raw ingredients, calculate the amounts of water and nutrients present in total raw food before cooking.

2.Sum the nutrients.

3.Divide the nutrient sums by cooked weight to give the composition of cooked food per 100 g. The water content of the cooked food is calculated (total water in raw ingredients – loss of weight on cooking).

A worked example of this calculation is given in Appendix 6. Table 3.3 on page 41 gives additional information, which may assist in the development of variations to this calculation.

Internal checks on selected values

Internal checks on the nutrient profiles developed for each food are especially important when values from several sources are used for a single food.

For the proximate composition, the sum of the components should ideally equal 100 g; in practice, a range of 97 to 103 g is permissible. If the summated values fall outside this range, one should first rescrutinize the calculation of protein values (was the appropriate factor used?) and mode of expression of starch (as g starch or as monosaccharide?). If the summated values are still outside 97 to 103 g, suspicion must fall on particular analysed values, which must be rescrutinized at the archival and data source levels.

Fatty acids should not exceed 95 percent when expressed as a percentage of total fat (because of the glycerol present in triacylglycerols (triglycerides)); when expressed as g per 100 g of food, they should not exceed total fat multiplied by the appropriate factor (see Table 9.2).

Total amino acids should not exceed 6.25 g per g of nitrogen to a level that is larger than any correction for the gain of water on hydrolysis (see Chapter 7). The total will be considerably less than this in foods with high levels of non-protein nitrogen or large amounts of amide. The checks on the recovery of amino acids may require rescrutiny of the data source, because many published papers do not report analytical recovery, especially of nitrogen from the ion-exchange column.

Summary of the compilation process

An overview of the compilation process is given in Table 10.4. Each stage of preparation demands detailed scrutiny of the preceding stages, and frequently requires a return to the data source level. The quality assessments become more clearly defined and established as the iterative compilation proceeds.

Producing an integrated estimate of data quality

Many data users require some indication of the quality of the data included in the various databases so that when data are being combined there is confidence that the quality of the data is comparable across different databases. This is particularly important where the data are interchanged electronically between databases.

Producing an integrated assessment of data quality involves a series of judgments about a data source and information about the food in question.

Although both sampling and analytical criteria, in principle, need to be considered, in practice it is often best to start with the analytical aspects.

The use of a well-documented method justifies a high quality rating whereas the use of a method without description or referencing gives the data a low rating. In addition, evidence that the method was controlled by a quality assurance programme, with the use of appropriate standards or SRMs, where available, further supports a high rating, whereas the absence of such evidence leads to a lower rating.

A low rating does not in itself mean that the values with this rating are incorrect, merely that the authors (or the journal) have not presented evidence that their data should inspire confidence.

A well-designed sampling protocol that was planned to meet certain confidence limits, for example 95 percent (implying that 95 percent of samples would have values within 5 percent of the given value) would represent a very high quality of sampling. However, in practice such protocols are extremely rare and will often only apply to a limited range of nutrients. Sampling protocols with confidence limits of 90 percent are probably the highest standard that one can reasonably expect, and for resource reasons will only be available for foods that are major components of the diet.

Most sampling protocols have lower confidence limits, and sample numbers of between 10 and 20 give a reasonable measure of confidence, except for those nutrients that are very variable or unstable such as vitamin C, folates and many trace inorganic constituents.

Analyses of single samples with no evidence of a sampling protocol other than convenience have a very low level of confidence. Many users consider that “any value” for a food or nutrient that forms a minor component of the diet is better than no value. Thus, one could argue, for example, that values on a few samples of caviar or champagne could be used in a database. Analogously, a few analyses on a proprietary product that is subject to rigorous quality control would have a good confidence limit.

Quality assessments and quality codes

Quality or confidence codes are a formalized approach to the acceptance of data (see Table 10.3), originally suggested by Exler (1982) and given in Tables 10.5 and 10.6. In this approach, a numerical value is given to the data for each criterion, and the values are combined and translated into a confidence code. Like all systems, it is arbitrary and can be used only as a guide. The preferred approach is statistically based; an appropriate number of food samples have been collected, and analysis has employed well-documented methods (with defined performance characteristics) that have been subjected to collaborative trial. Holden, Bhagwat and Patterson (2002) have described the evolution of the Exler approach. In this, quality is recognized as an integration of sampling and analysis, with a number of objective questions for each of the original five categories: sampling plan, number of samples, sample handling, analytical method and analytical quality control (see Box 10.1). It should be emphasized that the documentation related to the calculation of quality codes must be available in the archival and/or reference database.

Each of the evaluation categories is intended to have clear and objective questions that have Yes/No/Unknown answers. Each of the five is marked out of 20 on a continuous scale, giving a maximum score of 100. The methodological aspects are constructed using the advice of the best current practice by expert groups. The aggregated scores from the evaluation categories are used to provide the confidence codes.

The development of schemes for the evaluation is an ongoing one and, as will be evident, is highly dependent on the proper documentation of compositional studies. It is important to remember that confidence codes are not real numbers but guides for the users of the data. The confidence that can be ascribed to analytical values is, in the final analysis, determined by how accurately the value obtained predicts the value in the food; for this, statistical characterization of the composition of the food is essential. One caveat here is that these codes are categories and should not be manipulated arithmetically as if they were real numbers.

Changes to values

Once a user database has been disseminated and the data are in use, it is important to retain a record of the values even after they have been changed. Burlingame (1992) describes the importance of the “changes” feature of the New Zealand food composition database. There are at least three reasons for changing values in a database: they may be updated with the acquisition of more values for the calculation of a mean; they may be corrected if an incorrect value is identified; or the need for modification may be due to real changes in the composition of the food (e.g. arising from new fortification legislation). In all cases, it is useful to document the reason for change and retain the old values in a “changes” database, representing an audit trail for the database. One example of how this might be useful is when national food composition surveys are conducted over time; if the nutrient intakes of the population vary from one survey to the next the “changes” database will allow the differentiation between true changes in intakes and changes simply related to corrections and updates made to the database.

Obsolete foods

As with the “changes” feature, it is important to keep an audit trail of food records, even when a food is no longer represented in the food supply. The food code is usually used as the “key” in a relational database management system. Often these codes also are used in dietary assessment projects, applications software packages and other important ongoing activities where compositional data are used. It is therefore prudent to maintain original food codes permanently, and never reuse them for other foods, even when the foods to which they were originally assigned become obsolete.

Chapter 11

Guidelines for the use of food composition data

There are two schools of thought about food tables. One tends to regard the figures in them as having the accuracy of atomic weight determinations; the other dismisses them as valueless on the ground that a foodstuff may be so modified by the soil, the season or its rate of growth that no figure can be a reliable guide to its composition. The truth, of course, lies somewhere between these two points of view.

(Widdowson and McCance, 1943)

A food composition database or table is a scientific tool and must be treated as such. Even the best food composition database or table is of little value if it is used incorrectly. The compilers are responsible for ensuring that the database meets users’ requirements and they must also define for the user the limitations of the database, so that the data are not used inappropriately. However, correct use is the responsibility of those who train the users, and of the users themselves.

Effective use requires training and expertise, the level of which depends on the sophistication of the database or tables concerned (see Chapter 1 for a discussion of levels of data management). Even simplified food tables designed for lay use require some background knowledge of weights and measures, and of terms such as “kilojoules” and “energy”. More sophisticated databases require an understanding of modes of expression, food descriptors and concepts such as edible portion. A professional nutritionist or dietitian must become familiar with the principles of sampling, analytical methodology and data management, and be aware of common mistakes that can arise in database usage. The professional user also requires training in database evaluation for specialized uses (e.g. a research project). A training programme covering all of these areas should probably form a unit in any tertiary or professional course specializing in nutrition. Wageningen Agricultural University and UNU/FAO/INFOODS have run specialized short training courses on the production, management and use of food composition data in centres around the world since 1992, and information about forthcoming courses can be found on the INFOODS Web site (INFOODS, 2003). Overall, considerable responsibility rests with those who train users of food composition databases (Greenfield, 1991b).

Ultimately, it is the users, particularly the professional users, who bear responsibility for using the database correctly and particularly those users who have the responsibility for updating and supplementation of an existing database for their own organization. They must familiarize themselves with all aspects of the database or tables: coverage, methods of analysis, method of compilation, sources of values, differing types of values, coding, food nomenclature and modes of expression. They must understand the use of factors in calculating derived values (such as protein, energy value and vitamin equivalents) and the different levels of reliability attached to values for different nutrients. Arithmetical checks should be run to ascertain the accuracy of calculated values (e.g. fatty acid levels in a food, calculated from the food’s fat content and the fatty acid composition [see Appendix 5]). Any computer program developed for use with the database should be carefully checked for accuracy. Finally, the user must ensure that any research report based on a database or set of tables fully documents the database or tables used, together with any supplemental food values used (Perloff, 1983). Several journals (Journal of Food Composition and Analysis, 2003a; Journal of the American Dietetic Association, 2003; and Nutrition and Dietetics, 2003) now require the identification of nutrient databases and software in all published articles, with the following standard presentation suggested by the Citation Task Force aligned with the United States’ National Nutrient Databank Conference:

Cite software developers parenthetically in the text after the first mention of a software package. Software citations should include the name, version number, and release date of the software as well as the name and headquarters location (city and state) of the software developer. If software incorporates a nutrient database, provide information in the text about the database. This should include the release date for the database, a description of substantial modifications made to the database, and an explanation of how missing nutrient data for foods were handled (i.e., indicate whether values were extrapolated and evaluate the effect of any missing values on dietary totals for the nutrients of interest).

This practice could usefully be adopted by all journals dealing with dietary studies of humans. Failure to give such information means that a study as published can never be independently replicated.

The quality of future databases will improve only if all users are well trained and vigilant.

Limitations of the use of food composition databases

Several studies have compared values obtained from the chemical analysis of composite diets with values computed by use of food composition tables or databases, with greatly varying findings (Stock and Wheeler, 1972; Acheson et al., 1980; Stockley et al., 1985; Wolf, 1981; McCullough et al., 1999). Arab (1985) demonstrated the difficulties of making international comparisons, owing to variations in both nomenclature and composition of foods. Limitations in the use of food composition databases can be summarized as:

a)variability in the composition of foods;

b)partial or limited coverage of food items;

c)partial or limited coverage of nutrients;

d)inappropriate database or food composition values;

e)errors arising in database use;

f)incompatibility of databases;

g)differences in software packages;

h)limitations of methods for measuring food intake.

Variability in the composition of foods

Foods as biological materials exhibit natural variations in the amounts of nutrients contained. This variability is increased by different methods of plant and animal husbandry, storage, transport and marketing. Processed foods, despite being subject to quality control during production, also vary, in part because of variations in the composition of ingredients but also because of changes in formulation and production. Some composite foods such as margarines are routinely reformulated with the least-cost procedure that will maintain technological qualities of the product within a defined price range but may alter the nutrient content.

For many foods the limits of natural nutrient variation are not defined. Similarly, variations introduced as the food moves from production through retail sale to consumption are not known for many nutrients, because of the low priority (and hence lack of resources) devoted to food composition research. However, sufficient information exists to support some general statements about the major sources of variation in the nutritional composition of foods.

Meats. The major sources of variation in animal products are the proportion of lean to fat tissue and the proportion of edible to inedible materials (bone, gristle). The distinction between edible and inedible is subject to cultural and personal idiosyncrasies. Variations in the lean–fat ratio affect levels of most other nutrients, which are distributed differently in the two fractions.

Fruits and vegetables. In plant foods, genetics, husbandry and storage are major sources of variation. Water content is particularly affected by storage conditions, and changes in water content are associated with changes in all other constituents, primarily as a result of changes in nutrient density. Husbandry conditions, geochemistry (soil composition) and fertilizer use alter vitamin and mineral contents, especially of trace elements; levels of illumination affect sugars, organic acids, carotenoids and vitamin C levels. The level of phytochemicals in plant foods varies even more than nutrient levels because it is heavily dependent on factors such as pests and pesticides (Eldridge and Kwolek, 1983).

Cereal. Flours and grains vary less than do fruits and vegetables because they can be stored only if their water content lies within a narrow range. However, their protein content can vary by a factor of two, depending on variety and fertilizer usage. Of course, fertilizer and soil type will produce some variations in mineral content. Cereal enrichment/fortification practices in some countries markedly affect contents of B vitamins, iron, calcium and folate.

Milk. The major variation is in fat content and fat-soluble vitamins. Most industrialized countries have rigid standards for fat content, and the collection of milk from large herds minimizes differences due to stage of lactation. Considerable variation would occur in the composition of milk from small herds, which comprise the majority of those in developing countries. Levels of carotenes in milk may vary considerably, depending on time of year and whether the herds are fed concentrates or are at pasture. In some countries, milk is fortified, e.g. with vitamins A and D.

Processed foods. Variations in ingredients and formulation are common, although most manufacturers have rigid specifications for ingredients and use quality control procedures that sometimes pertain to nutrient levels. However, in many cases the requirement is to maintain specified levels of nutrients, and most additions include “overages” to allow for losses during handling and storage. Despite quality control, many processed foods exhibit the same variations seen in “natural” foods.

Composite dishes. Human diets include a wide range of composite dishes, prepared either by a food service (such as a restaurant or workplace canteen) or in the home. Composite dishes show the greatest variations in composition and therefore represent the least reliable data in a food database. Nonetheless, if a database is to be used in nutritional studies of individuals as members of groups, then data on these foods will be required. Recipe formulation and actual cooking method are the major sources of variation.

Calculated compositional data. The results of calculations will incorporate variations such as those listed above in the analytical data for the ingredients used, as well as variability in yield and retention factors.

The variations summarized above are a major constraint on the usage of food composition databases. A database is unlikely to predict within narrow limits the composition of a particular sample of food, because the limits will vary according to the food item and to the nutrient. Furthermore, the limits can be defined only if the value for each nutrient is accompanied by some measure of variation within that food. Beaton (1987) carried out simulation computations with United States food composition data (for which standard error data are published) using model diets. Variability appeared to produce a smaller bias in nutrient intakes computed for diets composed of many as opposed to few foods. This work also indicated the need to analyse or replicate analyses of foods that are major suppliers of dietary nutrients.

Ideally, all food composition databases should contain estimates of variability. Thus, the ideal composition database would have to be derived from sufficient numbers of analytical values to permit definition of the natural limits of variation and the distribution of the variance. Databases are in development that may meet these statistical requirements (ILSI, 2003). However, even such an ideal database would only predict the expected range of composition for any individual food.

Thus, natural variations in foods need to be understood by all users in all sectors, as they limit the predictive accuracy of nutrient intake calculations. Additionally, when using a compositional database for statutory purposes or to define standards against which to compare an individual food sample, this natural variation must be taken into consideration.

For some nutrients, a database is, at best, an approximate quantitative guide. Examples are vitamin C and folates, and sodium (and chloride) because of the wide use of salt as an additive. In many cases trace elements can be predicted only semi-quantitatively.

Limited coverage of food items

In industrialized countries the number of branded processed foods available is of the order of 10 000; furthermore, “new” products are being introduced continuously. The total number of foods consumed, if composite dishes are included, is probably of the order of 100 000. It is therefore unlikely that a database can be truly comprehensive for more than a short time. Clearly, priorities must be assessed when foods are selected for inclusion. Nevertheless, users require an increasing amount of brand name data in food composition databases because many manufactured foods are unique in their composition and/or have no generic equivalent (McDowell, 1993).

If the criteria discussed in Chapter 3 are applied to the selection, the database will include data for generic foods or major types of product. Thus biscuits (cookies) can be identified by brand name and type (sweet, semi-sweet, etc.), and a biscuit can be assigned to a type if the specific brand is not included. In most nutritional studies, the error produced by this approach is acceptable. For a computerized database application, software can probably be designed that will guide the user to the most appropriate alternative item. A cumulative record of items for which alternatives were sought would aid the assessment of priorities for items to be inserted in the database.

Coverage of nutrients

The assignment of priorities to specific nutrients for inclusion in a database is discussed in Chapter 4. Complete coverage of all nutrients requires high levels of laboratory instrumentation, and many nutrients remain problematical from the analytical viewpoint. Complete coverage of all the nutrients in well-documented samples is therefore uncommon. Furthermore, nutritional interests change with time; for example, in 1967–68 most dietitians in the United Kingdom did not require values for “unavailable carbohydrate” (dietary fibre), whereas by 1974 all were seeking such data avidly. Some interests in nutrients parallel analytical methodology; the advent of gas chromatographs permitted detailed characterization of fatty acid composition; automatic liquid chromatography heightened interest in amino acids, and high-pressure liquid chromatography in the analysis of free sugars. Improvements in inorganic analysis using atomic absorption spectroscopy have increased interest in trace elements.

If the first priority is given to proximates and major nutrients (as suggested in Chapter 4), new databases will lack certain data for some years. Even if a massive, comprehensive analytical programme is attempted, priorities must still be set according to the importance of a food in the provision of a nutrient. Assessment on the grounds of probable concentration alone is inadequate; low levels of a nutrient in a food that is regularly consumed are more important than high levels in a rarely consumed food such as a luxury item. Both frequency of consumption and nutrient concentration must be judged against the normal range of total intake of the nutrient in question. This assessment often shows that a certain food makes a virtually negligible contribution to total consumption of the nutrient in question, and consequently, analytical work on the food for that nutrient is difficult to justify.

Missing values can be a source of grave error, however. Stockley (1988) reviewed studies of errors associated with missing values in databases, citing underestimates of B vitamin intake ranging from 1.5 percent to 14.3 percent. Further, only 69 percent of total polyunsaturated acids analysed in duplicate diets were obtained, improving to 89 percent when missing values in the tables used were filled in. Cowin and Emmett (1999) compared nutrient intakes from a food intake study in the United Kingdom calculated from the fifth edition of the United Kingdom tables (Holland et al., 1991) with those calculated from the same database with missing values filled with “guesstimates”. They found that of the 1 027 foods recorded in the dietary survey, 540 had missing data for one or more nutrients. The nutrient intakes of over 90 percent of the subjects were altered by the use of the guesstimate-filled database. Underestimates using the uncorrected database ranged from 0.04 percent to 14.7 percent, the effect of missing data being proportionately greater at the lower end of the nutrient intake distribution. Further, in the European Prospective Investigation into Cancer and Nutrition (EPIC) project (Riboli et al., 2002), differences of up to 25 percent were found for dietary fibre intakes when missing values were treated as zero (Charrondiere, Vignat and Riboli, 2002). This kind of discrepancy will cause misranking of subjects within a nutrient intake distribution.

Clearly, then, zero must not be used for missing values in computations. If the database compilers have not supplied guesstimates, then a practical alternative would be for the user to assign estimated values to fill these gaps, or alternatively to substitute averages derived from known values for foods of the same type. Estimates prepared by careful interpretation of data on related foods are acceptable in nutritional studies, provided that their use is clearly noted. If computations of intake have to be made using zeros for missing values, the summation should be marked with a “not less than” sign and the programme must be written accordingly.

Slimani, Riboli and Greenfield (1995) have pointed out the need for tailored databases for nutritional epidemiology studies; examples of developing such a database include those of Hankin et al. (1995) for the Pacific Islands (using borrowed, calculated and commissioned analytical data), Salvini et al. (1996) for an Italian study, and Schakel (2001). A useful paper by Buzzard, Schakel and Ditter-Johnson (1995) describes procedures for quality control in database maintenance and use.

Inappropriate database or food composition values

An inappropriate database may be used as a result of lack of insight or lack of a purpose-designed database. The United States and United Kingdom food composition tables are probably the most common “default” databases used around the world, because of their ready availability in computerized form and their comprehensive coverage of foods and nutrients.

An opportunity to test the databases arose in Australia, where the first all-Australian database of original analytical data for Australian foods analysed in Australian laboratories was produced in the mid-1980s; prior to that time, United Kingdom or United States data had been used. In a comparison of the food supply data for 1990–91 in the new Australian tables (Department of Community Services and Health, 1989–91) with those in the United Kingdom and United States tables, it was found that the latter tables overestimated fat from meats by 60 percent and total fat by 15–22 percent. They also overestimated the iron, zinc, retinol activity, vitamin C and magnesium in the Australian food supply, while calcium was 35 percent higher using United Kingdom data and thiamin 59 percent higher using United States data (Cashel and Greenfield, 1995). The disparity arose because of differences in the gross composition for foods, as well as in the nutrient composition.

Another problem is the application of out-of-date data food composition databases. An interesting study by Hulshof et al. (1996) investigated the reasons for dietary change observed between the first Dutch National Food Consumption Survey (DNFCS), carried out in 1987–88 and the second one in 1992. The apparent decrease of 13 g in fat intake per person per day over the period was reduced to 11 g when artefactual changes in the food composition database were identified. About half of the reduction in fat intake was due to true changes in food choices and the other half to true changes in foods. All food composition databases tend to be “out-of-date” in view of the inevitable delays between the stages of collecting foods for analysis and entering validated data for nutrient composition into the database management system, and this study highlighted the need for the careful preparation and updating of a database prior to its use for national references for dietary studies. It also illustrated the usefulness of having a data audit trail – a system that records changes and reasons for changes in the data.

Errors arising in database use

Studies reported by Danford (1981) and Hoover (1983a) found considerable differences between results for a single day’s nutrient consumption when processed by several different food composition databases, even though all the databases were founded on the USDA handbook of food composition values. These problems have recurred in more recent studies, the situation being complicated by the proliferation of calculation software packages in the United States, each with different modifications of the nutrient database (Lee, Nieman and Rainwater, 1995; McCullough et al., 1999). Thus, software differences have to be added to the list originally identified by Hoover (1983a) as sources of error in database use: differences in conversion of household measures to standard weights, miscoding of food items and problems in identifying the food items exactly. Similar studies in France (Herbeth et al., 1991) identified differences in databases available in the country as the main source of error.

Hoover and Perloff (1983, 1984) have developed a series of procedures for testing the accuracy of use of a food composition database: procedures for updating the database, for calculating nutrients for a simple recipe, for reporting baseline data, for reporting nutrients for various portion sizes and for executing the computation of a dietary intake record. This quality control tool can be adapted for different kinds of nutrient database. It is also a useful model for a teaching tool.

Use of these standardized procedures revealed that inclusion of abundant descriptive detail of the foods reduced mismatching of foods with database food items (Hoover and Perloff, 1983). This indication that confusion of food nomenclature is a major source of error in database use highlights the need for improved methods for food nomenclature.

Errors arising in the use of composition data include the following:

a)failure to record sufficient details regarding the food (e.g. cooking or processing method);

b)failure to note whether the total food or edible portion only was weighed;

c)use of nutrient data for raw instead of cooked foods;

d)errors in calculating fatty acid intakes arising from the use of fatty acids per 100 g of total fatty acids instead of per 100 g food or the use of an incorrect conversion factor;

e)failure to adjust for water, vitamin and mineral losses when calculating nutrient intake from a recipe;

f)failure to note the identity of fats and oils used in recipe foods or foods cooked in fat;

g)failure to include provitamin A compounds when calculating vitamin A intakes;

h)failure to recognize difference in values as a result of nutrient definitions, e.g. available as opposed to total carbohydrate;

i)errors in matching nutritionally different foods when substituting for missing foods in the tables/database;

j)mistakes in conversions (volume to weight, portion description to weight).

Incompatibility of databases

Epidemiologists are often concerned with comparisons of diet among countries, or among populations. The incompatibility of databases often limits conclusions that can be drawn from such comparisons. Deharveng et al. (1999) compared the food composition tables of the nine European countries participating in EPIC in terms of availability, definition, analytical methods and mode of expression of the nutrients of interest for this epidemiological study. Although most nutrients in the tables had been analysed and expressed in a compatible way, some nutrients were not comparable (e.g. folate, dietary fibre, carbohydrates, carotenes). Other problems identified included out-of-date methods of analysis and the inclusion of data for foods collected over 20 years earlier. The authors concluded that purpose-built food composition tables were needed to analyse the large amount of dietary data being reported in EPIC.

Differences in software packages

Nowadays, the majority of users outside the major research centres that can afford to develop their own calculation programs will use the nutrient database integrated into the software package they purchase. This highlights the need to identify both the package and the database separately in publications. Software producers often incorporate additional foods or components into databases or may select certain nutrient data (e.g. niacin only, instead of niacin equivalents, when calculating dietary niacin status). This means that users must be trained to evaluate software packages prior to purchase, especially when purchasing a package for use by a large number of users (e.g. throughout a health care system, such as a group of hospitals, or for allied health use throughout an entire province or state).

The range of functions currently needed in dietary analysis tools is huge and is discussed in detail by Weiss (2001) and Stumbo (2001). They include: entering client records; facilities for updating the food composition databases; searching and displaying foods for nutrient composition by 100 g and by common serving size; ranking foods in terms of provision of nutrients; calculating the nutrient content of recipes, meals, diets, food intakes (from dietary records or food frequency questionnaires) and menus; multiplying or dividing food and nutrient intakes by factors such as days, meals or other variables of interest; comparing nutrient intakes with dietary recommendations; performing computations such as averaging, or dividing group intake data for foods and nutrients into deciles; printing or displaying results as tables, lists or graphs; storing calculated records or exporting them for further statistical analysis; calculating and printing product labels for nutrients, ingredients and comparisons with dietary references; costing products, meals and diets; printing labels for meals and clients; developing research, therapeutic or hospital diets, menus and food purchase lists according to different costs; adjusting menus to meet nutritional goals.

Limitations of methods for measuring food intake

The most accurate way to assess the nutrient intake of a person is to analyse an exact duplicate of the foods eaten over the survey period. This approach is seldom used because of obvious practical problems, in addition to the costs and the time involved in the analyses. Estimation of nutrient intakes by the application of food consumption data to food composition data is the method of choice. Indeed, computations of this sort probably constitute the major use of food composition databases at present.

All ways of estimating the amounts of foods consumed are associated with some degree of error. A full discussion of this topic is beyond the scope of this book, but readers are referred to several publications (Bingham, 1987, 1991; Gibson, 1990; Willett, 1998; Margetts and Nelson, 1997). A prominent problem with all dietary methods is the high prevalence of underreporting, estimated by Macdiarmid and Blundell (1998) to range up to 70 percent in certain groups.

Clearly, errors in the measurement of food intake add to errors arising from differences between the composition of the food consumed and the values recorded in the database. At the same time, the accuracy of nutrient intakes calculated from food composition data cannot be improved by attention to the database alone. The quality of the results depends on the quality of the database, the accuracy with which foods can be identified, the quality of the food consumption data, and the accuracy with which the food composition database and the programs (or calculations) are used (Figure 11.1).

Evaluation of a database, tables or software

One task that invariably falls to the professional nutritionist, particularly the nutritionist involved in a research project, is the choice of a database. Because of the many commercial diet analysis programs that are now available for the calculation of nutrient intakes, nutritionists require training in the evaluation and selection of databases; indeed, such training should form part of any professional or degree course in nutrition. In general, the options available for the nutritionist are (adapted from the suggestions of Perloff [1983]):

1.to computerize a set of tables or to make up a computerized database from several sets of tables that are available (in this case, criteria for the selection of values must be provided. Programs for calculating nutrient intakes will have to be written);

2.to link up to an existing computerized database via a modem;

3.to purchase a computerized database on disk, CD or online and prepare computer programs to calculate nutrient intakes from the base plus consumption data;

4.to purchase a database plus programs;

5.to contract to provide consumption data to a database user who will compute nutrient intake data for a fee.

In considering these options, the primary concerns of the user should be to choose a database that is appropriate, that contains reliable data for foods closely matching those consumed, and that has accurate programs.

The suitability of the database can be determined by putting it through standardized tasks based on the functions discussed above (Hoover and Perloff, 1983, 1984). Other considerations will include the cost, speed, ease and convenience of use, the degree of training required of the operator, and hardware requirements.

Chapter 12

Current needs and future directions

Since the first edition of this book was published (Greenfield and Southgate, 1992) there have been many dramatic changes in the world that are of great significance to the areas of food production, management and use. These are outlined below.

First, it is especially noteworthy that the final report of the International Conference on Nutrition (ICN) produced a World Declaration and Action Plan for Nutrition (FAO/WHO, 1992) that contained references throughout to the need for data on the nutrient composition of foods, especially in the sections under Section IV, “Strategies and actions”. Specifically under Section IV, 9, j, we read “Support and encourage … the development and use of local food composition information”. The ICN was followed up by means of countries developing their Plans of Action for Nutrition and, later, Implementation Reports on these plans. New Zealand’s National Plan of Action for Nutrition (MOH, 1996) emphasized, “Food composition provides essential information for effective monitoring of food and nutrition. In order to remain current food composition needs to continue to be updated and expanded to include new local and appropriate international food composition values.”

Second, the activities of INFOODS have now moved centre stage with the centralization of this important function within FAO. FAO had earlier reduced its involvement in food composition work after the publication of the food composition tables for the Near East (FAO, 1982) but, in 1994, FAO renewed its commitment to improving the quality and availability of food composition data in developing countries. As part of this new effort, FAO joined UNU in the coordination of INFOODS. With the amalgamation of UNU and FAO interests, INFOODS began to operate out of the FAO Rome headquarters in 1998 (see Chapter 1).

Third, in 1993 the First International Food Data Base Conference was held in Sydney, Australia (as an official satellite to the International Union of Nutritional Sciences [IUNS] International Congress on Nutrition) and proceedings were published (Greenfield, 1995). The series of international food data conferences has continued, each with published proceedings: Finland in 1995 (Finglas, 1996), Rome in 1999 (Burlingame, 2000), Slovakia in 2001 (Burlingame, 2002), and the United States in 2003 (Pennington and Stumbo, 2004). In 1997, IUNS established a Task Force for the International Food Data Conference, to oversee the arrangements by selecting convenors and venues and assisting with publicity and provision of resources (IUNS, 2003). These conferences and their published proceedings have done much to promote food composition research internationally. INFOODS (2003) hosts the Web site for the International Food Data Conference.

Fourth, access to personal computers became almost universal, with the wide accessibility of the Internet in the early 1990s creating infinite possibilities in terms of making information about food composition available worldwide. The first edition of this book (which was in preparation from 1983 to 1992) was mostly prepared by sending messages through telex and posting drafts back and forth through airmail, while the current edition has almost all been prepared by means of e-mail correspondence and attachments. Many food composition data or databases can now be accessed or downloaded from the Internet. Some are free, e.g. Nutrition Society of Malaysia (2003), LATINFOODS (2003) and USDA (2003a). Others are for purchase online, for example the German database (Souci-Fachmann-Kraut, 2003). In Australia and New Zealand, a free simplified food composition database of local foods that enables calculation for nutrition-labelling purposes is available via the Internet (FSANZ, 2003; Crop & Food Research, 2003), and many food composition programmes have their own Web sites, for example the Danish Food Composition Databank (Danish Veterinary and Food Administration, 2003). User software can also be purchased via the Web, with direct downloading of the software a possibility. Databases and software can even be downloaded on to palmtop devices.

While there is always a danger that the availability of food composition data over the Internet could lead to the downloading of inappropriate data, or posting of poor quality data (or data with no source identified), the Internet nevertheless represents an unlimited potential force for good in the area of food composition.

Burlingame et al. (1995b) were the first to document the tremendous potential of food images as tools to support the development of food composition data. It would be particularly useful to see more data linked to images of both foods and labels on the Web. An exemplary site is the United States Food and Drug Administration’s online Regulatory fish encyclopedia (FDA, 2003), which shows photographs of aquatic foods in their raw state and as prepared raw for retailing, together with taxonomic information and images of isoelectric focusing gel strips for unique identification. Although this site is not linked to compositional data on fish it does demonstrate the exciting possibilities available for foods in general.

The development of online training courses in food composition data production, management and use would appear to be an essential next step.

Fifth, increasing interest in nutritional epidemiology continues to be a driving force in the demand for more and improved food composition data. Major prospective epidemiological studies are beginning to yield results that demonstrate the importance of this approach to analysing the relationships between food and health. Epidemiological studies need to produce tailored databases (Slimani, Riboli and Greenfield, 1995). Multicentre multinational studies need specific food composition databases that will produce comparable results (i.e. not attributable to artificial differences among the different national databases) (Deharveng et al., 1999; Charrondiere et al., 2002)

Sixth, international standards development and the increasing tendency to harmonize food regulations worldwide have been major forces in developing improved methods of analysis and quality assurance programmes to ensure that data from all parts of the world are more reliable and compatible. The joint FAO/WHO Codex Alimentarius has become the global reference point for all countries in formulating and harmonizing food standards and ensuring their global implementation (FAO/WHO, 1999). Other events, for example, the merging of food markets in Europe, have created the need to harmonize food laws and compliance with them (Goenaga, 1994). Buss et al. (1998) identified the food composition priorities and resources in relation to the European Union, while a group in Europe has been very active in comparing analytical methods and developing certified reference materials (Finglas, 1996; Vahteristo et al., 1996; van den Berg et al., 1996), as has another in Asia (Puwastien, 2000). These developments have achieved much to improve laboratory performance and data quality.

The principal objective of the INFOODS initiative (under whose aegis this book has been produced) is the development of an international network of food data systems, dependent on the development and potential integration of compatible local, national and regional collections of food composition data. A list of INFOODS Regional Data Centres is provided in Appendix 1.

Compatibility does not require the adoption of the same format or the development of one database system that meets all present and future needs; it merely means that the data can be used together (Southgate, 1985) and interchanged and interpreted without ambiguity or loss of information (Klensin, 1992). Although some essential features such as the modes of expression and nomenclature of foods and nutrients must be the same, one of the most important requirements for compatibility is that the data be of high quality – a user must have confidence that the data are fit for the task at hand.

The thesis developed in the first edition of this book was that the production of sound compositional data depended on an integrated series of activities involving the data users, the analysts who generate the data and the database compilers. Sound data quality must be built into the programme from its inception. As described throughout this book, there have been some considerable movements to advance this aim.

Further needs for study

The preparation of the revised edition of these guidelines revealed a number of topics whose further study would advance the development of compositional databases. They are discussed below, following the order of their emergence in these guidelines.

Food composition data as a basis for quantitative nutrition studies

It is essential to recognize that a sound compositional database that is both comprehensive and representative of available foods is an essential basic tool for virtually all quantitative nutrition research, dietary evaluation and development of food and nutrition policies.

The validity of nutritional epidemiological studies depends on accurate food consumption and food composition data. Failure to understand the relationships between diet and health or disease are often due to inadequacies in food composition or food consumption data. Thus, a food composition database programme should be an integral part of any national nutrition research programme as it is, for example, in the USDA’s Human Nutrition Program (USDA, 2003d), which states:

The mission of the Human Nutrition Program is to conduct basic and applied research to identify and understand how nutrients and other bioactive food components affect health. The ultimate goal of this food-based agricultural research is to identify foods and diets, coupled with genetics and physical activity, that sustain and promote health throughout the life cycle. The research components of this program include: nutrition requirements; diet, genetics, lifestyle, and the prevention of obesity and disease; nutrition monitoring; composition of foods; health promoting intervention strategies for targeted populations; health promoting properties of plant and animal foods; bioavailability of nutrients and food components (e.g. phytonutrients and phytochemicals).

International harmonization of food composition programmes

Programmes for the collection of food composition data used to vary widely among countries, the differences often reflecting historical differences in how nutrition developed within individual communities. The international need for this large body of information demanded a degree of harmonization and the development of compatible standards of data quality, which in turn required that some common principles for the organization of nutrient composition studies of foods be developed.

In the course of the reading and consultations for the revision of these guidelines, it became clear that the most important organizational principle was still the integration of the efforts of the users (real and potential), of those involved in sampling and analysis, and of the compilers. The involvement of these three major elements in all stages of the programme is probably the most effective way to achieve high data quality. Data quality can be “grafted on” by the compilers at a later stage, but this approach invariably results in the rejection of work that would have met the desired standards had they been introduced earlier. Quality assurance programmes within the analytical laboratory are essential, but they need to be incorporated into the programme as a whole. This statement is as true today as it was when the first edition was written.

Foods that require investigation

The coverage of foods in all existing databases is very limited, compared with the numbers of foods consumed. This situation is likely to persist for the foreseeable future because the resources required to prepare truly comprehensive databases are considerable. It is therefore vital that priorities are properly assessed when future analytical studies are planned and that reanalyses are undertaken only when good evidence indicates nutritionally significant changes in composition or when new information on nutrients is needed.

There are three broad groups of foods for which information is conspicuously limited, and for which analytical work would be worthwhile.

Uncultivated foods. These foods are prominent in many communities and can assume great importance in times of food shortage following the failure of cultivated crops. Systematic compositional studies of uncultivated foods now assist nutrition studies of populations consuming them (e.g. Brand-Miller et al., 1993; Kuhnlein et al., 1979; Kuhnlein et al., 2002). Such studies could also provide information on species that may be suitable for further development (e.g. Dawson, 1998).

Individual cultivars. Many studies have demonstrated that different cultivars of the same species can have very different nutrient contents (Huang, Tanudjaja and Lum, 1999). With the advances in food biotechnology, the documentation of the composition of the existing food biodiversity, cultivar by cultivar, should be a priority (Kennedy and Burlingame, 2003) and a prerequisite before embarking on the development of genetically modified cultivars, as was recently recommended by the International Rice Commission (FAO, 2002; Kennedy, Burlingame and Nguyen, 2003).

Cooked foods and composite dishes. Foods are most often consumed in this form. In most databases direct analytical information is limited, forcing reliance on calculations from recipes. While this approach has its uses, there is a need to supplement and, ideally, replace calculated values with analytical data. Such studies will require careful attention to the design of sampling protocols.

Nutrients that require investigation

The generation of analytical values to fill the gaps found in most nutrient databases depends in part on the availability of suitable methods, which will be discussed later. Nutritional priorities determine which nutrients should be studied. Data for carbohydrates and dietary fibre in foods are now available worldwide, although gaps still remain for many foods in most countries. Methods for fatty acid analysis are now well established, and many new fatty acid data and data compilations have been produced (Quigley et al., 1995; Exler, Lemar and Smith, 2003; Mann et al., 2003). Data are still badly needed on the folate values of foods, especially given the recognition of the importance of folate in the neurological development of the foetus and the introduction of mandatory or voluntary fortification of foods with folic acid. More data on carotenoids (both provitamin A carotenoids and others that are not precursors of vitamin A) have been collated into databases (Chug-Ahuja et al., 1993), as have data on phytoestrogens, although such data are mainly from only a few sources. Other bioactive components of great interest and that need research have been summarized by Pennington (2002).

Research on bone mass and osteoporosis have revealed that data are badly needed on vitamin D in foods. Interest in this vitamin has resurged in recent years and some compilations of data are now recognized as being out of date, with new data only being produced slowly (J.M. Holden, US Nutrient Data Laboratory, personal communication, 2002). More data are also needed for vitamin K in foods given the increasing awareness of this nutrient’s significance in bone health (Buttriss, Bundy and Hughes, 2000; Bolton-Smith et al., 2000; Shearer and Bolton-Smith, 2000).

Research studies on sampling

An experimental basis is required for the design of sampling protocols. Despite the importance of variability within foods, formal studies on the factors involved in the variability of food components and the magnitude of their effects have been restricted to a few major commodities and have rarely been performed for nutritional reasons. Such studies could be incorporated, with advantage, into studies of the factors affecting the nutrient composition of many important foodstuffs.

The effects of sample handling are frequently studied during the course of a food composition study, and it would be valuable if these investigations could be conducted in a more formal way, making the information suitable for publication. Such information would be useful to all engaged in similar work.

Food nomenclature

The detailed studies of food nomenclature undertaken by McCann et al. (1988) and Truswell et al. (1991) and the formal studies of food classification undertaken for the Eurocode system (Arab, 1985; Arab, Wittler and Schettler, 1987) were central to controlling a major source of error in the use of nutrient data, that is, the identification of food items. This work developed further with LanguaL (Pennington et al., 1995; Møller and Ireland, 2000b). These systems can develop a degree of “elegant complexity” that makes them difficult to use accurately and consistently. It is therefore important to devise some formal procedures for evaluating nomenclature systems as they evolve. Some authors speculate that a single, internationally acceptable system of food nomenclature may not be an achievable goal (Burlingame, 1998). Nevertheless, this important work continues through an INFOODS-convened international technical committee, with the task of overviewing and focusing the work done on food classification and description in order to harmonize as far as is possible (INFOODS, 2003).

Needs for improved analytical methods

Since the first edition of this book was published in 1992 there has been an explosion of analytical methods development, particularly stimulated by the now worldwide acceptance of compositional standards for foods, and of the requirements of nutrition labelling in many countries (Government of Canada, 2002; EC, 1990; United States Code of Federal Regulations, 2003; FAO/WHO, 2001). This explosion makes it more difficult for a single analyst to be expert in methods across the board, and has created an even more urgent need for analysts, compilers and users of nutrient databases to share their knowledge and information.

The validation of methods for vitamin analyses is urgently needed, especially for the carotenoids (both the vitamin A-active and the non-vitamin A-active), folates and vitamin D. In all cases procedures that permit the separation and measurement of the different forms are required. This information, together with estimates of the biological activity of different vitamers, would provide better estimates of the vitamin activity of foods than are currently available. All methods for vitamin analysis are time-consuming and therefore costly; efforts to devise more rapid specific procedures should be given high priority.

For some inorganic nutrients speciation is an important determinant of bioavailability, and its measurement could be useful (e.g. haem and non-haem iron).

The methodology for determining dietary fibre is developing rapidly; indeed, significant progress has been made during the preparation of these guidelines. The stage has not been reached, however, where the methods can be applied routinely to a wide variety of matrices; this is a legitimate goal for research.

For many methods there is a need to extend the range of food matrices covered, not necessarily because the methods are inappropriate, but simply because their wider applicability has not been assessed. A food composition study frequently covers a wide range of foods, and for that reason it would be helpful if the applicability of some methods could be extended to a greater variety of matrices. Well-tested methods with broad applicability are needed. In the long run it is hoped that instrumental methods can be further developed that do not destroy or invade the food – methods such as NMR, NIR and so forth offer the main potential in this regard.

Nutritional analysis is a specialized branch of food analysis, and since the first edition of this book was published many new, comprehensive and extremely useful analytical textbooks and manuals have been produced (see Appendix 7).

Data quality assurance

The importance of a quality assurance programme in the analytical laboratory was explained in Chapter 8. Such programmes have benefited from more collaborative studies and from improved availability of standards and standard reference materials (SRMs) as outlined above, but more work is still required.

The range of SRMs discussed in Chapter 8 still requires expansion, particularly for the more labile nutrients, and for “new” components of interest such as phytochemicals.

Database management systems

Typewritten and spreadsheet-prepared food composition tables, with their two-dimensional formats allowing little or no documentation for each value, are being superseded. Relational database management systems provide facilities for the compilation of thoroughly documented food composition information, including analytical values down to the finest level of disaggregation. These systems can provide flexible user interfaces for selecting, viewing and editing the data and documentary information in formats that are convenient and meaningful for the users. The information is stored in data structures that are designed to minimize redundancy and to support extensions to documentary information as and when these are defined in data management guidelines. Similarly, facilities for calculating and manipulating component values will be extended according to user requirements, preferably defined as internationally accepted guidelines (Unwin and Becker, 2002). A large-scale acceptance of international standards for interchanging food composition data will also facilitate rapid and simple interchange of food composition data (Klensin, 1992).

Research needs for the compilation process

The greatest need is for more food composition data to be published in the scientific literature and for the standard of published data to improve. This could be achieved by requiring more documentation of the food samples analysed and, specifically, heightened scrutiny of analytical methods in the refereeing process. Details of quality assurance steps taken should be provided also. At present the methods sections of many papers rarely meet even the basic criterion of providing sufficient detail for a competent worker to repeat the work described. It is important that this minimum standard be preserved and, preferably, improved.

Formal procedures for the scrutiny of analytical data, from both published and unpublished sources, require further development. Such research should produce more objective indices of data quality that estimate the probability that the data are sound. Currently, errors can arise if intuitive data quality indices are manipulated as if they were real numbers. Formal analysis of the value judgments applied in the compilation process should lead to more objective and consistent assessments of data quality. Some steps towards these goals have been made, for example, the development of multinutrient data evaluation systems (Holden, Bhagwat and Patterson, 2002)

The application to the compilation process of the requirements for good scientific practice will provide a basis for quality. These include: independent replication of data, maintaining professional standards, documenting data, best practice in data management, questioning one’s own data and fully documenting and preserving all data sources (Office of Science and Technology, 1998; Office of Research Integrity, 1998).

Use of food composition data

A database can be interrogated in a number of ways; at a simple level the composition of a single food item can be selected for information or scrutiny, but in the majority of cases data are required for combinations of food items. The accuracy with which a database predicts the composition of such combinations of foods is currently an area requiring research. All databases have limits of predictive accuracy determined by the variations in food composition. Future research needs to define and go beyond these limits. In addition, large-scale epidemiological studies (Riboli, 1991) have particular needs in the use of food composition databases. For example, the need to analyse dietary intake data on the basis of individual ingredients rather than composite foods may require specialized applications.

At present, intuitive assessments suggest that the principal requirements are for better data on variations in the nutrient composition of major foodstuffs, for elimination of missing values and for inclusion of more food items in the database. Formal studies are required to estimate the importance of these three elements, however, before substantial resources are committed to their resolution.

In countries where nutritional labelling of foods is common, reliable data from the food industry could be a major factor in improving database accuracy in use.

Training and education

Perhaps most importantly, the objectives of international harmonization of food composition data and data management can only be achieved by training and education. Education and training programmes will develop a network of workers with common goals and standards who will contribute to the development of common approaches to the organization of food composition programmes, to food nomenclature and nutrient analysis and expression, as well as to food sampling and data quality assurance programmes. Data will become more compatible as their quality improves.

Courses in nutrient analysis of foods are now becoming more common in the training of analytical chemists, food scientists, nutritionists and dietitians, including at undergraduate level. Further, the movement of INFOODS into the basic work programme of FAO has led to the development of international short courses in food analysis and, in collaboration with Wageningen University, in food composition data, production, management and use.

The next long-awaited development is to see nutrient analysis of foods adopted as an essential component of the core training of food professionals such as dietitians and nutritionists because they are often the compilers of databases, as well as major users. Online training would be a very desirable development for the future, made possible by the development of the Internet, the widespread availability of computers and the fact that computer literacy is now an integral part of school education.

Conclusion

Finally, there is still a need for a fundamental change in attitudes towards the place of food composition work within the nutritional sciences themselves. Quantitative data on the composition of foods form the basis for virtually all quantitative human nutrition research and for the development of food and nutrition policies at the national and international levels. Food composition databases represent the primary scientific resource from which all other studies flow. It is vital for the development of the nutritional sciences that this key resource be maintained and developed as part of the activity of nutrition research as a whole.

Appendix 1

INFOODS regional data centres

INFOODS

(International)

Coordinator: Barbara Burlingame

Food and Agriculture Organization of the United Nations

Viale delle Terme di Caracalla

00100 Rome

Italy

Tel.: (+39) 06 57053728

Fax: (+39) 06 57054593

E-mail: Barbara.Burlingame@fao.org

AFROFOODS

Coordinator: Professor Hettie Schonfeldt

Sensory and Nutritional Sciences

Animal Nutrition and Animal Products Institute

Private Bag X2

Irene 0062

South Africa

Tel.: (+27) 12 6729351

Fax: (+27)12 6651551

E-mail: hschon@idpi1.agric.za

Subregions:

SOAFOODS

(Botswana, Djibouti, Lesotho, Malawi, Mauritius, Namibia, South Africa, Swaziland, Zambia, Zimbabwe)

Coordinator: Ms Pauline Zharare

Institute of Food, Nutrition and Family Science

University of Zimbabwe

PO Box MP 167 Mount Pleasant

Harare

Zimbabwe

Tel.: (+263) 4303211 ext. 1413

Fax: (+263) 4336491

E-mail: zhararep@compcentre.uz.ac.zw

ECAFOODS

(Eritrea, Ethiopia, Kenya, Madagascar, Somalia, Sudan, Uganda, United Republic of Tanzania)

Coordinator: Dr Wilbald Lorri

Tanzania Food and Nutrition Centre (TFNC)

PO Box 977

Dar-es-Salaam

United Republic of Tanzania

Tel.: (+255) 22 2118138

Fax: (+255) 22 2116713

E-mail: wlorri@muchsac.tz

WAFOODS

(Benin, Burkina Faso, Côte d’Ivoire, Gambia, Ghana, Liberia, Mali, Niger, Nigeria, Senegal, Sierra Leone, Togo)

Coordinator: Dr Esther Sakyi-Dawson

Department of Nutrition and Food Science

University of Ghana

PO Box 134

Legon, Accra

Ghana

Tel.: (+233) 21 500389/24 367242

Fax: (+233) 21 500389

E-mail: esakyid@xmail.com

CAFOODS

(Burundi, Cameroon, Central African Republic, Chad, Congo, Democratic Republic of the Congo, Gabon, Rwanda, Seychelles)

Coordinator: Dr Mbome Lape

Food and Nutrition Research Centre

Ministry of Scientific and Technical Research

Yaounde

Cameroon

LUSOFOODS

(Angola, Mozambique, etc., not yet fully established)

NAFOODS

(Algeria, Libyan Arab Jamahiriya, Mauritania, Morocco, Tunisia)

Coordinator: Dr Gharbi Tahar

National Institute of Nutrition

11 Rue Djebel Lakhdar (Bab Saadoun)

Tunis

Tel.: (+216) 1 570684

Fax: (+216) 1 570795

E-mail: esakyid@xmail.com

ASEANFOODS

(Brunei Darussalam, Cambodia, Indonesia, Lao People’s Democratic Republic, Malaysia, Myanmar, Philippines, Singapore, Thailand, Viet Nam)

Coordinator: Dr Prapasri Puwastien

Institute of Nutrition

Mahidol University of Salaya

Nakhon Pathom 73170

Thailand

Tel.: (+66) 2 8002380 ext 410/4410217

Fax: (+66) 2 4419344

E-mail: nuppw@mahidol.ac.th

CARICOMFOODS

(Anguilla, Antigua and Barbuda, Bahamas, Barbados, Belize, Bermuda, British Virgin Islands, Cayman Islands, Dominica, Grenada, Guyana, Jamaica, Montserrat, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, Suriname, Trinidad and Tobago, Turks and Caicos Islands)

Coordinators: Dr Fitzroy Henry and Dr Pauline Samuda

Caribbean Food and Nutrition Institute

University of the West Indies

PO Box 140, Kingston 7

Jamaica

Tel.: (+001) 809 9271540

Fax: (+001) 809 9272657

CARKFOODS

(Afghanistan, Azerbaijan, Kazakhstan, Kyrgyzstan, Tajikistan, Turkmenistan, Uzbekistan)

Coordinator: Dr Musa Aidjanov

Institute of Nutrition

Klochkova 66

480008 Almaty

Kazakhstan

Tel.: (+7) 327 2429203

Fax: (+7) 327 2420720

EUROFOODS

(Austria, Belgium, Croatia, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Israel, Italy, Luxembourg, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey, United Kingdom)

Coordinator: Professor Clive West

Department of Human Nutrition

Wageningen Agricultural University

PO Box 8129

6700 EV Wageningen

The Netherlands

Tel.: (+31) 317 484317

Fax: (+31) 317 483342

E-mail: Clive.West@wur.nl

Subregion:

CEECFOODS

(Bulgaria, Croatia, Czech Republic, Hungary, Poland, Lithuania, Romania, Slovak Republic, Slovenia)

Coordinator: Dr Fanny Ribarova

National Center of Hygiene, Medical Ecology and Nutrition

Department of Food Chemistry

15 Dimitar Nestorov Street

1431 Sofia

Bulgaria

Tel.: (+359) 2 5812238/265

Fax: (+359) 2 958 1277

E-mail: f.ribarova@nchmen.government.bg

LATINFOODS

President: Dr Elizabete Wenzel de Menezes

Departamento de Alimentos e Nutrição Experimental

Faculdade de Ciências Farmacêuticas

Universidade de São Paulo, USP

Av. Prof. Lineu Prestes 580, Bloco 14 de Conjunto das Quimicas

CEP 05508-900

São Paulo

Tel.: (+55) 11 30913647

Fax: (+55) 11 38154410

E-mail: wenzelde@usp.br

Subregions:

CAPFOODS

(Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua, Panama)

Coordinator: Ana Victoria Román

Unidad de Tecnología de Alimentos y Agroindustria

Instituto de Nutrición de Centroamérica y Panamá (INCAP)

Calzada Roosevelt, Zona 11

Apartado Postal 1188

Ciudad de Guatemala

Guatemala

Tel.: (+502) 472 3762

Fax: (+502) 473 6529

E-mail: aroman@incap.ops-oms.gt

MEXCARIBEFOODS

(Cuba, Mexico, Dominican Republic)

Coordinator: Miriam Muñoz de Chávez

Centro de Investigación en Ingeniería y Ciencias Aplicadas (CIICAP)

Universidad del Estado de Morelos (UAEM)

Torre de Investigación 6to piso

Avenida Universidad 2001

Cuernavaca, Morelos

Mexico

Tel.: (+52) 73 297084

Fax: (+52) 73 297084

E-mail: mmchavez@prodigy.net.mx

SAMFOODS

(Argentina, Bolivia, Brazil, Chile, Colombia, Ecuador, Paraguay, Peru, Uruguay, Venezuela)

Coordinator: Saturnino de Pablo

Instituto de Nutrición y Tecnología de los Alimentos, INTA

Universidad de Chile

Macul 5540

Santiago

Chile

Tel.: (+56) 2 6781431

Fax: (+56) 2 2214030

E-mail: sdepablo@uec.inta.uchile.cl

MEFOODS and GULFOODS

(Cyprus, Egypt, Jordan, Lebanon, Palestine, Syrian Arab Republic and the Arab Gulf countries)

Coordinator: Dr Abdulrahman O. Musaiger

Bahrain Centre for Studies and Research

PO Box 496

Manama

Bahrain

Tel.: (+973) 754757

Fax: (+973) 754678

NEASIAFOODS (formerly MASIAFOODS)

(China [mainland], Japan, Mongolia, Republic of Korea, Taiwan Province of China, Hong Kong Special Administrative Region, Macao Special Administrative Region)

Coordinator: Professor Yang Yuexin

Department of Nutrition,

Institute of Nutrition and Food Safety

Chinese Center of Disease Prevention and Control

29 Nan Wei Road

100050 Beijing

China

Tel.: (+86) 10 63131246

Fax: (+86) 10 63131246

E-mail: MASIAFOOD@yahoo.com.cn or Yxyang@public3.bta.net.cn

NORAMFOODS

(Canada, Mexico, United States of America)

Coordinator: Joanne Holden

Research Leader, Nutrient Data Lab

USDA, ARS, BHNRC

4700 River Road, Unit 89

Riverdale, MD 20737

United States

Tel.: (+1) 301 7348498

Fax: (+1) 301 7348491

E-mail: hni01jh@rbhnrc.usda.gov

OCEANIAFOODS (24 countries and territories)

(American Samoa, Australia, Cook Islands, Federated States of Micronesia, Fiji, French Polynesia, Guam, Kiribati, Marshall Islands, Nauru, New Zealand, Niue, Northern Mariana Islands, Palau, Papua New Guinea, Pitcairn Islands, Samoa, Secretariat of the Pacific Community, Solomon Islands, Tokelau, Tonga, Tuvalu, Vanuatu, Wallis and Futuna Islands)

Coordinator: Dr Nelofar Athar

Crop & Food Research

Private Bag 11600

Palmerston North

New Zealand

Tel.: (+64) 06 3517066

Fax: (+64) 06 3517050

E-mail: atharn@crop.cri.nz

SAARCFOODS

(Bangladesh, Bhutan, India, Maldives, Nepal, Pakistan, Sri Lanka)

Coordinator: Professor Jehangir Khan Khalil

NWFP Agricultural University

Peshawar

Pakistan

Tel.: (+92) 91 921 6855 or (+92) 91 9216847

Fax: (+92) 91 921 6520

E-mail: jkhalil@brain.net.pk or jkhalil@psh.paknet.com.pk or khaliljk@hotmail.com

Appendix 2

Calculation of sample size

Chapter 5 introduced the issue of calculating the sample size needed to estimate the population mean with a reasonable level of confidence.

The optimum sample size is based formally on calculation from the following equation (Proctor and Meullenet, 1998):

where

x = sample mean

μ= population mean

SD = standard deviation of the sample mean

n = sample size

The equation can be rearranged as follows:

Application of this equation requires knowledge of some parameters that will be only available if the analyst has some preliminary information about the food. This ideally should come from pilot analytical studies to determine the mean and standard deviation, from data in the literature or, if such data are not available, from intuitive guesses.

The values for α define the confidence limits required. If a 95 per cent confidence interval is required, α equals 5 percent, i.e. 0.05. The degree of freedom (df) is defined as n – 1. Thus, for a sample size of 10, df = 10 – 1 = 9.

The value for t is taken from standard statistical tables (Student’s t table), using the required value of α and a guesstimate of sample size.

Accuracy is the required closeness of the estimated value to the true value (unknown). A sample mean within 10 percent of the population mean would represent an accuracy of 0.1. In other words, the required confidence interval is x ± 0.1x.

Examples of value for t:

For a sample size of 10, α = 0.05, df = 9, t = 2.262. Thus t^2 = 5.1166.

For a sample size of 20, α = 0.05, df = 19, t = 2.093. Thus t^2 = 4.3806.

Examples of sample sizes calculated from literature values:

The examples below use the data reported by Greenfield, Makinson and Wills (1984) for moisture, fat and cholesterol in 24 samples of retail French fries. These data illustrate the fact that different nutrients, because they show different variances, need different sample sizes to achieve the same level of confidence.

Table A2.1 summarizes the relevant data and calculations.

This shows that for accuracy of 0.1, ten samples (a commonly used sample size) would be inadequate to achieve a mean with the required confidence in any of the three cases. A sample size of around 13 would be adequate for moisture and 37 for fat; a sample size of 76 would be required for cholesterol, which showed the greatest variability. This can be explained by the fact that some of the French fries had been fried in vegetable oils with virtually no cholesterol.

If the calculation is carried out to produce confidence limits for accuracy of 0.05 then a sample size of 50 would be needed for water, 146 for fat, and over 300 for cholesterol.

The examples show that the sample size will be larger for nutrients that show greater variability than for less variable nutrients. In practice most designers of sampling protocols have to make intuitive judgements in calculating the sample size to be collected.

Appendix 3

Methods of preparation of foods for analysis

Documentation of sample preparation is as important as other aspects of the analytical protocols. Care should be taken to separate edible portions and inedible portions (refuse, waste) carefully, and record descriptions and weights of all parts. Sample preparation is also the appropriate time to record common measures or portion sizes with the description (e.g. slice), linear dimensions and weight. Finally, if a volume measure is possible (e.g. all liquids, powders, granular substances), the food’s density should be measured and recorded.

Homogeneous foods

- Solids:

-Friable: crumble and mix.

-Sticky: freeze and crush at low temperature.

-Hygroscopic: take portions rapidly into pre-weighed sealable containers for weighing.

- Emulsions. Take by weight rather than volume; warm and mix.

- Liquids with suspended solids. Homogenize, or sample during gentle mixing.

Reduction by quartering

Large items, if symmetrical, can be reduced in size by this technique. The principle is that the quarter should be representative of the whole. Any symmetrical food should be cut into quarters, and one-quarter of each batch taken for processing for analysis. Oval or elongated foods (e.g. potato or cucumber) should be cut into eighths, and two-eighths taken for a quarter, because each end may represent different parts of the plant (e.g. stalk and flower).

- Large item foods. Foods consisting of fairly large, separate, but similar portions, such as loaves of bread or joints of a meat, should be quartered and sampled then processed for analysis.

- Food lots of small items (flour, rice, legumes, small fruits, chopped mixed units). These foods are quartered as follows: the bulk is tipped into a uniform pile on a clean, inert surface and turned over several times with a polythene or glass spatula. The pile is levelled, and then divided into four equal segments. Two opposing segments are taken and the other two discarded. The remaining segments are mixed and further reduced in the same way.

- Segmented foods (the purchased item consists of a number of individual units). When sampling packets of biscuits, cartons of eggs, batches of bread rolls, etc., it is usual to take every fourth item to form a composite sample. For sliced loaves, it is adequate to take every fourth slice and one end slice, which then must be thoroughly crumbed before further reduction. The principle is to keep the crust–crumb ratio the same as in the original loaf (see below).

Preparation of analytical samples for particular food types

- Cereals:

-Flours and grains. The units are mixed thoroughly on a clean, dry, inert surface with a polythene or glass spatula. The combined mass may be quartered (see above). Large analytical samples for inorganic analysis (ashing or wet digestion) should be taken at this point. Large grains (e.g. maize) may then need to be reduced in a hammer or ball mill. No reductions of fine flours should be necessary.

-Unsliced breads. Individual loaves are quartered, one quarter of each being taken, weighed, sliced, dried at ambient temperatures and weighed again. The air-dried quarters are ground with a pestle and mortar, then mixed well in a bowl with a spatula.

-Cakes, pastries, pies, cooked cereals, cereal-based puddings. Large items must be quartered. Quarters or small items should be chopped and mixed thoroughly in a bowl with a spatula. A large analytical portion should be taken for inorganic analysis and the remainder should be homogenized mechanically. If vitamin C is to be analysed (in fruit pies, for example) an unhomogenized analytical portion should be taken into metaphosphoric acid within a few seconds, but the remaining mixture can be thoroughly homogenized. Items resistant to homogenization can be frozen and crushed inside a polythene bag with a mallet (Osborne and Voogt, 1978).

-Biscuits. Every fourth item should be taken from the packet or batch, crushed with a pestle and mortar, and mixed together; then a large analytical portion should be taken for inorganic analysis. A grinder may be necessary for further reduction if nuts and/or dried fruit are present.

-Breakfast cereals. These can usually be quartered then crushed with a pestle and mortar; analytical portions can then be taken for inorganic analysis. High-fat, high-sugar cereals may require freezing and crushing in a polythene bag.

- Meats and fish (raw, cooked and processed). The fat and muscle of some meats are more conveniently analysed separately and the results combined to produce the final values. The edible portion of each unit is chopped coarsely with a sharp knife (fish is flaked with a fork) and mixed thoroughly in a bowl with a spatula. A portion is removed, frozen and crushed in a polythene bag, and used for inorganic analyses. The remainder of the analytical sample is minced and mixed thoroughly again; portions are taken for further analyses. Care must be taken to avoid fat separation during mixing.

- Vegetables:

-Dry seed legumes. These can be treated as grains, with a large analytical portion taken for inorganic analysis before grinding. Loose seed coats must be carefully mixed into the mass of the food sample.

-Leafy vegetables and vegetable inflorescences. Small leafy vegetables such as Brussels sprouts should be mixed together in a bowl, chopped coarsely and mixed again briefly. A large portion should be taken for inorganic analysis and another portion into metaphosphoric acid for vitamin C analysis. Large tight-leaved vegetables (e.g. cabbage, iceberg lettuce) must be quartered. All large leafy vegetables must be chopped coarsely and mixed, and this must be done very quickly. After the mixing, analytical portions should be taken for analyses of vitamin C, vitamin A, carotenes, vitamin E and inorganic nutrients; the remainder can be chopped further. Stalks are often difficult to reduce and may have to be chopped separately and reintegrated into the food sample.

-Roots and tubers. Large items may be quartered; quarters should be diced in a mechanical chopper for about 20 seconds and mixed rapidly. Portions can then be removed for all analyses.

-Others. Some vegetables, such as cucumber and tomato, must be treated as fruit.

- Fruit. Large fruits (e.g. pineapples or watermelons) and medium-sized ones (e.g. apples) must be quartered. Small fruits (e.g. cherries) should be quartered by the method used for particulate foods. Quarters should be coarsely chopped and combined, and unhomogenized analytical portions should be taken for immediate vitamin C and inorganic analyses. The remaining mixture can then be homogenized to produce an analytical sample for other analyses. Unripe bananas, and possibly some other fruits, should not be mechanically homogenized vigorously, because starch may break down to sugars. Dried fruit may be difficult to homogenize mechanically and may require manual chopping.

- Milk and milk products:

-Liquid and evaporated milk. The contents of units should be swirled together gently in a stoppered glass or polythene container.

-Dried milk. This should be treated as flour.

-Cheese. The texture of the cheese will determine its treatment. Units of friable cheese can be crumbled and mixed; soft cheese should be mashed and mixed; hard or rubbery cheeses should be grated on a polythene grater.

-Yoghurt, cream, ice cream, condensed milk, very soft cheese. Units should be blended in a bowl with a spatula. Those containing fruit and/or nuts should be mechanically homogenized after a large analytical portion has been taken for inorganic analysis.

-Butter. See Fats below.

- Eggs:

-Fresh. Fresh eggs should be shelled and mixed briskly with a fork; after analytical portions are taken for inorganic analyses, the remainder is homogenized mechanically.

-Dried. Dried eggs should be handled as flour.

- Fats and oils:

-Oils. Units should be gently warmed, if necessary, then swirled at 30 °C.

-Fats. Units of butter, margarine, lard or drippings should be softened over a warm water bath and then blended together gently. Units of suet can be crumbled and mixed with a fork. In homogenizing low-fat spreads, care must be taken to prevent breakdown of the fat/water emulsion.

- Nuts. Batches of nuts should be ground separately with a pestle and mortar, then mixed together thoroughly in a bowl. An analytical portion should be taken for inorganic analyses and the remaining mixture should be homogenized mechanically for further analyses.

- Sugars, syrups and confectionery:

-Sugars. Refined sugars should be treated as flour.

-Syrups. Syrups should be taken by weight rather than volume. Sticky syrups should be warmed and gently but thoroughly blended.

-Confectionery. Confectionery samples should be frozen and crushed on a chilled surface or blended in liquid nitrogen, which is then allowed to evaporate in a cold room. Any mixing of crushed units must also be done in a cold room.

- Sauces:

-Viscous sauces. Units should be gently warmed and blended thoroughly together.

-Fluid sauces. These should be swirled together.

-Biphasic sauces (e.g., salad dressings). These items must be thoroughly homogenized and mixed. Test portions should be taken for inorganic analyses and then the mixture should be rehomogenized for further analyses.

- Beverages. Carbonated beverages can be degassed by application of reduced pressure or by being poured from one vessel to another. Specific gravity should be measured by weight of measured volume; units should be mixed by swirling.

- Prepared composite foods and dishes. This is the form in which most foods are consumed. Items should be briefly homogenized, carefully mixed, then rehomogenized. It can be assumed that laboratory homogenization will not introduce any contamination greater than that arising during domestic or commercial food preparation. Care is required to blend in the individual pieces of muscle, fat, vegetables, etc., which may be found in mixed prepared foods. Portions for vitamin C assay are best taken from the mixed homogenate before it is rehomogenized. If the prepared foods are hot, speed is essential to prevent moisture loss. Total meals or diets can be handled in the same way.

Some practical equipment requirements for handling and preparation of laboratory and analytical samples

- General:

Trays (for carrying foods)

Bowls (0.5 litre to 4 litre capacity)

Spatulas

Chopping boards (polythene, wood)

Kitchen knives, knife-sharpener

Can-opener

Spoons (various sizes)

Plastic sieves, colanders

Oven thermometer, meat thermometer

Electric heat-sealer (for freezer bags)

Large sheets of strong plastic (to cover benches, mix particulate foods)

Kitchen cutlery and tableware

- Homogenizers:

-Common domestic equipment:

Domestic food processor (can be equipped with titanium or other special blades)

Coffee-bean grinder

Food blender

Bamix (hand-held homogenizer)

Mincer (hand, electric)

Graters, especially with non-metal cutting edges

-Laboratory equipment:

Sorval Omnimix

Turrax

Waring blender

Ato-Mix

Automatic pestle and mortar

Knife mill

Ball mill

Hammer mill

Robot Coupe blender (available in sizes appropriate for food service)

Appendix 4

Examples of procedures for the preparation of analytical samples

Root vegetables

Food sample collection procedure: Replicate purchases of approximately 1 kg each were made in the towns that were major distribution centres in a country. The places of purchase in the town were randomly chosen by volume of sales from the various types of outlet (supermarket, greengrocer, farmgate stall, etc.)

Laboratory procedure:

1.Opposing quarters of each purchase diced quickly in a domestic food processor and mixed quickly in a bowl with a plastic spatula

(a)2 × 20 g taken into metaphosphoric acid for immediate vitamin C analysis

(b)2 × 5 g taken into hot 80% v/v ethanol for sugars, starch and dietary fibre analysis

(c)2 × 10/20 g (larger portion if food is very low in folate) taken into 1 percent w/v buffered ascorbate for folate analysis

(d)large portions taken for ashing for inorganic constituents, analysed over a period of weeks

(e)analytical samples freeze-dried and stored for amino acid analyses

(f)remaining material mixed, diced, frozen, stored at –20 °C and analysed for remaining B vitamins within two weeks

2.Remaining quarters sliced, homogenized and blended thoroughly together

(a)2 × 10 g taken for overnight moisture analysis.

(b)remainder frozen, stored at –20 °C, and analysed for total nitrogen, phosphorus, chloride, sulphur, fat and carotenoids

Meat

Example: Twenty meat cuts purchased, two from each of ten regions; purchases distributed between butchers and supermarkets in the ratio 7:3, evenly distributed throughout the regions. One cut from each region remained to be analysed raw; one from each region to be analysed grilled.

Raw

Each cut weighed and measured, including width of superficial fat, then dissected into edible (fat and muscle) and inedible (bone and gristle) portions, weighed separately.

1.The ten muscle samples were chopped coarsely and mixed thoroughly together in a bowl

(a)100 g removed, deep-frozen and crushed; crushed sample shaken to mix it further

(i)

2 × 20 g taken for ashing and analyses of inorganic constituents

(ii)

remainder stored at –20°C in heat-sealed polythene bag with minimum headspace for check analyses

(b)remaining fresh mix minced and mixed thoroughly

(i)

2 × 10 g taken for moisture analysis

(ii)

2 × 50 g heated with alcoholic KOH solution and frozen for retinol analysis

(iii)

2 × 50 g taken immediately for thiamin analysis

(iv)

analytical samples stabilized with an antioxidant and stored at –30°C for fatty acid analysis

(v)

analytical samples deep-frozen for other B vitamin analyses (performed within two weeks), fat, total nitrogen, other minerals, vitamins D and E

(vi)

cholesterol and other sterols stored at –30°C in sealed container under nitrogen

2.The ten fat samples were treated similarly.

Cooked

Cuts were weighed before and after grilling, then treated in the same way as raw cuts, with lean and fat being analysed separately (Paul and Southgate, 1977).

Appendix 5

Calculations of fatty acids in 100 g food and 100 g total fatty acids

When the fatty acids provided by a given weight of food are being calculated, allowance must be made for the fact that the total fat in a food includes triglycerides (of which a proportion is glycerol, i.e. not fatty acid), phospholipids and unsaponifiable components such as sterols.

In foods where the total fat is virtually all triglyceride, a correction factor based on the average chain length of the fatty acids present is adequate. The factors for foods containing appreciable amounts of phospholipids and unsaponifiable matter depend on the class of foodstuff. Suggested values for these factors are given in Table A5.1.

These factors are used as in the following examples:

If 100 g goat milk contains 4.5 g fat,

then

4.5 × 0.945 = 4.25 g total fatty acids in 100 g goat milk

When individual fatty acid data are available, the values can be converted from a g/100 g food basis to g/100 g total fatty acid basis. For example, if 100 g goat milk contains 1.15 g palmitic acid, the following equation is applied to calculate palmitic acid in g/100 g total fatty acids:

100/4.25 × 1.15 = 27 g/100 g total fatty acid

When data on fatty acids per 100 g total fatty acids and total fatty acids are available, they can be converted to a g/100 g food basis. For example, if we know that the palmitic acid level in goat milk is 27 g/100 g total fatty acids and the total fatty acid value is 4.25 g/100 g food, the following equation is applied to calculate palmitic acid in g/100 g food:

4.25 × 27/100 = 1.15 g /100 food

Source: Paul and Southgate, 1978.

Appendix 6

Calculation of the composition of dishes prepared from recipes

The method of calculation is as follows. The weights of the raw ingredients are used to calculate the total amounts of nutrients in the dish. A correction for wastage as a result of ingredients left on utensils and in the vessels used in preparation is made at this stage. The weight of the raw dish is then measured, using a scale weighing to about 1 g (a less accurate scale may be used if the total weight of ingredients is over 500 g). The dish is then cooked and the dish reweighed. (A minor correction to allow for the difference between weighing the dish hot and at room temperature is not usually necessary.) The difference in weight is taken as being accounted for by water, and the composition of the cooked dish is calculated as follows. Divide the total nutrients in the dish calculated from the raw ingredients by the weight of the cooked dish and multiply by 100. The water content of the raw ingredients less the loss in weight on cooking divided by the weight of the cooked dish gives the water content of the cooked dish if it is required. The detailed procedure for calculation of the nutrient content of a multi-ingredient food is outlined below.

1.Select or develop an appropriate recipe.

2.Collect the weight and nutrient content data for each ingredient.

3.Correct the ingredient nutrient levels for weight of edible portions where appropriate.

4.Correct the ingredients for the effects of cooking:

either

-if data for the cooked ingredients are available, use yield factors to adjust from raw to cooked weights;

or

-if data for the cooked ingredients are not available, use data for the uncooked ingredients and apply yield factors to adjust for weight changes and retention factors for nutrient losses or gains during cooking.

5.Sum the weights of the ingredients to obtain the weight of the recipe.

6.Sum the nutrient values of the ingredients to obtain the nutrient value of the recipe.

7.Adjust the recipe weight and nutrient levels to reflect changes in fat/water contents when the whole mixture is cooked; make any additional refuse adjustments; apply retention factors if available for the whole recipe.

8.Determine the quantity of prepared food produced by the recipe.

9.Determine the final values per weight (e.g. per 100 g), volume (e.g. per cup) or serving portion, as desired.

Source: Rand et al., 1991.

Appendix 7

Essential book list for food composition databases

AOAC. 1990. Official methods of analysis of the Association of Official Analytical Chemists. 15th edition. Washington, DC, Association of Official Analytical Chemists.

AOAC International. 1995. Official methods of analysis of AOAC International. 2 vols. 16th edition. Arlington, VA, USA, Association of Analytical Communities.

AOAC International. 2000. Official methods of analysis of AOAC International. 17th edition. Gaithersburg, MD, USA, Association of Analytical Communities.

AOAC International. 2002. Official methods of analysis of AOAC International. 17th edition. 1st revision. Gaithersburg, MD, USA, Association of Analytical Communities.

AOAC International. 2003. Official methods of analysis of AOAC International. 17th edition. 2nd revision. Gaithersburg, MD, USA, Association of Analytical Communities.

Ball, G.F.M. 1994. Water-soluble vitamin assays in human nutrition. London, Chapman & Hall.

Ball, G.F.M. 1998. Bioavailability and analysis of vitamins in foods. London, Chapman & Hall.

Belitz, H.D. & Grosch, W. 1999. Food chemistry. 4th edition. Berlin. Springer.

Christie, W.W. 2003. Lipid analysis. Bridgwater, UK, The Oily Press.

De Leenheer, A.P., Lambert, W.E. & Van Bocxlaer, J., eds. 2000. Modern chromatographic analysis of vitamins. 3rd ed. New York, USA, Marcel Dekker.

Efiok, B.J.S. 1993. Basic calculations for chemical and biological analysis. Arlington, VA, USA, AOAC International.

Eitenmiller, R.R. & Landen, Jr, W.O. 1998. Vitamin analysis for the health and food sciences. Cambridge, UK, Woodhead Publishing.

Gilbert, J., ed. 1984. Analysis of food contaminants. New York, USA, Elsevier Science Publishing.

Greenfield, H., ed. 1995. Quality and accessibility of food-related data. Proceedings of the First International Food Data Base Conference. Arlington, VA, USA, AOAC International.

Harris, D.C. 1997. Exploring chemical analysis. New York, USA, W.H. Freeman and Company.

James, C.S. 1995. Analytical chemistry of foods. London, Blackie Academic & Professional.

Journal of AOAC International. Arlington, VA, USA, AOAC International.

Journal of Food Composition and Analysis. London, Elsevier.

Kirk, R.S. & Sawyer, R. 1991. Pearson’s chemical analysis of foods. 9th ed. Harlow, UK, Longman Scientific and Technical.

Klensin, J.C., Feskanich, D., Lin, V. Truswell, A.S. & Southgate, D.A.T. 1989. Identification of food components for INFOODS data interchange. Tokyo, United Nations University (available at http://www.unu.edu/unupress/unupbooks/80774e/80774E00.htm).

Klensin, J.C. 1992. INFOODS food composition data interchange handbook. Tokyo, United Nations University (available at http://www.unu.edu/unupress/unupbooks/80774e/80774E00.htm).

Kramer, R. 1998. Chemometric techniques for quantitative analysis. New York, USA, Marcel Dekker.

Lawn, R.E., Thompson, M. & Walker, R.F. 1997. Proficiency testing in analytical chemistry. Cambridge, UK, Royal Society of Chemistry.

Macrae, R., ed. 1988. HPLC in food analysis. 2nd edition. London, Academic Press.

McCleary, B.V. & Prosky, L., eds. 2001. Advanced dietary fibre technology. Oxford, UK, Blackwell Science.

Meier, P.C. & Zund, R.E. 2000. Statistical methods in analytical chemistry. 2nd edition. New York, USA and Chichester, UK, Wiley.

Miller, D.D. 1998. Food chemistry: a laboratory manual. New York, USA, Wiley.

Nielsen, S.S., ed. 1998. Food analysis. 2nd edition. Gaithersburg, MD, USA, Aspen Publishers

Nollet, L.M., ed. 1996. Handbook of food analysis. New York, USA, Marcel Dekker.

Nollet, L.M., ed. 2000. Food analysis by HPLC. 2nd edition. New York, USA, Marcel Dekker

Pare, J.R. & Belanger, J.M.R., eds. 1997. Instrumental methods in food analysis. Amsterdam, Elsevier.

Pomeranz, Y. & Meloan, C.E. 1994. Food analysis: theory and practice. 3rd edition. New York, USA, Chapman & Hall.

Rand, W.M., Pennington, J.A.T., Murphy, S.P. & Klensin, J.C. 1991. Compiling data for food composition data bases. Tokyo, United Nations University Press (available at http://www.unu.edu/unupress/unupbooks/80772e/80772E00.htm).

Ratliff, T.A. 2003. The laboratory quality assurance system: a manual of quality procedures and forms. 3rd edition. New York, USA, and Chichester, UK, Wiley.

Rucker, R.B., Suttie, J.W., McCormick, D.B. & Machlin, L.J., eds. 2001. Handbook of vitamins. 3rd edition. New York, USA, Marcel Dekker.

Schlotke, F., Becker, W., Ireland, J., Møller, A., Ovaskainen, M.L., Monspart, J. & Unwin I. 2000. EUROFOODS recommendations for food composition database management and data interchange. COST Report EUR19538. Luxembourg: Office for Official Publications of the European Commission.

Scott, A.O. ed. 1998. Biosensors for food analysis. Cambridge, UK, Royal Society of Chemistry.

Shaw, P.E. ed. 1988. Handbook of sugar separations in foods by high performance liquid chromatography. Boca Raton, FL, USA, CRC Press.

Skoog, D.A. & Leary, J.J. 1998. Principles of instrumental analysis. 4th edition. New York, USA, Saunders College Publishing.

Sørensen, H., Sørensen, S., Bjergegaard, C. & Michaelsen, S. 1998. Chromatography and capillary electrophoresis in food analysis. Cambridge, UK, Royal Society of Chemistry.

Southgate, D.A.T. 1991. Determination of food carbohydrates. 2nd edition. Barking, UK, Elsevier Applied Science.

Southgate, D.A.T. 1995. Dietary fibre analysis. Cambridge, UK, Royal Society of Chemistry.

Stoeppler, M., Wolf, W.R. & Jenks, P.J., eds. 2000. Reference materials for chemical analysis: certification, availability, and proper usage. Chichester, UK, Wiley.

Sullivan, D.M. & Carpenter, D.E., eds. 1993. Methods of analysis for nutrition labeling. Arlington, VA, USA, AOAC International.

Taylor, J.K. 1987. Quality assurance of chemical measurements. Chelsea, MI, USA, Lewis Publishers.

Rand, W.M., Pennington, J.A.T., Murphy, S.P. & Klensin, J.C. 1991. Compiling data for food composition data bases. Tokyo, United Nations University.

Wernimont, G.T. 1985. Use of statistics to develop and evaluate analytical methods. Washington, DC, Association of Official Analytical Chemists.

Wetzel, D.L.B & Charalambous, G. 1998. Instrumental methods in food and beverage analysis. New York, USA and Oxford, UK, Elsevier.

Wood, R., Nilsson, A. & Wallin, H. 1998. Quality in the food analysis laboratory. Cambridge, UK, Royal Society of Chemistry Information Services.

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