Chapter 25. Food composition tables, nutrient requirements and food balance sheets
Chapter 26. Cereals, starchy roots and other mainly carbohydrate foods
Chapter 27. Legumes, nuts and oilseeds
Chapter 28. Vegetables and fruits
Chapter 29. Meat, fish, eggs, milk and their products
Different tools are used to assess the nutritional situation of groups of people, including families, communities and nations. Food composition tables provide a means to estimate the nutrient content of foods consumed by the population being studied. Tables of nutrient requirements or of recommended dietary allowances (RDAs) indicate either the suggested daily requirements of each important nutrient judged necessary to maintain satisfactory nutritional status or allowances intended to serve as goals for intakes of nutrients. These allowances often afford a margin of sufficiency; except for the energy allowance they are usually set somewhat above the physiological requirements of individuals. In general both suggested requirements and RDAs are designed for use by groups of persons, not by an individual subject. Assessment of the nutritional status of an individual needs to be based on determination of food consumption (translated into daily nutrient consumption using food composition tables), clinical examination, biochemical assessment, anthropometry and perhaps other tests.
Food balance sheets are used to provide data on food available nationally for the whole population. FAO assists many countries in assembling data on estimates of food production, imports, exports and other food uses to provide an estimate of food that was available in a particular year for the population of the country. If the population figures are available, then mean available foods can be calculated. Through the use of food composition tables, these can then be translated into mean nutrient availability [for example, the daily (or annual) availability of energy, protein and each of the important micronutrients] per head of population.
Thus food composition tables, estimates of nutrient requirements or dietary allowances and food balance sheets are tools used in different ways and for different purposes by persons wishing to assess the nutritional situation of groups of persons or of nations.
A food composition table usually consists of a list of selected foods with figures for the content of selected nutrients in each food. Annex 3 provides a very limited table of the nutrient content of foods that are known to be quite widely used in developing countries. The annex is included in this publication so that using one volume the professional reader can, for example, estimate the nutrient consumption of certain groups of people or calculate the nutrient content of diets that are used or recommended, for instance for institutional feeding or emergency rations.
Many books provide much more comprehensive data on food composition and are appropriate for use in research or for nutrition surveys. These include the United States Department of Agriculture's massive Composition of foods - raw, processed and prepared, in several volumes, first published as USDA Handbook No. 8 in 1963 and revised in 1984; the various editions of and supplements to McCance and Widdowson's The composition of foods; food composition books concerning either certain geographic areas (some published by FAO) or particular developing countries; and others that deal with only certain nutrients.
The food composition table in Annex 3, taken from the recent FAO publication Food and nutrition in the management of group feeding programmes (FAO, 1993b), provides the content of nutrients per 100 µg edible portion of each food. The nutrients included, which have been selected as the most important for developing countries, are energy, protein, fat, calcium, iron, vitamin A, thiamine, riboflavin, niacin, folate and vitamin C. Readers wishing to know the content of other nutrients, for example zinc, selenium or biotin, in a food will have to refer to more detailed food composition tables.
Some tables list the nutrient content per "normal serving size" rather than by weight, and some provide data on the nutrients in various prepared foods rather than, as in this book, the raw food. Although the table in Annex 3 gives the nutrient content of both wheat flour and a prepared product, bread, in general most prepared foods are not included. Thus the nutrient content of maize is given but not that of the tortillas eaten in Central America or that of ugali, a maize-based dish consumed in East Africa. The foods, for simplicity, are listed in categories to allow easy use of the table.
A word of caution is needed for those using any food composition tables. The figures given for the content of a particular nutrient in a particular food are based on analyses of samples of that food. However, foods often vary in their nutrient content depending on the country and climate where they are grown, the type of food analysed, how the food is prepared before consumption (which varies among different cultural groups) and many other factors. It should also be recognized that analyses conducted even in very sophisticated laboratories have a margin of error, which is larger for some nutrients than for others. For example, tomatoes come in many different varieties, are grown in different soils in both tropical and temperate climates and can be picked green or ripe; therefore there is a wide variation in the amount of carotene (which can be converted to vitamin A in the body) in 100 µg of tomatoes consumed. The table in Annex 3 shows that a 100 µg edible portion of tomato contains 113 µg of vitamin A. Some tomatoes have much higher and others much lower content of vitamin A. Food composition tables are useful but must be used judiciously.
A great deal of research has been conducted to determine the needs or requirements of human beings for different nutrients. Nutrient requirements of course vary in certain groups of people, for example in children because they have added needs for growth and in women during pregnancy and lactation. Large comprehensive textbooks discuss in detail the research leading to the best estimates of the requirements of different individuals for each nutrient.
Many countries provide recommendations regarding the amounts of each of the important nutrients that should be consumed by their populations. In many cases these provide for levels of safety and take account of variations in requirements; often, therefore, the figures are somewhat higher than minimum requirements for health.
Generally recommended dietary allowances for a country provide only guidelines for the evaluation and development of good diets for the population. It is important to understand clearly that the values presented are not requirements, since many individuals are known to consume smaller amounts than those listed and still enjoy good health. On the other hand, it is recognized that the actual requirement for any nutrient is not precisely known. Recommended dietary allowances therefore must not be considered requirements but rather levels of intake that should be entirely adequate for essentially all members of the population. This kind of dietary guidance seems appropriate in affluent countries such as the United States. It may not be appropriate in many parts of the world where there are more urgent problems and where food and money are more limiting factors for many people.
For the purposes of this book Annex 1 provides recommended intakes of nutrients and "safe levels of intake", which apply to groups of persons and not to individuals. They pertain to healthy, not diseased people. These tables are designed to recommend, based on current knowledge, intakes of selected macronutrients and micronutrients that will maintain health, prevent deficiency diseases and allow adequate stores of nutrients in normal circumstances. The recommendations for children are for amounts of nutrients that allow proper growth, and those for women of child-bearing age take into account their special needs, including those of pregnancy and lactation.
Research workers and policy-makers in the developing countries should use, where available, tables of recommended dietary intakes or allowances that have been adopted in their own countries or geographic regions. Over 40 countries have such tables. They should all be used critically, and often in conjunction with publications from international organizations such as FAO, the World Health Organization (WHO) and the International Union of Nutritional Sciences (IUNS).
Many developing countries, using their own resources or with assistance from FAO or other organizations, have from time to time published food balance sheets, which are the best estimates that can be made from existing data of the total amount of food available for consumption by the human population in a particular year (or other period). Usually these estimates are based on the total quantities of food produced in the country, the foods imported and the changes up and down of food reserves or food stocks for the period. Deductions are made for foods such as cereals or legumes used as seeds rather than for consumption, for those used for livestock consumption (termed "animal feeds"), for those used for industrial non-food purposes (for example, fats or oils used for production of soap or for ethanol fuels) and for a waste factor. The final figures deduced are construed to represent the amount of food potentially available for consumption by the population of the country.
These figures then can be divided by the total mid-year population of the country to derive average per caput availability of food for the year, which can in turn be translated into per caput availability of nutrients using food composition tables. Total availability of energy and of other nutrients for the nation can also be calculated. These figures can then be compared with the calculated nutrient needs of the country to assess the adequacy of food availability. Most important, the data provide information on the dietary energy supply (DES), which combined with information about the distribution of food supplies allows an estimate of the number of people whose energy intakes are too low. The main limitation of DES is that it is not a direct assessment of food consumption. Food balance sheets also do not take into account age and gender factors, internal distribution differences within a country or seasonal variations in food availability.
Food balance sheets are often used to indicate a country's sufficiency and/or deficiency of food or particular nutrients. When prepared over successive years, they show trends in the country's food availability, indicating whether it is improving or declining and thereby allowing the country to institute appropriate policy to safeguard national food security and to channel agricultural production. The tables may also help the country to devise appropriate crop diversification policy to improve both agricultural income and production of nutritionally desirable foods. In addition the data indicate how much a country depends on its own food production relative to food importation and could thus contribute to the design of national food importation policy.
Food balance sheets for most poor developing countries are only very rough estimates of the food situation. The accuracy of the data used in the preparation of food balance sheets varies widely depending on the availability of good-quality data and the level of development of agrostatistics services. Generally they are much better in developed than in developing countries. Accurate census data on population are not available in many developing countries. Therefore the limitations of food balance sheets should be examined critically before the information is used in designing important agricultural, food security or economic policies in a particular country.
Early peoples lived mainly on foods obtained by hunting and gathering. Among the first crops to be planted and harvested were the cereal grains. Ancient civilizations flourished partly because of their abilities to produce, store and distribute these cereal grains: maize in the Americas before the arrival of Europeans; rice in the great Asian civilizations; and barley in Ethiopia and northeast Africa.
Foods with a predominantly carbohydrate content are important because they form the basis of most diets, especially for poorer people in the developing world. In the developing countries these foods usually provide 70 percent or more of the energy intake of the population. In contrast, in the United States and Europe often less than 40 percent of energy comes from carbohydrates.
Through the ages many plants from the family of grasses have been cultivated for their edible seeds; these are the cereal grains. Cereals form an important part of the diet of many people. They include maize, sorghum, millets, wheat, rice, barley, oats, teff and quinoa. A new cereal of considerable interest is triticale, a cross between wheat and rye.
Although the shape and size of the seed may be different, all cereal grains have a fairly similar structure and nutritive value; 100 µg of whole grain provides about 350 kcal, 8 to 12 µg of protein and useful amounts of calcium, iron (though phytic acid may hinder absorption) and the B vitamins (see Table 33). In their dry state cereal grains are completely lacking in vitamin C and, except for yellow maize, contain no carotene (provitamin A). For a balanced diet, cereals should be supplemented with foods rich in protein, minerals and vitamins A and C. (Vitamin D can be obtained through exposure of the skin to sunlight.)
The structure of all cereal grains (Figure 15) consists of:
· the husk of cellulose, which has no nutritive value for humans;
· the pericarp and testa, two rather fibrous layers containing few nutrients;
· the aleurone layer,`which is rich in protein, vitamins and minerals;
· the nutrient-rich embryo or germ, consisting of the plumule and radicle attached to the grain by the scutellum;
· the endosperm, comprising more than half of the grain and consisting mainly of starch.
FIGURE 15. Cross-section of a wheat kernel
TABLE 33
Content of certain nutrients in 100 g of selected cereals
Food |
Energy |
Protein |
Fat |
Calcium |
Iron |
Thiamine |
Riboflavin |
Niacin |
(kcal) |
(g) |
(g) |
(mg) |
(mg) |
(mg) |
(mg) |
(mg) | |
Maize flour, whole |
353 |
9.3 |
3.8 |
10 |
2.5 |
0.30 |
0.10 |
1.8 |
Maize flour, refined |
368 |
9.4 |
1.0 |
3 |
1.3 |
0.26 |
0.08 |
1.0 |
Rice, polished |
361 |
6.5 |
1.0 |
4 |
0.5 |
0.08 |
0.02 |
1.5 |
Rice, parboiled |
364 |
6.7 |
1.0 |
7 |
1.2 |
0.20 |
0.08 |
2.6 |
Wheat, whole |
323 |
12.6 |
1.8 |
36 |
4.0 |
0.30 |
0.07 |
5.0 |
Wheat flour, white |
341 |
9.4 |
1.3 |
15 |
1.5 |
0.10 |
0.03 |
0.7 |
Millet, bulrush |
341 |
10.4 |
4.0 |
22 |
3.0 |
0.30 |
0.22 |
1.7 |
Sorghum |
345 |
10.7 |
3.2 |
26 |
4.5 |
0.34 |
0.15 |
3.3 |
The embryo is the part of the grain that sprouts if the grain is planted or soaked in water. It is very rich in nutrients. Although small in size, the embryo often contains 50 percent of the thiamine, 30 percent of the riboflavin and 30 percent of the niacin of the whole grain. The aleurone and other outer coats contain 50 percent of the niacin and 35 percent of the riboflavin. The endosperm, although by far the largest part of the grain, often contains only one-third or less of the B vitamins. Compared with other parts, it is also poorer in protein and minerals, but it is the main source of energy, in the form of a complex carbohydrate, starch.
Processing
Cereal grains are subjected to many different processes during their preparation for human consumption. All of the processes have in common that they are designed to remove the fibrous layers of the grain. Some processes, however, are also intended to produce a highly refined white product consisting mainly of endosperm. Another common characteristic shared by all the processes is that they reduce the nutritional value of the grain.
Traditional methods of processing, involving the use of a pestle and mortar or stones, usually produce a cereal grain that has lost some of its outer coats but retains at least part of the germ, including the scutellum. Although very careful and prolonged processing using traditional methods can yield a highly refined product, such preparation is unusual. Light milling, similar to home pounding, also produces a product that retains most of the nutrients. Mechanization of this type has the additional advantage of taking a great burden off the woman of thehousehold, as it is usually the woman who is responsible for pounding grain.
Heavy milling to produce a highly refined product is undesirable from a nutritional point of view. Highly milled cereals such as white maize flour, polished rice and white wheat flour have lost most of the germ and outer layers and with them most of the B vitamins and some of the protein and minerals. Millers are, however, servants of the public, and the public increasingly demands products that are pure white, have a bland, neutral taste and are easily digestible. These demands led, in the first half of the twentieth century, to a vast increase in the production of highly refined cereal flours and white rice. The millers have responded to public demand by devising "improved" milling machinery that separates more and more of the nutritious parts from the grain, leaving the white endosperm.
The percentage of the original grain that remains in the flour after milling is termed the extraction rate. Thus an 85 percent extraction flour contains 85 percent (by weight) of the whole grain, 15 percent having been removed. Therefore, a high-extraction flour has lost little of the nutrients in the outer coats and germ, whereas a low-extraction flour has lost much. The advantages of low-extraction flours over high-extraction flours from the trade point of view are that they are whiter, and so more popular; they have less fat, and hence less tendency to become rancid; they have less phytic acid, which possibly means that minerals from associated foods are absorbed better; and they have better baking qualities. The disadvantages of low-extraction flours to the consumer are that they contain less B vitamins, minerals, protein and fibre than high-extraction flours.
In many countries food fashions begin among the wealthier people. As long as the new food fashion remains confined to those with a high income, it need not do much harm, for they can afford a better all-round diet which compensates for the nutrients lost to fashion. However, the white-flour fashion has spread to all levels of society, rich and poor, in many countries. In addition, highly milled rice spread across Asia quite rapidly over 80 years ago.
Where preference for white flour or highly milled rice leads to the consumption of a staple cereal rendered deficient by milling, widespread ill health could be, and has been, the result among those who do not include in their diet other foods that make up for this deficiency. Much misery, suffering and death resulted directly from the introduction of milled cereals to the people of Asia around the beginning of the twentieth century, when the disease beriberi became highly prevalent (see Chapter 16).
Increasing industrialization and urbanization in developing countries have brought with them a much greater use of bread because of its convenience for workers eating away from home.
Manufactured cereal products are being increasingly sold as baby and breakfast foods. In developing countries these products are mainly imported. They may be convenient but they are relatively expensive and have no inherent advantage, from a nutritional point of view, over local cereals prepared in a traditional manner. However, they may be highly advertised, considered as prestige foods and falsely regarded as more nutritious than local foods. Their use should be discouraged for those who cannot really afford them.
Legislation requiring millers to put additional vitamins into cereal flours exists in some countries and can be effective. This procedure does not work as well with rice because it is most commonly bought and eaten in its granular form, whereas maize and wheat and most other cereals are more often bought as flour. Attempts have been made in Asia to add vitamins in a concentrated form to a few artificial granules and then to mix these with rice. This method has not proved entirely successful, partly because one of the B vitamins, riboflavin, is yellow and lends a colour unacceptable to those who want a uniformly white product.
Maize
Maize (Zea mays) is a very important food in the Americas and much of Africa. It was first cultivated in the Americas, and it was an important food of the great Aztec and Mayan civilizations long before the arrival of Columbus and the colonizers. Seeds were brought to Europe and later to Africa, where maize is now the most important part of the diet in many areas. Maize is popular because it gives a high yield per unit area, it grows in warm and fairly dry areas (much drier than those needed for rice, although not as dry as those where sorghum and millet can be raised), it matures rapidly and it has a natural resistance to bird damage. The United States is the largest producer of maize, but much of what is grown there is used to feed domestic animals.
Nutrient content. Maize grains contain about the same amount of protein as other cereals (8 to 10 percent), but much of it is in the form of zein, a poor-quality protein containing only small amounts of lysine and tryptophan. The association noted between maize consumption and pellagra (see Chapter 17) may be due in part to a deficiency of these amino acids. Whole-grain maize contains 2 mg niacin per 100 g, which is less than that in wheat or rice and about the same as the amount in oats. The niacin in maize is in a bound form and not entirely available to humans. In Mexico and some other countries maize is treated with an alkaline solution of lime which releases the niacin and helps prevent pellagra; maize treated with lime is used for making tortillas, an important food in Mexico.
New varieties of maize, for example opaque-2 maize, have now been developed with an improved amino acid pattern.
Processing. Milling reduces the nutritive value of maize just as it does that of other cereals. The increased popularity and use of highly milled maize meal as opposed to traditionally ground or lightly milled maize in Africa could create a problem, since the highly milled product is deficient in B vitamins (Table 34); it is necessary to eat 600 g of highly milled maize to obtain the amount of thiamine present in 100 g of lightly milled maize. The vitamin B constituents lost in milling may be replaced in maize meal, as in other cereal flours, by fortification. Enrichment of this kind has been effective in many countries. Legislation to ensure an adequate level of B vitamins in cereal flours may be feasible and worthwhile for more countries to adopt.
Rice
Rice, like other cereals, is a domesticated grass; wild varieties have existed for centuries in both Asia (Oryza sativa) and Africa (Oryza glaberina). Rice is a particularly important food for much of the population of China and for many other countries in Asia, where close to half the population of the world lives. It is also important in the diets of some peoples in the Near East, Africa and to a lesser extent the Americas. Much of the rice is produced in small irrigated fields or paddies in Asia, but some is grown in rain-fed areas without irrigation.
Nutrient content. The outer layers and the germ together contain nearly 80 percent of the thiamine in the rice grain. The endosperm, though constituting 90 percent of the weight of the grain, contains less than 10 percent of the thiamine. Lysine and threonine are the limiting amino acids in rice.
TABLE 34
Effect of milling on vitamin B content of maize (mg per 100 g)
Level of processing of maize |
Thiamine |
Riboflavin |
Niacin |
Whole grain |
0.35 |
0.13 |
2.0 |
Lightly milled |
0.30 |
0.13 |
1.5 |
Highly milled (65 percent extraction) |
0.05 |
0.03 |
0.6 |
Processing. After harvesting, the rice seeds or grains are subjected to different milling methods. The traditional home method of pounding rice in a wooden mortar and winnowing it in a shallow tray usually results in the loss of about half of the outer layers and germ, leaving a product containing about 0.25 mg thiamine per 100 g. The procedure of milling and subsequently polishing rice, which produces the highly esteemed white rice on sale in many shops, removes nearly all the outer layers and germ and leaves a product containing only about 0.06 mg thiamine per 100 g. This amount is grossly deficient. In Asia, many poor people have a diet consisting mainly of rice for much of the year. A person eating 500 g of highly milled polished rice per day would get only 0.3 mg thiamine. The same quantity of home-pounded or lightly milled rice would provide approximately 1.25 mg thiamine, which is about the normal requirement for an average man.
Fortification is one method of adding micronutrients. Another way of providing highly milled rice that is reasonably white and yet contains adequate quantities of B vitamins is by parboiling. This process is usually done in the mill, but it can be done in the home. The paddy, or unhusked rice, is usually steamed, so that water is absorbed by the whole grain, including the endosperm. The B vitamins, which are water soluble, become evenly distributed throughout the whole grain (Figure 16). The paddy is dried and dehusked, and it is then ready for milling in the ordinary way. Even if it is highly milled and polished, the parboiled grain still retains the major part of its thiamine and other B vitamins.
The solubility of the B vitamins has its disadvantages. Rice that is washed too thoroughly in water loses some of the B vitamins, which are dissolved out. Similarly if rice is boiled in excess water, a considerable proportion of the B vitamins is likely to be discarded with the water after cooking. Rice should therefore be cooked in just the amount of water it will absorb. If any water is left over it should be used in a soup or stew, since it will contain valuable B vitamins which should not be wasted.
Wheat
Wheat (genus Triticum) is the most widely cultivated cereal in the world and its products are very important in human nutrition. In many parts of the world where wheat cannot be grown it is imported and is becoming an increasingly important part of the diet, especially for the urban population. However, importation of wheat, like that of all products, must be offset by adequate exports to prevent a drain on a country's foreign exchange.
Bread, usually made from wheat flour, is a popular convenience food. When purchased, it saves time and fuel for poor families. Pasta is also becoming increasingly popular in some developing countries.
Nutrient content. Wheat provides a little more protein than does rice or maize, about 11 g per 100 g. The limiting amino acid is lysine. In many industrialized countries wheat flour is fortified with B vitamins and sometimes iron and other nutrients.
FIGURE 16. Effects of milling and parboiling on thiamine in rice
Processing. Wheat is usually ground and made into flour. As with other milled cereals, the nutrient content depends on the degree of milling, i.e. the extraction rate. Low-extraction flours have lost much of their nutrients. In some developing countries where wheat is being increasingly used, the bakers have encouraged the trend towards highly refined products
because white wheat flour has better baking qualities. Traders also prefer the highly milled product because it stores better. Its low fat content reduces the chances of rancidity, and its low vitamin content makes it less attractive to insects and other pests.
Millets and sorghum
Millets and sorghum are cereal grains widely grown in Africa and in some countries in Asia and Latin America. Although less widely grown than maize, rice and wheat, they are important foods. They survive drought conditions better than maize and other cereals, so they are commonly grown in areas where rainfall is low or unpredictable. They are valuable food crops because they nearly all contain a higher percentage of protein than maize and the protein is also of better quality, with a fairly high content of tryptophan. These cereals are also rich in calcium and iron. Because they tend to be ground at home and not in the mill, they are less frequently subjected to vitamin, mineral and protein losses. However, in many areas of Africa, they are being replaced by rice and maize, although they usually continue to be grown for making beer. In some parts of Asia millets are regarded as low-class foods for poor people.
Many millet and sorghum varieties have the disadvantages of susceptibility to attack by small birds and a tendency to shed their grain. Losses are often high. In some countries millets and sorghum are used mainly to feed animals.
Sorghum (Sorghum vulgare or Sorghum bicolor) is believed to have originated in Africa but is now cultivated in many countries. It is also called guinea corn or durra, and in India it is known as jowar. There are many varieties of sorghum; most grow tall and have a large inflorescence, but there are also dwarf varieties. The grain is usually large but varies in colour and shape with the type. Sorghum requires more moisture than millets but less than maize. Sorghum is a nutritious food and many varieties have a higher protein content than other cereals.
There are several species of millet. The most important in Africa are bulrush millet (Pennisetum glaucum), also called pearl millet, and finger millet (Eleusine coracana). The former, as the name implies, looks rather like a bulrush, but the inflorescence may be much longer and thicker, some times 1 x 8 cm. The inflorescence of finger millet is shaped like a rather flaccid hand. The seeds are smaller than those of bulrush millet. It is very commonly used for making beer.
Other cereals
Oats. Oats are not important in the diets of most developing countries. The crop is grown in a few cold highland areas, where it is locally prepared and not usually milled. Oats are a good cereal containing rather more protein than maize, rice or wheat, but they also contain a considerable quantity of phytic acid which may hinder absorption of iron and calcium. Oatmeal is imported for use in porridge and is used in some manufactured infant foods.
Rye. Rye is little grown in Africa, Asia and Latin America, and even in Europe it is not an important item of the diet. It has nutritive properties similar to those of other cereals and is sometimes added to bread.
Barley. Barley is grown in some of the wheat-growing districts of Africa and in highland areas of Asia and South America. In these places it is usually consumed as a stiff porridge after home preparation. In Europe it is now used mainly for animal feeding and in the preparation of alcoholic beverages such as beer and whisky.
Triticale. This new cereal is a cross between wheat and rye. It has promise of high yields and good nutritive value. It is particularly suited to temperate climates.
Teff. Teff (Eragrostis tef) is an important cereal in Ethiopia, where it is held in special regard although it gives a relatively low yield per unit area. It is usually ground into a flour, cooked and eaten as injera, a type of baked pancake. The nutritive value of teff is similar to that of other cereal grains, except that it is richer in iron and calcium. The high consumption of teff in parts of Ethiopia may be an important reason why iron deficiency anaemia is rarely reported there.
Quinoa. Quinoa is a millet-like cereal grain which is grown in South America in the Andes, particularly in the altiplano. It grows well even where rainfall is low, soil is not very fertile and nights are very cold. As a food it has a special place in the diets of some Andean peoples.
A number of edible tubers, roots and corms form an important part of the diet of many peoples in different parts of the world. In tropical countries cassava, sweet potatoes, taro (cocoyam), yams and arrowroot are the most important foods in this class. In the cooler parts of the world the common potato is also widely grown.
These food crops are usually relatively easy to cultivate and give high yields per hectare. They contain large quantities of starch and are therefore a fairly easily obtainable source of food energy. As staple foodstuffs, however, they are inferior to cereals because they consist of about two-thirds water and have much less protein, as well as lower contents of minerals and vitamins. They usually contain less than 2 percent protein, whereas cereals contain about 10 percent. Taro and yams, however, contain up to 6 percent good-quality protein.
Cassava
Although cassava (Manihot esculenta), also known as yuca or manioc, originated in South America, it is now widely grown in many parts of Asia and Africa, mainly for its starchy tuberous roots which may grow to an enormous size. Readily established from cuttings, it will grow in poor soil, requires relatively little attention, withstands adverse weather conditions and until recently was not greatly afflicted by pests or disease. However, in some parts of Africa, notably Malawi, cassava plants in the fields have been attacked and destroyed by mealy bug.
Energy yields per hectare from cassava roots are often very high, potentially much higher than from cereals. The leaves of the plant are eaten by some societies and are nutritious. However, cassava has the great disadvantage of containing little but carbohydrate. It is especially unsuitable as the main source of energy for the infant or young child because of its low protein content. It should therefore be supplemented liberally with cereals and also with legumes or other protein-rich foods. However, in non-arid areas where the main food and nutrition problems arise from shortage of total food and deficient energy intake, cassava should be encouraged because of its high yields and other agricultural advantages.
Cassava contains less than 1 percent protein, significantly less than the 10 percent in maize and other cereals. It is not surprising, therefore, that kwashiorkor resulting from protein deficiency is much more common in young children weaned on to cassava than in those weaned on to millet or maize. Cassava also has considerably less iron and B vitamins than the cereal grains.
Cassava, particularly bitter varieties, sometimes contains a cyanogenic glucoside. This poisonous substance is present mainly near the outer coat of the tuber, so peeling cassava helps reduce the cyanide. Cassava that is soaked in water or boiled in water that is then discarded also has reduced cyanide levels. In addition, toxicity can be reduced by pounding, grating and fermentation of the cassava roots. Toxic effects tend to occur where these practices are not used. Cassava consumption has also been linked with goitre and iodine deficiency disorders (see Chapter 14).
Cassava leaves are frequently used as a green vegetable. Their nutritive value is similar to that of other dark green leaves. They are an extremely valuable source of carotene (vitamin A), vitamin C, iron and calcium. The leaves also contain some protein. To preserve the maximum quantity of vitamin C in the leaves, they should not be cooked for longer than about 20 minutes.
Cassava tubers may be eaten roasted or boiled, but more often they are sun-dried after soaking and then made into a powdery white flour. In some countries cassava is milled commercially. In some of this processing the end product is tapioca, which is mainly cassava starch. In West Africa cassava is used to make fufu (a boiled, mashed product). In some countries, for example Indonesia, cassava is regarded as a poor persons' food, and in others as a famine food.
Sweet potatoes
Sweet potatoes originated in the Americas and are now widely grown also in tropical Africa and Asia, usually from stem cuttings. Like cassava, the irregularly shaped, variously sized tubers contain little protein. They contain some vitamin C, and the coloured varieties, especially the yellow ones, provide useful quantities of carotene (provitamin A). Sweet-potato leaves are often eaten and have properties very similar to those of cassava leaves. However, the leaves should not be picked to excess since, as with other tuber crops, this may reduce the yield of tubers.
Yams
There are innumerable varieties of yams (genus Dioscorea), some of which are indigenous to Africa, Asia and the Americas. They vary in shape, colour and size as well as in cooking quality, leaf structure and palatability. Besides the many domesticated varieties a number of wild varieties are eaten.
Yams are more extensively grown in West Africa than in East Africa. In Nigeria, for example, yams are still an important root crop despite an increase in the popularity of cassava. Yams require a warm, humid climate and soil rich in organic matter; these requirements limit their cultivation.
The proper cultivation of yams entails initial deep digging and subsequent staking of the twining vine-like plant. The work involved is more arduous than for cassava, and the yields, though high, are usually a little lower than cassava yields. Yams usually contain about twice as much protein (2 percent) as cassava, although very much less than cereals.
Taro or cocoyams
Taro (Colocasia sp.) originated in Asia but is quite extensively grown in areas where there is fairly high rainfall spread over much of the year. It is widely grown and consumed in the Pacific islands. In Africa, taro is common in forest areas (e.g. in the Ashanti country of Ghana) and on mountain slopes where precipitation is high (e.g. Mount Kilimanjaro). Taro is often grown in association with bananas or plantains (e.g. by the Buganda) or together with oil-palms. The plant has large "elephant ear" leaves. Both the tubers and the leaves are eaten. The nutritive value of taro is similar to that of cassava. In some areas taro is being replaced by tania or new cocoyam (Xanthosoma sp.), a somewhat similar but more robust plant originally from South America, which outyields taro.
Potatoes
Potatoes were first taken to Europe from South America and became a cheap, useful, high-yielding alternative to the existing main staples, just as cassava replaced millet in parts of Africa and Asia. However, the mistake of relying almost entirely on one crop was emphasized by the great Irish potato famine of the nineteenth century: when the potato crop failed because of a blight, over one million people died and even larger numbers emigrated. Potatoes remain a very important food of people living in the Andean countries of South America. Much research on this crop has been conducted in Peru. From Europe, potatoes travelled to Africa and Asia, where they are grown in higher cool areas. If well cultivated in the right soil and climate, they can give a very high yield per hectare.
Like other starchy tubers, potatoes contain only about 2 percent protein, but the protein is of reasonably good quality. Potatoes also provide small quantities of B vitamins and minerals. They contain about 15 mg vitamin C per 100 g, but this amount is reduced in storage. The keeping quality of potatoes is not good, unless they are stored carefully.
Arrowroot
Arrowroot, which is grown in areas with adequate rainfall, is liked by certain peoples in Africa and Oceania. The nutritive value of arrowroot is similar to that of potatoes. The roots are eaten in a variety of ways, often roasted or boiled.
Bananas and plantains
Strictly speaking, bananas and plantains should be discussed with fruits; from the nutritional point of view, however, they are more appropriately considered under starchy foods. It is difficult to differentiate among the many varieties of plantain and of banana. For the purposes of this book, plantains may be described as bananas that are picked green and are cooked before eating. Plantains contain more starch and less sugar than bananas, which are usually eaten raw like other fruits.
Bananas and plantains originally grew wild in damp, warm forest areas. They have probably been used as food by humans since earliest times. Bananas and plantains are now cultivated extensively in many of the humid tropical areas. Some peoples such as the Buganda in Uganda and the Wachagga in the United Republic of Tanzania depend on plantains as their main food.
A 100-g portion of green bananas or plantains provides 32 g carbohydrate (mainly as starch), 1.2 g protein, 0.3 g fat and 135 kcal. Plantains also have a high water content. Their very low protein content explains why kwashiorkor commonly occurs in young children weaned on to a mainly plantain diet. Bananas usually contain about 20 mg vitamin C and 120 mg vitamin A (as beta-carotene equivalent) per 100 g. For this reason fresh fruits and vegetables are much less important in the diet for those whose staple food is banana than for those whose staple is a cereal or root. Bananas are, however, low in their content of calcium, iron and B vitamins. As bananas supply only 80 kcal per 100 g, about 2 kg must be eaten to provide 1 500 kcal.
Plantains are usually picked while they are still green. The skin is peeled off and they are then either roasted and eaten, or, more commonly, cut up, boiled and eaten with meat, beans or other foods. Plantains are frequently sun-dried and made into a flour.
Sago
Sago (Metroxylon sp.) is almost pure starch and comes from various forms of the sago palm. The trees are widely grown in Indonesia, but sago as a food is particularly popular in certain Pacific islands. Sago has low protein content.
Sugar
Sugar, as sold in shops, is almost 100 percent sucrose and is essentially pure carbohydrate. In Africa, Asia and Latin America nearly all locally produced sugar comes from sugar cane, while in Europe and North America some comes from sugar beet.
In areas where much sugar cane is grown, the consumption of sugar or sugar-cane juice (chewed cane) is often high. In other parts of the world the consumption of sugar tends to rise with economic advancement. In the United States and the United Kingdom in 1995 about 18 percent of energy consumed came from sugar (sucrose), mainly in sweetened foods. In contrast, in many African countries less than 5 percent of energy comes from sucrose. Sugar is a good and often inexpensive source of energy and can be a valuable addition to bulky energy-deficient diets. Contrary to popular belief, customary consumption of sugar is not related to obesity, diabetes, hypertension or any other non-communicable disease. Frequent sugar consumption can be associated with dental caries when coupled with poor oral hygiene, but sucrose is no more cariogenic than other fermentable sugars.
White sugar contains no vitamins, protein, fat or minerals. Many people find that its sweet taste adds to the enjoyment of eating. The yields of energy per hectare of land are very high on productive sugar estates.
Honey
From time immemorial honey has been extensively gathered in developing countries from wild hives. Now more and more hives are being kept, often in hollowed and suspended pieces of tree-trunk or in other more managed ways. The incentive to keep bees tends to be the high price of beeswax rather than just the honey.
Honey has gained the false reputation of being of special nutritive value. In fact it contains only sugar (carbohydrate), water and minute traces of other nutrients. Although merely a source of energy, it has sensory value as a pleasant food for humans.
Beans, peas, lentils, groundnuts and their like belong to the botanical family of Leguminosae. Their edible seeds are called legumes or pulses. Agriculturally the plants of this group have the advantage of being able to obtain nitrogen from the air and also add some to the soil, whereas most other plants take nitrogen from the soil and do not replace it. Legumes usually thrive best when they can get water early in their growth and then have a warm dry spell for ripening. They are therefore often planted at the end of the rains to ripen early in the dry season.
In Africa, Asia and Latin America the seeds are usually left on the plant to reach full maturity and are then harvested and dried. Some may be picked earlier and eaten while partly green, as in Europe and North America.
The dried seeds can be kept and stored in much the same way as cereals.
Some varieties are susceptible to attack by weevils; spending a small amount of money on insecticides to prevent this is definitely economically sound. However, care must be taken to ensure that an excess of insecticide is not applied, that the insecticide is relatively safe and that the beans are well washed before cooking.
The legumes are very important from a nutritional point of view because they are a widely available vegetable food containing good quantities of protein and B vitamins in addition to carbohydrate. Some legumes, such as groundnuts and soybeans, are also rich in oil. They usually supplement very well the predominantly carbohydrate diet based on cereals. Most legumes contain more protein than meat, but the protein is of slightly lower quality because it has less methionine. However, when pulses and cereals are eaten together at one meal they supply a protein mixture containing good quantities of all the amino acids, which improves the protein value of the diet. Legumes also contain some carotene (provitamin A) and ascorbic acid if eaten green. Similarly, dried legumes allowed to sprout before eating have good quantities of ascorbic acid. Some legumes contain antivitamins or toxins (see Chapter 34).
Unless there is a very good reason for introducing a new crop such as soybeans, it is more sensible to encourage increased production and consumption of whatever legume is already grown and popular in any area. The local people will have a taste for it, and agricultural conditions are usually suitable. It is also highly important to try to introduce beans (and other pulses) into the diet of children at an early age. Children are just as able as adults to digest beans easily.
Beans, peas, lentils and grams
A wide variety of beans, peas, lentils, grams, etc. are grown and are important in the diet of people in Asia, Africa and Latin America. All three regions have indigenous legume varieties but also grow varieties that originated on other continents.
There are many kinds of beans. Haricot or kidney beans (Phaseolus vulgaris) were originally from the Americas but are now widely grown in Asia and Africa. Broad beans (Vicia faba) are more common in temperate areas. Lima beans (Phaseolus lunatus) originated from Peru but are eaten all over the tropics and subtropics. Mung beans (Phaseolus aureus), indigenous to the Indian subcontinent, are small seeds but very popular. Scarlet runner beans (Phaseolus multiflorus) are popular as a fresh vegetable in Europe and North America, but the large mature seeds are eaten dried in many countries.
Lentils (Lens esculenta) and some similar legumes often known as grams are very important in the diets of people in many developing countries. Lentils have been cultivated for food by humans for thousands of years. The plants are of small size, as are the seeds. Grams include the important pigeon pea (Cajanus cajan), chickpea (Cicer arietinum) and green gram or mung bean (Phaseolus aureus). In many South Asian countries various dhals made from these legumes form a significant part of the diet, providing important nutrients to supplement the staple food, which may be rice or wheat. In many parts of Africa both cowpeas and pigeon peas are grown and consumed. The pigeon pea is perennial and relatively drought resistant. Lathyrus sativus, another drought-resistant legume, is grown widely in India, but consumption of large amounts can lead to the severe toxic condition called lathyrism (see Chapter 34). Winged bean (Psophocarpus tetragonolobus) is another important legume with a very high protein content (35 percent), but it is not yet widely grown.
Peas are commonly consumed as a green vegetable (fresh, canned or frozen) in Europe and North America and by more affluent people elsewhere. In developing countries the seeds are allowed to mature and are dried and consumed in the same manner as other legumes.
These legumes (excluding soybean) all have a somewhat similar nutritive value, but the mature beans are eaten in a variety of ways and have different flavours and other culinary qualities. Most legume seeds usually contain about 22 percent protein (as opposed to 1 percent in cassava roots and 10 percent in maize) and good quantities of thiamine, riboflavin and niacin; in addition they are richer in iron and calcium than most of the cereals.
The large number of other legume seeds of various shapes, colours and sizes on sale at food shops or marketplaces in almost any village or town in tropical countries is evidence of an appreciation of dietary variety and culinary finesse. Culture and local taste importantly determine how these foods are eaten.
Soybean
Soybean (Glycine max) originated in Asia, but now the main producers are the United States and Brazil. However, the soybeans produced by these countries are mainly used commercially for oil and as animal feeds. Asia still produces much of the soybeans for direct human consumption. They are not widely grown in Africa or Latin America.
Soybeans contain up to 40 percent protein, 18 percent fat and 20 percent carbohydrate. The protein is of a higher biological quality than that from other plant sources.
Soybeans, used in a wide variety of ways, are very important in the diets of the Chinese and in those of some other Asian countries. In China soybeans are made into a variety of tasty dishes which supplement the staple food of rice or other cereal. Soy products such as tofu (soybean curd) and tempeh (a fermented product) are important in Indonesian cuisine and popular elsewhere. Soybeans have not become a popular food in Africa or Latin America, where there is little local knowledge of the best methods of preparing them. People lacking experience with soybeans find them difficult to prepare and cook.
Where soybeans are grown they can be locally processed for use in the country as an enrichment of cereal flours, as an infant food or for institutional and school-feeding purposes. The oil can be exported and the protein-rich residue cake can be utilized in the country.
Groundnuts (peanuts, monkey-nuts)
The term "groundnut" is a misnomer since, although botanically a nut, the groundnut (Arachis hypogaea) is a true pulse, a member of the Leguminosae family. It originated in Brazil but is now extensively grown in warm climates around the world. It is an unusual plant in that the flower stalk bearing the ovary burrows into the ground, where a nut containing the seed or seeds develops.
Groundnuts contain much more fat than other legumes, often 45 percent, and also much more niacin (18 mg per 100 g) and thiamine, but relatively little carbohydrate (12 percent). The protein content is a little higher than that of most other pulses (27 percent). Groundnuts are an unusually nutritious food with more protein than animal meat. They are energy dense because of their oil, and they are rich in vitamins and minerals. As suggested in Chapter 9, if every child, woman and man in Africa ate a handful of groundnuts per day in addition to their normal diet, Africa would be rid of most existing malnutrition.
Groundnuts are fairly widely grown in the tropics. Farmers should produce them for home consumption as well as for cash crops, since they form a very useful addition to the primarily cereal or root diets of many poor families. They supply much-needed fat, which is high in energy and assists in the absorption of carotene as well as serving other functions. In predominantly maize diets, relatively small quantities of groundnuts, with their high content of niacin and also of protein (including the amino acid tryptophan), can prevent pellagra. When groundnuts are added to children's diets, their high protein and energy content serves to prevent protein-energy malnutrition.
However, groundnuts are often grown mainly as a cash crop even in developing countries. The world's largest producer is the United States. Groundnuts are usually utilized for oil extraction, and the residue, groundnut cake, is used for animal feed. In the United States a good proportion is consumed as peanut butter. In many countries groundnuts are consumed roasted, boiled or cooked in other ways.
Groundnuts, if damaged during harvesting or if poorly stored in damp conditions, may be attacked by the mould Aspergillus flavus. This fungus produces a poisonous toxin known as aflatoxin, which has been shown to cause liver damage in animals and to kill poultry fed on infected groundnuts. It may be toxic also for humans and may be a cause of liver cancer (see Chapter 34).
Bambara groundnut
The bambara groundnut (Voandzeia subterranea) originated in Africa and is grown widely. It resembles the groundnut physically but is not nutritionally similar, having only 6 percent fat. Its protein content of 18 percent is a little lower than that of most other pulses, but it has about the same mineral and vitamin content as beans. Because of the lower fat content the crop is not in great demand for oil production. Therefore, instead of being sold as a cash crop, it is more often used locally for food.
Coconut
Coconut is the most important nut crop in Africa. Its origins are uncertain. The nut, being light and impervious to water, no doubt drifted across many seas to germinate on a new shore. It is now extensively cultivated. The tree that bears it is a picturesque and highly useful plant, apart from the food it provides for humans. When it is green, the nut contains about half a litre of water; this is a very refreshing and hygienic drink, but apart from a little calcium and carbohydrate, it has no nutritive value. The white flesh, however, is rich in fat.
The flesh of the coconut is usually sun-dried into copra. The oil from copra is used both for cooking and for making soap. Copra itself is used in the tropics and elsewhere as an addition to many dishes. It is an important ingredient in a variety of cuisines from Thailand to Saudi Arabia. Coconut oil has the disadvantage of containing a relatively high proportion of saturated fatty acids. The coconut sap in many countries is fermented to yield alcoholic beverages.
Cashew nut
The cashew nut is produced on a small tree that originated in dry areas of the Americas. It is widely grown in the tropics, and the nuts are mainly exported. They are rich in fat (45 percent) and contain 20 percent protein and 26 percent carbohydrate. The edible swollen stalk of the nut contains good quantities of vitamin C. Cashew nuts are a useful local food but too expensive for most people.
Sesame
Sesame, or simsim (benniseed in West Africa), is grown fairly widely throughout the world and is largely used for oil extraction. The seeds, which are of various colours, contain about 50 percent fat and 20 percent protein. They are also rich in calcium and contain useful quantities of carotene, iron and B vitamins. Sesame seeds can form a nutritious addition to the diet.
Sunflower seeds
Sunflowers are grown mainly as a cash crop, but some of the seeds and some of the oil are eaten locally. The oil has the advantage of being relatively high in polyunsaturated fatty acids. The seeds contain about 36 percent oil (less than sesame), 23 percent protein and some calcium, iron, carotene and B vitamins.
Red palm oil
The product from oil-palm (Elaeis guineensis) is discussed in Chapter 30 with other oils and fats.
Other oilseeds
A number of other oil-rich seeds are eaten or used for oil extraction. These include pumpkin seeds, melon seeds, oyster nut (Telfairia pedata) and cottonseed. The last is a major source of oil in the cotton-growing areas of Asia, Africa and Latin America. In West Africa and elsewhere, shea butter (Butyrospermum parkii), butternut and several other oilseeds are used in the diet. Most of these grow on indigenous trees.
The foods called vegetables include some fruits (e.g. tomatoes and pumpkins), leaves (e.g. amaranth and cabbage), roots (e.g. carrots and turnips) and even stalks (e.g. celery) and flowers (e.g. cauliflower). Many of the plants from which these various edible parts are taken are unrelated botanically. However, "vegetable" is a useful term both in nutrition and in domestic terminology.
In developing countries, nearly all types of vegetables are eaten soon after they are harvested; unlike cereals, tubers, starchy roots, pulses and nuts, they are rarely stored for long periods (with a few exceptions such as pumpkins and other gourds).
It is not uncommon for rural people in parts of Asia, Latin America and Africa to forage for an important proportion of the vegetables they consume. With increasing population, however, the availability of wild fruits and vegetables is decreasing. Therefore vegetables are obtained from the farm or the household garden or from the marketplace, neighbours or small stalls along the roadside. When rural families with low income move to an urban environment they may resent having to purchase vegetables, because they are used to being able to gather wild ones or grow their own. They may therefore spend relatively little on this component of the diet. In any case, vegetables are rarely a prestige food, and in few societies are they high on the list of food preferences.
Vegetables are a very important part of the diet. They are nearly all rich in carotene and vitamin C and contain significant amounts of calcium, iron and other minerals. Their content of B vitamins is frequently small. They usually provide only a little energy and very little protein. A large proportion of their content consists of indigestible residue, which adds bulk or fibre to the faeces.
In many tropical diets the dark green leaves are the most valuable vegetables because they contain far more carotene and vitamin C, as well as more protein, calcium and iron, than pale green leaves and other vegetables. Thus amaranth is much superior to cabbage or lettuce. Leaves from pumpkin, sweet potato and cassava plants, as well as many wild edible leaves, are also excellent.
An increase in the consumption of green leaves and other vegetables could play a major part in reducing vitamin A deficiency, which is often prevalent in children, and could contribute to lessening the prevalence of iron deficiency anaemia in all segments of the population but especially in women of child-bearing age. Increased vegetable consumption would also supply additional calcium and vitamin C which would prevent the rare disease scurvy and perhaps also assist the healing of ulcers and wounds. Vitamin C also enhances iron absorption.
It is not possible here to describe the individual properties of the many vegetables commonly eaten in developing countries. A few, such as pumpkins, can be stored for several months with little loss of nutritive value; others, such as leaves and even tomatoes, are frequently sun-dried, but with considerable loss of vitamin content. The vitamin C content of vegetables is also lowered by prolonged cooking.
Vegetables grown in home and school gardens could be a valuable source of food for the family and the school and could make an important nutritional contribution, particularly to micronutrient intake. Home gardens can be raised with spare family labour and the participation of women and children. It is therefore important for most rural households and virtually every school to devote more time to growing vegetables. A community garden near the village source of water is often a useful adjunct to the villagers' own backyard gardens. In towns, even the smallest piece of land behind a house could, with the assistance of waste water, yield a valuable supply of vegetables all year round. The allocation of allotment plots for vegetable growing deserves serious consideration by town councils and other urban authorities. Even people living in flats can grow certain varieties in pots kept on their verandas.
A wide variety of fruits grow wild or are cultivated in tropical countries. The varieties available at any one time in a given area depend on the climate, the local tastes for fruit, the species cultivated and the season.
The main nutritive value of fruits is their content of vitamin C, which is often high. Some fruits also contain useful quantities of carotene. Fruits (except the avocado and a few others) contain very little fat or protein and usually no starch. The carbohydrate is present in the form of various sugars. Fruits, like vegetables, contain much unabsorbable residue, mainly cellulose. The citrus fruits, such as oranges, lemons, grapefruits, tangerines and limes, contain good quantities of vitamin C but little carotene. In contrast, papayas, mangoes and Cape gooseberries (Physalis peruviana) contain both carotene and vitamin C.
Papayas are a useful fruit, especially for those who cultivate a piece of land for a few years and then move on to new land. The papaya grows rapidly and may yield fruit after one or two years. The mango, on the other hand, grows slowly, but once established (and it may establish itself) needs no care and yields fruit for half a century. Guavas, which are quite widely grown, contain five times as much vitamin C as most citrus fruits, as well as useful amounts of carotene.
The avocado requires special mention because, unlike other fruits, it is rich in fat, a substance that is lacking in many tropical diets. It could with benefit be much more widely grown and eaten and fed to children.
Bananas are widely grown and eaten in tropical countries. They contain fair quantities of carotene and vitamin C, and they are rich in potassium. In East Africa plantains or bananas are commonly picked when green. Cooked and eaten as a mainly starchy food, they form the staple diet of many people. When bananas are ripe their starch is converted into other sugars.
A few fruit-trees would be a useful addition to all households, both urban and rural.
Foods of animal origin are not essential for an adequate diet, but they are a useful complement to most diets, especially to those in developing countries that are based mainly on a carbohydrate-rich staple food such as a cereal or root crop. Meat, fish, eggs, milk and dairy products all provide protein of high biological value, which is often a good complement to the limiting amino acids in plant foods consumed. These products are also rich in other nutrients. The iron provided by meat and fish is easily absorbed and enhances the absorption of iron from common staple foods such as rice, wheat or maize. However, foods of animal origin are usually relatively expensive and not within the purchasing power of poorer families. Some wealthier people in both developing and industrialized countries consume large quantities of these foods; in consequence their intake of fat, especially saturated fat, may become excessive, increasing the risks of heart disease and obesity. Americans consume about 80 kg of meat per person per year - almost 0.25 kg per day.
Meat is usually defined as the flesh (mainly muscles) and organs (for example, liver and kidneys) of animals (mammals, reptiles and amphibians) and birds (particularly poultry). Meat is sometimes subdivided into red meat (from cattle, goats, sheep, pigs, etc.) and white meat (mainly from poultry). The animals providing meat may be domesticated or wild. The amount of meat consumed often depends mainly on cultural factors, on the price of meat in relation to incomes and on availability.
Meat contains about 19 percent protein of excellent quality and iron that is well absorbed. The amount of fat depends on the animal that the meat comes from and the cut. The energy value of meat rises with the fat content. The fat in meat is fairly high in its content of saturated fatty acids and cholesterol. Meat also provides useful amounts of riboflavin and niacin, a little thiamine and small quantities of iron, zinc and vitamins A and C. Offal (the internal organs), particularly liver, contains larger quantities. Offal has a relatively high amount of cholesterol. In general all animals - wild and domestic, large and small, birds, reptiles and mammals - provide meat of rather similar nutritional value. The main variable is the fat content.
Worldwide, a vast range and variety of animal products are eaten. Not all of them are popular everywhere, of course. Certain foods that are popular in some parts of the tropics and East Asia - such as locusts, grasshoppers, termites, flying ants, lake flies, caterpillars and other insects; baboons and monkeys; snakes and snails; rats and other rodents; and cats and dogs - are not found in European or North American diets. Similarly, the French liking for frogs' legs and horse meat and the English and Japanese taste for eels and raw oysters are not shared by many people living elsewhere. Liked or disliked, however, all these foods are nutritious and contain protein of high biological value.
Contaminated meat can lead to disease. There is a need for improvements in conditions associated with production of meat both for local or family consumption and more importantly for commercial sale. For meat to be safe for human consumption, hygienic practices are essential at all levels, from the farm, through the slaughterhouse, to the retailer and into the kitchen. Most countries have regulations governing meat hygiene and authorities responsible for applying the regulations, but their effectiveness varies widely.
Fish and seafoods, like meat, are valuable in the diet because they provide a good quantity (usually 17 percent or more) of protein of high biological value, particularly sulphur-containing amino acids. They are especially good as a complement to a cassava diet, which provides little protein.
Fish varies in fat content but generally has less fat than meat. Fish also provides thiamine, riboflavin, niacin, vitamin A, iron and calcium. It contains a small quantity of vitamin C if eaten fresh. Small fish from the sea and lakes such as sardines and sprats (dagaa in the United Republic of Tanzania, kapenta in Zambia) are consumed whole, bones and all, thus providing much calcium and fluorine. Dried dagaa, for instance, may contain 2 500 mg calcium per 100 g. Fish offal is not usually consumed as part of any diet anywhere. However, fish liver and fish oils are very rich sources of the fat-soluble vitamins A and D. The amount varies, usually with the age and species of fish.
Wherever water is available, fish provide a simple way of increasing protein consumption. The stocking of dams, the construction of fish ponds and better and more widespread fishing in rivers, lakes and the sea should all be given greater encouragement.
There is much regional variation in the variety of sea creatures people will eat. Encouraging children in coastal districts to collect sea urchins, sea slugs, limpets and the numerous other edible sea creatures, just as inland children collect locusts and lake flies, would considerably improve poor diets. The introduction of swimming lessons in youth clubs and as a community development activity would encourage development of this pastime as well as fishing both for pleasure and for profit; fear of the water because of inability to swim is a deterrent to these activities, particularly among people who do not live beside water.
The egg is one of the few foods containing no carbohydrate. Just as the foetus in the mother's uterus draws nutrients from the mother's blood in order to grow and develop into a human being, so the bird embryo draws all its nutrients from within the egg. It is not surprising therefore that eggs are highly nutritious. Each egg contains a high proportion of excellent protein, is rich in fat and contains good quantities of calcium, iron, vitamins A and D and also thiamine and riboflavin.
Eggs are an essential part of the reproductive cycle of birds, so it is hardly surprising that their consumption, particularly by females, is forbidden by taboos in many societies. The irony is that eggs are often more easily available than most other high-quality foods. In developing countries it is not often that a family can afford to kill a cow or even a goat for food, but eggs are small and frequently laid. They are also an easily prepared, easily digestible, protein-rich food suitable for children from the age of six months onward. Eggs do have a nutritional disadvantage: very high cholesterol content. The cholesterol is present in the yolk.
Production of eggs for family use should be encouraged wherever possible, even in the small garden or yard of an urban dwelling. Toddlers should be given priority in eating the eggs.
Cattle blood, which is regularly consumed raw by many pastoral peoples, particularly in Africa, is highly nutritious. It is rich in protein, has high biological value and contains many other nutrients. It is a particularly valuable source of iron. It is also a good source of nutrients in its processed form, usually a type of sausage.
Animal milks and other dairy products are highly nutritious and can play an important part in human diets for both children and adults. The composition of milk varies according to the animal from which it comes, providing the correct rate of growth and development for the young of that species. Thus, for human infants, human milk is better than cows' milk or any other milk product. Exclusive breastfeeding without other foods or liquids is the optimum means of feeding for the first six months of an infant's life (see Chapter 7). Continuing breastfeeding for many more months is of great value, while the baby is introduced to other foods. If breastmilk remains an important food for the child into the second or even third year of life, then animal milk is not necessary in the child's diet.
The composition of human and cows' milk is compared in Chapter 7 (Table 7). Except for certain vitamins, the composition of human breastmilk is fairly constant, regardless of the diet of the mother. Maternal malnutrition will not cause a mother to produce milk of markedly lower nutrient content, but it will reduce the quantity she can produce. A few nutrients such as thiamine and vitamin A may be low if mothers are deficient in these nutrients.
Caseinogen and lactalbumin, proteins of high biological value, are among the most important constituents of cows' milk. The carbohydrate in cows' milk is the disaccharide lactose. Fat is present as very fine globules, which on standing tend to coalesce and rise to the surface. The fat has a rather high content of saturated fatty acids. The calcium content of cows' milk (120 mg per 100 ml) is four times that of human milk (30 mg per 100 ml), because calves grow much more quickly and have a larger skeleton than human babies and therefore need more calcium. When a human infant is fed entirely on cows' milk the excess calcium does no good but causes no harm. It does not produce a rate of growth beyond the optimum. The excess is excreted in the urine.
Milk is also a very good source of riboflavin and vitamin A. It is a fair source of thiamine and vitamin C, but it is a poor source of iron and niacin. The mother usually provides her infant with a store of iron before birth. However, this store is exhausted by about the sixth month of life, and if feeding of milk alone is prolonged, iron deficiency anaemia may develop.
The amount of thiamine in human milk varies more than the other constituents and is largely dependent on the mother's intake of this vitamin. Infantile beriberi may occur in infants breastfed by thiamine-deficient mothers. The vitamin A content of human milk is to some extent dependent on the diet of the mother.
Despite the variation in the composition of milk from different animals, all milk is rich in protein and other nutrients and constitutes a good food for humans, especially children. Although most animal milk for human consumption comes from cows, in certain societies the milk of buffaloes, goats, sheep and camels is important. Some peoples have taboos against milk.
In many parts of the world, milk is more often consumed sour or curdled than fresh; in fact, some people dislike fresh milk. There is no need to alter this habit, for curdled milk keeps longer, retains its nutritive value and may be more digestible and more hygienic than fresh milk. However, it is much safer to drink milk that has been boiled and kept in a clean container, because milk can provide a vehicle for the transmission of some disease-causing organisms.
Pasteurization of milk carried out efficiently in a large, well-run dairy greatly reduces the risk of pathological organisms spreading, provided that the milk is placed in clean containers destined for direct delivery to the consumer. However, in many small towns where pasteurization is not well controlled, the milk may be insufficiently heated, the containers may not be well cleaned, and the milk may go from the plant into large churns for bottling elsewhere in insanitary surroundings. The consumer should not be overconfident in all milk labelled "pasteurized", since it is not necessarily free from pathological organisms.
In many countries where cows' milk is a normal item of the diet, it is customary to wean infants from breastmilk on to a diet in which cows' milk plays an important part. This is a valuable practice, for it helps ensure that the child will receive a balanced diet that provides all the requirements for growth, development and health.
Some people limit their milk consumption because of lactose intolerance, a condition resulting from low levels of the digestive enzyme lactase, which is responsible for digesting lactose, the main carbohydrate in milk. It is probably normal for human adults to have low levels of intestinal lactase, and the condition is very common in non-white peoples. Research shows that most lactose-intolerant persons can in fact consume milk in moderate quantities (perhaps three to five cups of milk per day) without developing symptoms.
Skimmed milk and dried skimmed milk
Skimmed milk is milk from which the fat has been removed, usually for making butter. In its dried form (DSM), it is a familiar product in many countries. It contains nearly all the protein of milk, as well as the carbohydrate, calcium and B vitamins. It is an excellent food, especially for those on predominantly carbohydrate diets and those who have extra needs for protein. In some places DSM is supplied to those with special needs through clinics and health centres. It is extensively used in hospitals and dispensaries as the basis for the treatment of protein-energy malnutrition (PEM). It is also issued at child-welfare clinics to prevent this most devastating form of malnutrition. Skimmed milk is an excellent food to add to any diet, but it is particularly useful in the diets of children and pregnant and lactating women. However, it is not a suitable substitute for whole milk for infants. It is sometimes added to dietary supplements such as, for example, corn (maize)/soybean/milk mixture (CSM).
Whole powdered milk
This product, as the name implies, is whole milk that has been dried. Unlike DSM, it contains fat. It is suitable for infants when no breastmilk is available.
Evaporated and condensed milks
These are milks that have had much of their water removed but that are still liquid. Condensed milk is sweetened by the addition of sugar, whereas evaporated milk does not contain added sugar. Many brands of condensed milk have vitamins added. These brands should be preferred to those that do not have vitamins added, especially if they are used in the diets of young children. They are not suitable as breastmilk substitutes for infants.
Yoghurt and soured or fermented milks
Many different organisms are used in the process of making yoghurt and fermented milks. These products are easy to prepare, are highly nutritious, have enhanced keeping quality and are a little less likely than fresh milk to harbour pathogenic organisms. Their use should be encouraged.
Casein
Casein is the protein from milk. It tends to be rather expensive. It is commonly mixed as part of a formula or mixture for treatment of children with PEM (see Chapter 12).
Cheese
The making of cheese no doubt arose from the desire of farm people to preserve some of the excess milk of the summer. Numerous processes are used, but essentially cheese is made by letting milk clot and subsequently removing some of the water. Salt and other flavourings may be added. Cheese-making is an excellent way of using any excess milk produced during the seasons when milk yields are high.
Butter and ghee
Butter and ghee are both milk products, but being mainly fat they are discussed in Chapter 30, "Oils and fats".
In general adults should consume at least 15 percent of their energy intake from dietary fats and oils, and women of childbearing age should consume at least 20 percent. Active individuals who are not obese may consume up to 35 percent and sedentary individuals up to 30 percent of energy from fat as long as saturated fatty acids do not exceed 10 percent of the energy intake and cholesterol intake is limited to 300 mg per day. Annex 1 gives levels of fat intake for low-income countries calculated according to the recommended range (15 to 35 percent) of dietary energy from fat.
Infants fed human milk or formula usually receive 50 to 60 percent of their total energy from fat. Infants should receive breastmilk, but if they do not, the fatty acid composition of infant formula should correspond to the range found in the breastmilk from omnivorous women. During complementary feeding up to two years of age or beyond, the diet should provide 30 to 40 percent of energy from fat.
To achieve the recommended levels of fat intake, poor people, particularly in developing countries, would need to increase their intake of fat and oils. In contrast, most people living in rich industrialized countries would need to reduce their consumption of fat and oils, which now often provide 40 percent or more of the energy they consume.
The fat consumed in human diets is often divided into two categories: "visible" fat such as cooking oil and "invisible" fat such as the oil naturally present in cereals and legumes. Persons in developing countries who may get only 15 percent of their energy from fat will often obtain two-thirds as invisible fat and one-third as visible fat (or fat added to food). In contrast, in North America and Europe, where mean intakes of fats are high, some 70 percent may be visible fat and 30 percent invisible fat.
A diet very low in fat tends to be unpalatable and dull. It is difficult to cook a really good meal without any fat or oil, although the desired amount is largely a matter of habit and taste. However, like animal proteins, fats are relatively expensive, so the diet of poorer people is often short of fat. Fat is important because weight for weight it provides more than twice as much energy as carbohydrate or protein, thus reducing the bulk of the diet. Fats and oils may be good sources of fat-soluble vitamins, and they assist with the absorption of other nutrients. Recent work has established that certain unsaturated fatty acids are essential for pre- and postnatal development of the brain in children and are also essential for health in adults.
Fats contain a variety of fatty acids. Fats derived from land animals (e.g. butter and lard) usually contain a high proportion of saturated fatty acids and are solid at room temperature. Fats derived from vegetable products and marine animals (e.g. groundnut and cod-liver oils) contain more unsaturated fatty acids; they are usually liquid at room temperature and are termed oils. Coconut oil is an exception in that it contains mainly saturated fatty acids. A high intake of saturated fatty acids may contribute to raised serum cholesterol levels, which in turn may increase the risk of coronary heart disease.
Butter consists mainly of the fat from milk. It usually contains about 82 percent fat, with a trace of protein and carbohydrate; the rest is water. Butter is rich in vitamin A and has a small amount of vitamin D, but the content varies with the time of year and the diet of the cow from which it was derived. Usually about 800 mg of retinol and 50 IU of vitamin D are present in 100 g of butter. Butter and margarine are increasingly used in diets in developing countries as the use of bread increases.
Developed as a substitute for butter, margarine is made from various vegetable oils that are partially hydrogenated to give a product with a consistency similar to that of butter. In most countries vitamins A and D are added so that the final product is nutritionally very similar to butter. If these vitamins have been added, they will usually be mentioned on the margarine container.
Ghee is made by heating butter to precipitate the protein, which is then removed. Ghee contains 99 percent fat, no protein or carbohydrate, about 2 000 IU of vitamin A per 100 g and some vitamin D. It has good keeping qualities and is much used in tropical countries in place of butter, because butter soon goes rancid if kept unrefrigerated in warm temperatures.
Lard is collected during the heating of pork. Like other similar animal fats (e.g. drippings, suet), it consists of 99 percent fat and contains no carbohydrate, proteins, vitamins or minerals.
Vegetable oils are the cooking fats most commonly used in Africa, Asia and Latin America, and there are many different kinds. Except for red palm oil, they have the disadvantage of containing no vitamins except vitamin E. They are mainly low in saturated fatty acids.
Commonly used vegetable oils are soybean, olive, maize, groundnut, sunflower, sesame, cottonseed and coconut oils. In their pure form, they are 100 percent fat and contain no water or other nutrients.
Red palm oil is widely produced in West Africa and in certain Asian countries (e.g. Malaysia). In West Africa it is important in human diets, but elsewhere it is exported for soap production and not much consumed locally. The oil contains large quantities of carotene, the precursor of vitamin A, commonly 12 000 µg per 100 g (with a range from 600 to 60 000 µg per 100 g). It is therefore a very valuable food wherever a shortage of vitamin A occurs in the diet. Vitamin A deficiency will not be a problem in areas where all members of the family consume even small quantities of red palm oil. Encouragement should be given to its wider cultivation and consumption.
It is essential that the human body receive water, yet the human taste prefers that much of this water be obtained in the form of beverages. These include beer, wine, spirits, fruit juices, tea, coffee, cocoa, synthetic sweetened soft drinks and aerated waters. Some of these beverages contain small amounts of drugs such as caffeine (tea, coffee and some colas) or alcohol in varying amounts (beer, wine and spirits), and some are sources of minerals and vitamins.
In most countries there are traditional beverages of great variety. In Africa many of these are made from cereal grains that have been soaked and sprouted. These beverages may or may not be alcoholic, and some are useful sources of B vitamins. In other parts of the world local beverages may be made from honey or coconut or any number of local products.
In the industrialized countries aerated soft drinks, often called "sodas", many with a cola base, are highly popular and consumed in huge quantities. In many parts of Africa, Asia, Latin America and the Near East, manufactured soft drinks and sodas are replacing traditional beverages. Most of these sodas provide no significant nutrients other than carbohydrates.
In contrast, fruit juices, either purchased or home-made from fresh fruit, usually contain useful amounts of vitamin C, and some provide carotene. They are good beverages, especially for children.
It is not uncommon to find mothers giving their babies and children orange squash or fruit-flavoured sodas because they were told at the clinic to give them fruit juice. These manufactured beverages are no substitute for fruit juice and will do the child no good; they are simply a waste of money.
Certain vitamin-rich proprietary beverages have been designed for infants and children. Their vitamin content is nearly always clearly stated on the label. They need to be used with caution, however. They are not necessary if the child is getting fresh fruit and vegetables, and they are often a very expensive way of providing vitamin C to a child. The advertising promoting them is pervasive, however, and can persuade mothers that they are useful.
Another major group of beverages comprises those usually consumed hot. Tea, which was probably first drunk in China, is now the favourite beverage of many people in Africa, the Near East and Europe. Coffee originated in Africa but is now drunk most in the Americas, Europe and the Near East. The two main types are Arabian, Coffea arabica, and robusta, Coffea canephora. In all regions of the world tea, coffee and to a lesser extent cocoa are popular beverages. All three provide small amounts of caffeine, which is a mild stimulant. None have any great nutritional significance. Tannin and polyphenols in tea may reduce iron absorption.
For thousands of years people from all continents have produced beverages that contain ethyl alcohol. Usually certain yeasts are used to ferment a local carbohydrate-rich food (for example, cereals or root crops), but fruits, palm sap, honey and other raw ingredients are also used. In the industrialized countries beer (often made from barley), wine (made from grapes) and various spirits (drinks with a relatively high alcohol content made by distillation) are very widely consumed, and this practice has spread to many countries of the South. Alcohol produces a good feeling for many who drink it, but it also impairs the senses, and it can be addictive. It can be claimed that alcohol consumed in moderation provides a sense of well-being and may improve social interaction; but alcohol in excess is a serious cause of automobile and other accidents, and alcoholism is a highly prevalent and very damaging disease in all continents of the world.
Animals and primitive men and women obtained most of their fluids in the form of water; then for thousands of years other beverages became the favourite drink for humans; and now there is almost a craze to drink "natural" or "spring" waters, either aerated or still. Many consumers believe that these waters, coming from springs, lakes, rivers or wells, have near-magical qualities and great nutritive value. This idea is false. Bottled water may contain small amounts of minerals such as calcium, magnesium and fluoride, but so does tap-water from many municipal water supplies. A study comparing popular brands of bottled water showed that they were in no way superior to New York tap-water. They have only the advantage of being safe in areas where tap-water may be contaminated. However, for low-income people bottled waters are very expensive, and boiling local water renders it safe at a much lower cost.
Salt consists mainly of sodium chloride. It is the only mineral salt that humans customarily consume in a chemically pure form. The body has a definite need for sodium and chlorine. The amount of sodium chloride in the body is regulated by the kidneys. In hot countries a person doing heavy work may lose 15 g of sodium chloride in body sweat in one day. Urinary excretion ranges from 1 to 30 g or more per day. Despite this loss, salt is not essential in the human diet unless sweating is profuse, because sufficient sodium and chlorine can be obtained from food alone. Nevertheless, nearly all people use salt, obtaining it by digging, making or buying it, however small the income. Certainly a salt-free diet is unpalatable. Adults usually consume about 10 g of salt a day, but there are enormous variations. A high intake of salt may contribute to the development of hypertension or high blood pressure in some individuals.
Other spices and flavourings are of less physiological or nutritive importance. In all countries, in all ages, people have added such items to their food to improve and vary its taste. In Africa, Asia and Latin America a variety of wild leaves are used, partly for flavour, partly as vegetables per se; hot chilies, both red and green, are frequently used; and pepper and curry powder are popular additions to the sauce or stew accompanying the staple food. Few of these flavourings have much nutritional importance, but all serve to make the food more pleasing to the taste. They therefore both increase the appetite and assist digestion by stimulating the secretion of saliva and intestinal juices. With the march of so-called civilization, many of the traditional and natural condiments and herbs are being replaced by proprietary sauces and flavourings. Some of these are artificial chemical agents (for example, monosodium glutamate) and some are based on traditional spices (garlic, cloves, ginger, etc.).
Humans are unique in the animal kingdom in that they harvest, store and process food that they have grown. Almost all animals harvest food, and many animals store it for later consumption, but they do not grow it or process it. In their evolution from the apes humans learned to grow food for their own sustenance and then to develop many processes to preserve the food or to increase its desirable characteristics, sometimes thus decreasing or improving its nutritional value.
People seek to preserve food and to improve its quality using a variety of techniques such as drying, canning, pickling, adding chemical preservatives, refrigeration, freezing and irradiation. The main aim of these processes is to allow foods to remain in good edible condition, without serious deterioration, for longer than would be possible if these preservation methods were not used. The processes include cooking; adding substances to improve the taste or appearance of the food; taking measures to make the foods more nutritious, for instance adding micronutrients or germinating grains; and removing undesirable constituents, including toxins. Some food processing techniques have multiple effects. For example, milling of cereal grains may make them less nutritious, but it may also make them easier to cook and digest and less likely to deteriorate on storage.
Today food processing includes both traditional and some more industrial and modern techniques. Almost all aspects of food processing have some relevance to nutrition. The effects of various processes, including cooking, on the nutrient content of foods are summarized in Table 35. In addition to those effects, milling and cooking break down cell walls so that nutrients are digested more easily.
Research, teaching and extension regarding modern techniques of food processing are within the domain of food scientists rather than nutritionists. Food science is a very important subject which is advancing rapidly not only in academic institutions but also in the food industry, where large manufacturers often have advanced food science laboratories. Many books deal with food science, and some are included in the Bibliography.
This chapter and Chapter 34 discuss those aspects of food processing that have an impact on the nutritional quality of foods consumed in developing countries or that influence their safety. Fortification of foods with nutrients is an aspect of food processing directly aimed at reducing deficiency diseases.
In ancient times and in traditional societies everywhere, cooking was and is the main food processing technique used. Humans learned to harness and make fire, and cooking their food became a way to improve the quality of their diets. Cooking techniques have changed much over the years in some societies and very little in others. Many people still cook over open fires and on traditional stoves, but in contrast now almost a majority of households in Western Europe and North America have a microwave oven in the kitchen, a relatively new invention. Similarly, industry uses both old and new cooking methods.
TABLE 35
How processing alters the nutrient content of foods
Nutrient |
Processes that decrease the amount |
Processes that increase the amount |
Other effects of processing |
Vitamin A |
Drying, especially in the sun: |
Fortification |
|
Boiling for a long time in contact with air (without a lid on the pan); |
|||
Frying for a long time or at high temperatures |
|||
Thiamine |
Washing of rice |
||
Riboflavin |
Leaving milk in daylight |
||
Folate |
Cooking (for example, in green leaves 35 percent and in potatoes25 percent of folate may be lost); |
||
Storage |
|||
Vitamin C |
Storage (except for citrus or baobab fruits), drying. canning and bottling, cooking and reheating of fresh roots, vegetables and fruits (for example, 40 percent of vitamin C in green leaves may be lost in cooking, as some passes into the water and some is destroyed by heat), |
Germinating of seeds (for example, of legumes) |
|
Chopping the foods into small pieces, preparing them long before cooking and cooking them long before eating |
|||
Minerals |
Milling |
Fortification (for example, salt may be fortified with iodine) |
Fermentation and germination increase the absorption of non-haem iron and other minerals;: |
Milling may remove some minerals but increase their absorption | |||
Carbo-hydrate, fats and protein |
Milling may reduce the amount of fat, protein and fibre |
Milling may increase the proportion of starch: the proportions of starch and |
Fermenting and malting alter sugar: |
Bottling and canning may add sugar; |
Fermenting may add alcohol | ||
Frying increases the fat content |
|||
Water |
Drying foods |
By decreasing water content, drying foods increases the concentration of other nutrients |
Cooking is practised by almost everybody, everywhere. Except for fruits and some vegetables, most groups of foods are generally cooked before being eaten. In many African and Asian countries even vegetables are seldom eaten uncooked, and there is little tradition of eating salads. The practice of cooking vegetables probably helps protect consumers from diseases
spread by faecal contamination including parasitic, bacterial and viral infections of the gastro-intestinal tract. Most tropical fruits are eaten raw, but the exposed peel is not consumed so they do not present the same risk of infections. Bananas, mangoes, papayas and citrus fruits, for example, are not dangerous because their peel is not eaten.
Cooking of food is a universal practice mainly because it improves the taste of food, makes inedible foods edible or makes foods more digestible. Cooking also kills organisms, including many disease-causing microorganisms in food. Cooking of high-starch foods including cereals (rice, wheat, maize, etc., which for most of humankind provide the bulk of the energy and even protein consumed) and also potatoes, yams and cassava makes these foods palatable and also more digestible. Cooking of some foods removes undesirable compounds such as antinutrients, for example trypsin inhibitors in soybeans and undesirable constituents in cassava.
There is more to cooking than merely roasting, baking, grilling, or boiling of foods as gathered or harvested. It usually also involves mixing of foods or perhaps more commonly adding food items to the main food being cooked, which may alter the nutritional value of the main food but is usually intended to make the food, dish or meal taste better. For example, fat is added in frying; salt, sugar, fruit and other products may be added to baked foods; and often the staple food such as potatoes may be cooked in a stew or soup with added onions, tomatoes and small quantities of meat. Cooking can be an art. It makes food tasty and desirable, and in most societies sharing a meal with family and friends is a pleasurable social occasion and is expected to do more than just fill the belly, assuage hunger and provide essential nutrients; it fuels feelings of mutuality and underpins the sense of community.
For all its good points, cooking can have some negative nutritional effects. Frying foods at very high temperatures can destroy some vitamins and can produce undesirable components such as carcinogens in the food. Smoking of food can also produce such substances. Boiling some items in water that is then discarded can remove water-soluble vitamins.
There is intense interest now in the use of traditional germinating methods to produce malted foods. For many years people in the United Republic of Tanzania and other countries have allowed sorghum, millet and other cereals to germinate by soaking the grains in water for some hours, then keeping them damp for two or three days, and finally drying them, often by spreading them in the sun. The dried cereal grains are then pounded using a traditional large pestle and mortar. The resulting flour is stored, and small amounts are used mainly for brewing local beer (pombe). The dried germinated flour, known as kimea, is also used to thin and sour traditional porridges made from maize for child feeding. The kimea thins the porridge because it produces the enzyme amylase, which breaks down the starch (see Chapter 6).
Physical methods
Cooling or freezing greatly prolongs the time it takes for many foods to spoil or become inedible. In this sense it is a very important method of food preservation. Refrigerators are now very common appliances in the homes of better-off people in developing countries and are found in the majority of houses in industrialized countries. Freezers are also widely used.
Other methods used traditionally, but also in industry, are drying and smoking of foods. Removal of water prevents or reduces the ability of organisms to grow and multiply on or in many foods. Organisms thus inhibited include moulds and their toxic products, such as aflatoxin, as well as microorganisms that spoil the food and produce undesirable odours and taste. Dry cereals store better, and dried fish remains edible for relatively long periods. Some foods, such as milk, are dried in factories so that the preserved product can be easily marketed, transported and made available for consumption.
Chemical methods
Food may be kept edible longer by the use of substances termed chemical preservatives. The most widely used in the home are salt (sodium chloride) and sugar, which most homemakers would not consider to be chemical preservatives. Foods with high levels of salt or sugar are less attacked by organisms and so are preserved. Industry also uses salt and sugar to preserve food.
Over 100 years ago chemicals not usually available in the home (as salt and sugar are) were introduced as food preservatives. Some of these are not now used because of fears of toxicity; others are deemed safe and are widely used. International meetings have discussed safety issues, and most industrialized countries have regulations which list permitted food preservatives and concentrations that can be used. Among the widely used preservatives are sulphur dioxide and benzoates, which are mainly effective in controlling moulds and yeasts, respectively. Baked goods such as bread are often preserved using propionic acid, which inhibits the attack and growth of moulds and then prolongs the time before spoilage. Meats, particularly salted meats such as bacon and ham, are further preserved with sodium nitrite and sometimes sodium nitrate.
Sterilization
Both in the home and in the factory, foods of almost all kinds are preserved by a process termed canning, although some are actually put in jars or bottles. In general the foods (vegetables, fruits, meat products and others) are sterilized by heating them to kill all living organisms and are sealed in a can or bottle while still hot. Sometimes salt and sugar are used as part of the process. Home canning or bottling of foods of animal origin, particularly meat or fish of any kind, can be risky. Highly resistant bacteria such as Clostridium botulinum can survive, produce toxins and cause very serious disease (see Chapter 34).
Microbiological methods
Fermentation, which involves chemical breakdown of substances by microorganisms such as yeasts and bacteria, is used traditionally to preserve foods or to improve their palatability in many countries, as is the case with soy products in Indonesia. The process is also used commercially, for example in the manufacture of yoghurt or commercial alcoholic beverages.
Fermentation using yeasts and other organisms which act on the carbohydrate in the food produces alcohol. Humans almost all over the world, without food science classes, have discovered this mechanism and have found that alcohol consumption can be mood changing and pleasant. Thus with any carbohydrate they have, they use some to make alcoholic beverages. The carbohydrate may be a common cereal such as wheat, rice, barley or sorghum, or it may be honey, used to make mead in ancient Britain and modern Africa; coconut sap to make coconut wine in Oceania; or cassava or plantain to make strong drinks called waragi and koinage in Uganda.
Yeast also acts on sugars to produce carbon dioxide gas in food. This principle is used to make bread.
In some foods non-disease-causing organisms are encouraged to multiply to sour the food. Souring results when the microorganisms produce acid from the carbohydrate. Souring foods to some extent prevents pathogenic or harmful organisms from multiplying in the food, which keeps it safer and makes it last longer. Common soured foods are dairy products such as sour milk and yoghurt; fermented soy products such as tempeh; and fermented cereal porridges, consumed in much of sub-Saharan Africa. In some cases souring enhances the nutrient content of the food.
In many countries, including China, pickling is widely used to preserve vegetables and vegetable products.
Other methods
A purely industrial method of food preservation is irradiation. In this process the food is exposed to radiation, usually gamma rays, which kills microorganisms and fungal spores. The food is then sealed and is safe until opened. Irradiation can also be used to prevent or delay sprouting of certain cereals, legumes or other seeds and so to increase their shelf-life. Although irradiated foods are generally regarded as safe, there remains considerable debate about possible hazards of the irradiation process itself.
Fortification is a form of food processing that is of special interest to nutritionists. When properly used it can be a strategy to control nutrient deficiencies. The terms "fortification" and "enrichment" are often used interchangeably. Fortification has been defined as the addition of one or more nutrients to a food to improve its quality for the people who consume it, usually with the goal of reducing or controlling a nutrient deficiency. This strategy may be applicable in nations or communities where there is a problem or a risk of a deficiency of the nutrient or nutrients concerned.
In some instances fortification may be the easiest, cheapest and best way to reduce a deficiency problem, but care needs to be taken to avoid its excessive promotion as a general panacea for controlling nutrient deficiencies. The pros and cons of fortification need to be weighed in each circumstance. Even so, as a strategy to control micronutrient deficiencies in developing countries fortification has often been underutilized, whereas in many industrialized countries it is generally overused. Ironically, it may add nutrients that are not generally lacking for consumers who are not at much risk of deficiency of those nutrients.
Outsiders should not rush into recommending fortification in a particular country. Local professionals need to be much involved in the planning, implementation and monitoring of a fortification programme. It is important to have a very clear picture of the local situation: nutrient deficiencies, food habits, food preparation practices, food processing facilities, marketing practices, etc. Food fortification is easier with one food, such as salt, and where there are very few manufacturers. Under other circumstances fortification is possible, could work and might have a major role in improving nutritional status and reducing the risk of deficiencies, even at the local level. In the past people have tried to find one ideal food to fortify with vitamin A or iron. Now it is recommended that countries consider fortifying several foods at the same time.
Two kinds of fortification that have been highly effective in many countries are the addition of iodine to salt (iodization) and the addition of fluorine to water (fluoridation). In the latter case (see Chapter 21) the fluoride in a municipal water supply is augmented to provide levels that are considered optimal (i.e. one part per million) in order to reduce the incidence of dental caries or tooth decay.
In industrialized countries, and to some extent in developing countries, fortification is used to adjust the nutrient content of processed foods so that nutrient levels are close to those of the food before processing. For example, highly milled cereals such as wheat flour may have nutrients added to replace those lost during the refining process. An alternative would be to insist, or even legislate, that cereals not be highly refined.
Micronutrients
Other chapters in this book describe the important micronutrient deficiencies and ways in which they have been, or can be, controlled. Food fortification offers an important strategy to help control, in particular, the three main micronutrient deficiencies, namely deficiencies of iodine, vitamin A and iron. In developing countries the greatest priority should be given to fortification with these nutrients. With iodine, fortification alone, in the form of salt iodization, is often the only strategy used. With vitamin A and iron, fortification should be used in combination with, not to the exclusion of, other interventions. Particular care needs to be given to possible toxic problems with vitamin A, especially in women who are pregnant or planning to conceive. The advantages of fortification over some of the other strategies for controlling vitamin A and iron deficiencies are often relatively ignored and deserve more attention.
As indicated elsewhere in the book, other micronutrient deficiencies are of some importance in some countries, and fortification may be a good strategy to reduce the prevalence of deficiencies of, for example, niacin, thiamine, riboflavin, folate, vitamin C, zinc and calcium.
Macronutrients
A somewhat different kind of fortification is the addition of macronutrients to enrich food. Enrichment could involve the addition of fat or oil to increase the energy density of a food; the addition of amino acids to cereal products to improve protein quality; or the addition of protein, sugar or oil (as well as micronutrients) to a formulated food, for example a manufactured weaning food, or to a food supplement such as corn (maize)/soybean/milk (CSM) for emergency feeding.
Criteria or principles for fortification
The following are some of the conditions, considerations and principles relevant for those planning to fortify one or more foods to improve nutritional status. They apply mainly to fortification as a strategy to tackle micronutrient deficiencies.
Known nutrient deficiency in the population. Dietary, clinical or biochemical data must show that a deficiency of the nutrient being considered exists to some degree in significant numbers of the population when they consume their usual diet, or that a risk exists.
Wide consumption of the food to be fortified among the at-risk population. The food or foods to be fortified must be consumed by significant numbers of the population who have a deficiency of the nutrient being considered for fortification. If the deficiency disease occurs only in the very poor but they seldom purchase the food that is fortified, then it will do little good. Thus, for example, fortifying a relatively expensive manufactured weaning food with vitamin A may not help the poor children who have the highest prevalence of xerophthalmia if their parents cannot afford to purchase that food.
Suitability of the food and nutrient together. Adding the nutrient to the food must not create any serious organoleptic problems. The items must mix well and this mixing must not cause an undesirable chemical reaction, any disagreeable taste, colour or odour changes or any other unacceptable characteristics.
Technical feasibility. It must be technically feasible to add the nutrient to the food to satisfy the preceding condition.
Limited number of food manufacturers. It is very helpful in a national or even a local fortification programme if there are relatively few manufacturers or processors of the food being considered. For example, if there are hundreds of salt producers, an iodization programme will face major problems. Similarly, if there are many millers, fortifying cereals will be difficult.
No substantial increase in price of the food. It is important to consider the impact of fortification on the price of the food to be fortified. If adding a nutrient greatly increases the price of the food, consumption of the food may decline, particularly among poor people whose families are at special risk of the deficiency. If fortification does increase the price of the food, then it is possible to consider subsidizing the cost.
Range of consumption of the food. Attention should be given to the usual range of consumption of the food being considered for fortification. If there is a very wide range between the smallest amount consumed, perhaps by 25 percent of the population, and the greatest amount consumed, perhaps by another 25 percent of the population, it may be difficult to decide the nutrient level for fortification. If large numbers of people at risk of the nutrient deficiency consume very small amounts of the food, then they may not benefit much from fortification. If significant numbers of individuals consume so much of the food that they may get toxic amounts of the nutrient, then the food may be unsuitable. In general there is a range of consumption of salt, and mean intakes may be 20 g per day, but practically no one consumes 200 g per day, every day. It is important to avoid a situation where people receive undesirable amounts of added nutrients, particularly in the case of fat-soluble vitamins or nutrients known to be toxic in large amounts.
Legislation. When a government is moving in earnest to control a serious micronutrient deficiency using fortification, the appropriateness of legislation needs to be considered. Many industrialized countries have legislation to ensure that required minimum levels of B vitamins and sometimes also iron are present in wheat flour and some other cereal products. Many countries in the North and South have legislation to require that all salt sold is iodized, usually at a particular level. Fluoridation of water supplies to certain levels has been mandated legally, sometimes by municipalities (as in the United States) and sometimes nationally.
Monitoring and control of fortification. Monitoring to provide information on fortification of foods is useful. It is particularly important where fortification is legislated. In this case failure to fortify adequately could lead to prosecution and penalization of delinquent food manufacturers. Monitoring by governments is dependent on the availability of laboratory facilities and the trained personnel. Many countries lack adequate laboratory facilities to monitor salt iodization, and salt merchants are often aware that they can sell salt that is not iodized at all or at the level required by law. A good monitoring system needs to include testing, perhaps at sentinel sites all over the country. In the case of fluoridation, cities often monitor the fluoride content of their water. It is useful if a national laboratory also monitors the level of fluoride in municipal water provided to consumers.
Methods of fortification and suitable foods
The technology of fortification is a complex topic, discussed in many publications. Many different methods are used, and the choice of method depends on both the nutrient and the food.
A frequently used technique in a flour or a fine-grained product involves adding a nutrient premix at a measured rate into the powdery food as it flows at some stage in the processing. Thorough mixing is necessary. This method is suitable for mills and large processing plants. For smaller processing facilities, or even at the village level, packages of premix are supplied. Instructions are given on the proportions to use (for example, one packet per 50 kg of food) and on methods to ensure a good mixture.
Difficulties have been encountered in fortifying rice because it is mainly eaten in a granular or whole-grain form. Therefore adding a powder, which is easy with wheat flour, is not possible with rice. At least two methods have been used. In one, rice grains are coated or impregnated with the nutrients to be used. In the second, artificial rice grains fortified with the desired nutrients are mixed with the rice. The artificial grains have to be well made so that they appear similar to ordinary rice grains. In the Philippines some decades ago it was reported that prior to cooking many housewives removed and threw away the fortified artificial rice grains because they had a yellowish colour from the added thiamine and riboflavin.
Some nutrients such as the B vitamins are relatively easy to add (although riboflavin has the disadvantage of being yellow). Although vitamin A deficiency is of great importance, vitamin A is less easily used than the B vitamins in fortification programmes, in part because it is fat soluble and not water soluble. It is also likely to become oxidized. The most simple means is to add vitamin A to cooking oils and margarine, but food technology has overcome the difficulties, and many different foods have been successfully fortified with vitamin A in both industrialized and non-industrialized countries.
For quite different reasons iron fortification of foods has presented serious challenges. Many different iron salts have been used. Often those that humans utilize best, such as ferrous sulphate, present the greatest difficulties and serious organoleptic problems. As discussed in Chapter 39, sodium iron EDTA is increasingly recommended.
Table 36 lists some nutrients that have been used and their food vehicles.
TABLE 36
Some types of food used as vehicles in fortification programmesa
Nutrient |
Types of food |
Comments |
Ascorbic acid |
Canned. frozen and dried fruit drinks, canned and dried milk products, dry cereal products |
Ascorbic acid must be protected from air if in neutral solution. |
Thiamine, riboflavin and niacin |
Dry cereals, flour, bread, pasta, milk products |
Rice and similar food grains may be impregnated or coated with the nutrient. |
Riboflavin may colour the food. | ||
Nicotinamide is usually preferred to nicotinic acid. | ||
Vitamin A or beta-carotene |
Dry cereal products, flour, bread, pasta. milk products, margarine, vegetable oils, sugar, tea, chocolate, monosodium glutamate |
Vitamin A must be protected from air and or added in water-miscible form to non-fatty products. (It may be added as gelatin based beadlets together with stabilizer as a coating on the food product or mixed into a simulated kernel such as rice.) |
Carotene may colour the products. | ||
Losses due to heat may be great in cooking oils. | ||
Vitamin D |
Milk products, margarine, dry cereal products, vegetable oils, fruit drinks |
See comments for vitamin A. |
Multiple sources of this vitamin may be undesirable. | ||
Calcium |
Cereal products, bread |
The quantity to be added usually limits the range of vehicles that can be used. |
Iron |
Cereal products, bread, canned and dried milk |
Availability varies with the form in which the iron is added. |
Iron may cause colour or flavour changes in the food. | ||
Iodine |
Salt |
Iodide is usually used. |
Iodate Is more stable In crude salt. | ||
Protein |
Cereal products, bread, cassava flour |
Protein concentrates of various types are used. |
The amount to be added usually limits the range of vehicles that can be employed. | ||
Amino acids |
Cereals, bread, meat substitutes |
Other vehicles have been proposed. |
The use of lysine cysteine or methionine is authorized in some areas. | ||
Interest In amino acid fortification has diminished from the early 1970s. |
a In addition, a wide range of nutrients have been added to infant foods and formulations.
