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Recent developments in protein quality evaluation

Faits nouveaux intéressant l'évaluation de la qualité des protéines
Novedades en la evaluación de la calidad de las proteínas

E. Boutrif

Dr E. Boutrif is Senior Officer, Food Quality and Consumer Protection Group, Food Policy and Nutrition Division, FAO, Rome.

Proteins are not alike. They vary according to their origin (animal, vegetable), their amino acid composition (particularly their relative content of essential amino acids), their digestibility, texture, etc. Good quality proteins are those that are readily digestible and contain the essential amino acids in quantities that correspond to human requirements.

Humans require certain minimal quantities of essential amino acids from a biologically available source as part of a larger protein/nitrogen intake. The required amounts of these amino acids vary with age, physiological condition and state of health. It is therefore important to be able to discriminate with both accuracy and precision the relative efficiency with which individual protein sources can meet human biological needs. This efficiency also has direct implications for the commercial value of the protein product.

Clinical human studies that measure growth and/or other metabolic indicators provide the most accurate assessment of protein quality. For reasons of both cost and ethics, such techniques cannot be used. Consequently, assay techniques designed to measure the effectiveness of a protein in promoting animal growth have been utilized. Since 1919, the protein efficiency ratio (PER) method, which measures the ability of a protein to support growth in young, rapidly growing rats, has been used in many countries because it was believed to be the best predictor of clinical tests. However, after decades of use, it is now recognized that PER overestimates the value of some animal proteins for human growth while it underestimates the value of some vegetable proteins for that purpose. The rapid growth of rats (which increases the need for essential amino acids) in comparison to human growth rates is the reason for this discrepancy.

For some time the use of an amino acid score has been advocated as an alternative to the PER. Although clearly the quality of some proteins can be assessed directly by using amino acid score values, that of others cannot because of poor digestibility and/or bioavailability. Consequently, both amino acid composition and digestibility measurements are considered necessary to predict accurately the protein quality of foods for human diets.

Essential amino acid requirements and casein content for children and rats
Besoins en acides aminés essentiels et teneur en caséine chez les enfants et chez les rats
Necesidades de aminoácidos y contenido de caseína en los niños y en las ratas

Essential Amino Acid

(2-5 yrs)

Laboratory rat2
(mg/g protein)






















Methionine and cystine




Phenylalanine and tyrosine
















1 FAO/WHO/UNU, 1985.

2 National Research Council, 1978. Based on a protein requirement of 12 percent plus an ideal protein (100 percent true digestibility and 100 percent biological value).

3 Steinke, Prescher and Hopkins, 1980.

4 A lower rat requirement of 40 mg/g protein for methionine and cystine has also been reported (Sarwar, Peace and Botting, 1985).

On the other hand, the methods currently used for measuring protein quality of foods were established when information was not extensively available on human amino acid requirements. Therefore, while results were not grossly in error, they did not accurately reflect human requirements. Since most of these methods use a rat assay, they in large part measure the amino acid requirements of the rat rather than the human. This is particularly misleading, since the rat appears to have a much higher requirement for sulphur amino acids than does the human (see Table). In addition to the higher requirement for sulphur amino acids, the rat also has a higher requirement for histidine, isoleucine, threonine and valine.


The Codex Committee on Vegetable Proteins (CCVP), while elaborating general guidelines for the utilization of vegetable protein products in foods, felt the need for a suitable indicator to express protein quality. It pointed out at Its first session in 1980 that PER might not be the most suitable means for protein quality evaluation. In the successive two sessions, the committee considered the suitability of other indicators such as the relative net protein ratio (RNPR) (a rat assay procedure) and the amino acid composition data (amino acid scores) corrected for crude protein digestibility/amino acid availability, but no decision was taken. At its fourth session (Havana, 2 to 6 February 1987), the committee noted improvements made in amino acid analysis and amino acid requirement pattern and discussed initial data from ongoing comparative studies organized by the United States Department of Agriculture (USDA) involving amino acid availability, nitrogen digestibility and protein nutritional assessment based on amino acid composition data. The committee concluded that an amino acid scoring procedure, corrected for true digestibility of protein and/or bioavailability of limiting amino acids, is the preferred approach for assessing protein quality of vegetable protein products and other food products. The results of the collaborative studies undertaken on the subject in 1987 and 1988 (Bodwell, Carpenter and McDonough, 1989; McDonough et al., 1990) and recent improvements in amino acid methodology enabled the committee at its fifth session (Ottawa, 6 to 10 February 1989) to endorse the use of the FAO/WHO/UNU (1985) suggested pattern of amino acid requirements of a two- to five-year-old child as the reference for calculating amino acid scores. The committee agreed that amino acid scoring (based on the amount of the single most limiting amino acid) corrected for true digestibility of protein (as determined by the rat balance method) is the most suitable routine method for assessing the protein quality of most vegetable protein products and other food products (Codex Alimentarius Commission, 1989). Because the methodology used to measure protein quality had broad implications beyond its purview, the CCVP recognized the need for the wider scientific community to address issues such as human requirements for essential amino acids, amino acid evaluation methodology, protein digestibility and amino acid availability. The committee accordingly recommended that a joint FAO/WHO Expert Consultation should be held to review the issues. This Expert Consultation on Protein Quality Evaluation was held in Bethesda, Maryland, USA from 4 to 8 December 1989.


The consultation was convened for the task of:

· reviewing present knowledge of protein quality evaluation;
· discussing various techniques used in evaluating protein quality;
· specifically evaluating the method recommended by the CCVP, i.e. amino acid score corrected for digestibility.

The consultation reviewed in particular the scientific basis for the adoption of the protein digestibility-corrected amino acid score method. It recognized that the most serious problem with the use of a rat growth assay in predicting protein quality in food is that rats have a higher requirement than humans for some amino acids. The PER is the official method for assessing protein quality of foods in Canada and the United States, but it has been severely criticized for not meeting the criteria for a valid routine test (Sarwar and McDonough, 1990). A major criticism of the PER assay is that it does not properly credit protein used for maintenance purposes. A protein source may not support growth and may have a PER near zero yet may still be adequate for maintenance purposes. Because of the error introduced by not making allowance for maintenance, PER values are not proportional to protein quality, i.e. a PER of 2,0 cannot be assumed to be twice as good as a PER of 1.0. The lack of proportionality of protein quality makes the PER method unsuitable for the calculation of utilizable protein, as in protein rating (protein in a reasonable daily intake, mass x PER).

The nutritive value of a protein depends upon its capacity to provide nitrogen and amino acids in adequate amounts to meet the requirements of an organism. Thus, in theory the most logical approach for evaluating protein quality is to compare amino acid content (taking bioavailability into account) of a food with human amino acid requirements. A number of comparisons have been made using reference patterns such as those derived from egg and milk protein. The first major change in procedure was substitution of a provisional pattern of amino acid requirements for the egg protein standard.

A hypothetical reference protein derived from the pattern of human amino acid requirements was proposed as a standard for comparison. Shortcomings have been recognized and progress has been made in accurately evaluating human amino acid requirements. Equally critical for success is the ability to obtain precise measurements of amino acid content in the protein sources. Finally, to improve on accuracy of scoring procedures, chemically determined amino acid contents may have to be corrected for digestibility or biological availability.

Conclusions and recommendations Methodology for determining the amino acid composition of proteins. The consultation concluded that modem amino acid analysis can provide data with a repeatability within a laboratory of about 5 percent and a reproducibility between laboratories of about 10 percent. It recommended that this variability be considered acceptable for the purposes of calculating amino acid score. To achieve such results requires careful attention to many aspects of the protocols, including replicating the complete analytical procedure.

The consultation also made the following recommendations.

· Further studies should be undertaken to standardize the hydrolytic and oxidation procedures and improve accuracy of the procedures to further reduce interlaboratory variation.

· Amino acid data should be reported as mg amino acid per g N or be converted to mg amino acid per g protein by use of the factor 6.25. No other food-specific protein factor should be used.

· FAO should update their publication Amino acid content of foods and biological data on proteins (FAO, 1970) and commission new analyses of foods where there are insufficient reliable data.

· Reliable national tables of amino acid composition of products that have been clearly defined in terms of composition and processing should be developed.

Amino acid scoring pattern. The consultation evaluated the existing evidence and arguments about the use of amino acid scoring patterns to evaluate protein quality and concluded that at present there is no adequate basis for the use of different scoring patterns for different age groups with the exception of infants. Therefore, it decided to make the following recommendations.

· The amino acid composition of human milk should be the basis of the scoring pattern to evaluate protein quality in foods for infants under one year of age.

· The amino acid scoring pattern proposed by FAO/WHO/UNU (1985) for children of preschool age should be used to evaluate dietary protein quality for all age groups except infants.

· The recommendations made here for the two amino acid scoring patterns to be used for infants and for all other ages must be deemed as temporary until the results of further research either confirm their adequacy or demand a revision.

· Further research must be carried out to confirm the currently accepted values of requirements of infants and preschool-aged children, which are the basis for the scoring patterns recommended by this consultation.

· Further research must be carried out to define the indispensable amino acids (IAA) requirements of school-aged or adolescent children and of adults.

· Given the urgency of these research needs and the magnitude of the task required, it is recommended that an FAO/WHO coordinated international research programme be established immediately to assist in the determination of human amino acid needs.

Digestibility of proteins. The consultation discussed in detail the various methods used for determining the digestibility of proteins and made the following recommendations.

· Studies should be undertaken to compare protein digestibility values of humans and rats from identical food products.

· Extensive evaluation of existing in vitro and in vivo methods in foods indicates that the rat balance method is the most suitable practical method for predicting protein digestibility by humans. Therefore, when human balance studies cannot be used, the standardized rat faecal-balance method of Eggum (1973) or McDonough et al. (1990) is recommended.

· Since the true digestibility1 of crude protein is a reasonable approximation of the true digestibility of most amino acids (as determined by the rat balance method), it is recommended that amino acid scores be corrected only for true digestibility of protein.

1 The proportion of food nitrogen that is absorbed is:

A = absorbed nitrogen,
F = faecal nitrogen,
F k= metabolic nitrogen and
I = nitrogen intake.

· For new or novel products or processes, digestibility values must be determined. However, established digestibility values of well-defined foods may be taken from a published data base for use in the amino acid scoring procedure. A data base should be established for all raw and processed products.

· Further research is encouraged to perfect and evaluate the most promising in vitro procedures for estimating protein digestibility, such as those of Satterlee, Marshall and Tennyson (1979) and Pedersen and Eggum (1983). Based on the above conclusions, the consultation agreed that the protein digestibility-corrected amino acid score method was the most suitable approach for routine evaluation of protein quality for humans and recommended that it be adopted as an official method at the international level.

The report of the consultation contains details of the recommended methodology for the evaluation of protein quality and a practical guide on how to apply this methodology for individual foods as well as food mixtures. Copies of the report may be obtained on request from the author.


Bodwell, C.E., Carpenter, K.J. & McDonough, F.E. 1989. A collaborative study of methods of protein evaluation: introductory paper, Plant Foods Hum. Nutr., 39: 3-11.

Codex Alimentarius Commission. 1989, Document ALINORM 89/30, Rome, FAO.

Eggum, B.O. 1973. A study of certain factors influencing protein utilization in rats and pigs. Publ. 406. Copenhagen, National Institute of Animal Science.

FAO. 1970. Amino acid content of foods and biological data on proteins. Nutr. Div. Pub. 24. Rome, FAO.

FAO/WHO/UNU. 1985. Energy and protein requirements. Report of a joint FAO/WHO/UNU Expert Consultation. WHO Tech. Rep. Ser. No. 724. Geneva, WHO.

McDonough, F.E., Sarwar, G., Steinke, F.H., Slump, P., Garcia, S. & Boisen, S. 1990. In vitro assay for protein digestibility: interlaboratory study, J. Assoc. Off. Anal. Chem., 73: 622-625.

McDonough, F.E., Steinke, F.H., Sarwar, G., Eggum, B.O., Bressani, R., Huth, P.J., Barbeau, W.E., Mitchell, G.V. & Phillips, J.G. 1990, In vivo rat assay for true protein digestibility: collaborative study. J. Assoc. Off, Anal. Chem., 73: 801-805.

National Research Council. 1978. Nutrient requirements for laboratory animals. No. 10, Washington, D.C., National Academy of Sciences.

Pedersen, B. & Eggum, B.O. 1983. Prediction of protein digestibility by an in vitro enzymatic pH-stat procedure, Z. Tierphysiol. Tierernähr. Futtermlttelkd., 49: 265-277.

Sarwar, G. & McDonough, F.E. 1990. Evaluation of protein digestibility-corrected amino acid score method for assessing protein quality of foods. J. Assoc. Off. Anal, Chem., 73: 347-356.

Sarwar, G., Peace, R.W., & Botting. H.G. 1985. Corrected relative net protein ratio (CRNPR) method based on differences in rat and human requirements for sulfur amino acids, J. Assoc. Off. Anal. Chem., 68: 689-693.

Satterlee, L.D., Marshall, H.F. & Tennyson, J.M. 1979. Measuring protein quality. J. Am. Oil Chem. Soc., 56: 103-109.

Steinke, F.H., Prescher, E.E., & Hopkins, D.T. 1980. Nutritional evaluation (PER) of isolated soybean protein and combinations of food proteins, J. Food Sci., 45: 323-327.

Faits nouveaux intéressant l'évaluation de la qualité des protéines

L'évaluation de la qualité d'une protéine est essentielle pour déterminer sa valeur nutritive et commerciale. Plusieurs techniques d'analyse ont été mises au point et utilisées à cette fin; elles se fondent principalement sur la mesure de l'efficacité de la protéine à promouvoir la croissance des animaux, le plus souvent à l'aide du coefficient d'efficacité protéique. Après l'avoir employé pendant plusieurs décennies, on a reconnu que ce coefficient surestime la valeur de certaines protéines animales et sous-estime celle de quelques protéines végétales. La nécessité de mettre au point une méthode plus précise et plus appropriée pour évaluer la qualité des protéines s'est imposée et a été portée à l'attention du Comité du Codex sur les protéines végétales, qui a constitué un groupe de travail spécial pour aborder le problème. Après plusieurs années d'activité intense, le groupe de travail a conclu que la méthode de l'indice des acides aminés corrigé pour tenir compte de la digestibilité des protéines constituait l'approche la plus appropriée pour évaluer la qualité des protéines contenues dans les matières protéiques végétales. Une consultation mixte FAO/OMS d'experts réunie en décembre 1989 a confirmé l'utilité de la méthode proposée pour l'évaluation courante de la qualité des protéines et a recommandé son adoption en tant que méthode officielle au plan international. La Consultation a en outre recommandé des recherches ultérieures sur les besoins en acides aminés des divers groupes d'âge et sur les valeurs de digestibilité des protéines chez l'homme comparées aux valeurs chez les rats. Le rapport de la Consultation, publié par la FAO, expose dans le détail la méthodologie recommandée et fournit un guide pratique sur la façon de l'appliquer à divers aliments et mélanges alimentaires.

Novedades en la evaluación de la calidad de las proteínas

La evaluación de la calidad de las proteínas constituye un factor esencial para determinar su valor nutricional y comercial. Con este fin se han elaborado y utilizado diversas técnicas de ensayo que se basan sobre todo en la medición de la eficacia de la proteína para estimular el crecimiento de los animales. La técnica del coeficiente de eficiencia de la proteína es probablemente la que se aplica de modo más generalizado. Tras haber sido empleada durante decenios, se reconoció que esta técnica sobreestimaba el valor de algunas proteínas animales y subestimaba el de otras proteínas vegetales. La necesidad de elaborar un procedimiento más exacto y apropiado para evaluar la calidad de las proteínas quedó así de manifiesto y se señaló a la atención del Comité del Codex sobre Proteínas Vegetales, el cual estableció un grupo especial de trabajo para examinar este problema. Después de varios años de intensa labor, el grupo de trabajo llegó a la conclusión de que el procedimiento del cómputo de aminoácidos corregido en función de la digestibilidad de las proteínas era el método más conveniente para evaluar la calidad de las proteínas en los productos proteínicos vegetales. Una Consulta Mixta de Expertos FAO/OMS, celebrada en diciembre de 1989, confirmó la utilidad del método propuesto como instrumento para las actividades ordinarias de evaluación de la calidad de las proteínas y recomendó que se adoptara como método oficial con carácter internacional. La Consulta formuló algunas otras recomendaciones con respecto a investigaciones ulteriores sobre las necesidades de aminoácidos de diversos grupos de edad y sobre los valores de la digestibilidad de las proteínas en los seres humanos comparados con los valores en las ratas. En el informe de la Consulta, publicado por la FAO, se detalla la metodología recomendada y se ofrece una guía práctica sobre el modo de aplicarla a alimentos y mezclas de alimentos.

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