Values (%) for true digestibility (TD) of protein and individual amino acids in some vegetable proteins were determined by the rat balance (fecal) method. Diets containing 8% crude protein (N X 6.25) from soaked and autoclaved samples of Trapper and Century field peas, lentil, pinto bean, seafarer bean, black bean or fababean and autoclaved samples of soybean, peanut, sunflower, rolled oat, rice + soybean and corn + pea were tested in two rat balance studies. In the case of blends, each protein source provided 50% of total protein. The beans, peas and lentil proteins were limiting in sulphur amino acids, tryptophan and threonine, whereas sunflower and rolled oat were most limiting in lysine. In beans, peas and lentil, the TD values of methionine (51-82), cystine (46-85), tryptophan (47-90) and threonine (62-84) were considerably lower than the TD values of total nitrogen (72-90). Similarly, in sunflower and rolled oat, the TD values of lysine (81-83) were lower than the TD values of total nitrogen (90-91). These data suggested that crude protein digestibility may not be a good predictor of bioavailability of limiting amino acids in vegetable proteins. Amino acid scores of the vegetable proteins were 62-96%. The corrections for true digestibility of protein and individual amino acids lowered the scores by 6-15 and 11-47 percentage units, respectively.

Values (%) for true digestibility of crude protein and individual amino acids in 20 selected foods were determined by the rat balance (fecal) method. The products were fed as the sole source of protein in diets containing 8% crude protein (N x 6.25). Lowest true protein digestibility values (79-84) were obtained for pinto beans, kidney beans and lentils; intermediate values (89-92) were obtained for chick peas, beef stew, skim milk (over heated), rolled oats, whole wheat cereal, and pea protein concentrate; and highest values (94-100) were obtained for sausage, macaroni-cheese, rice-wheat gluten cereal, skim milk, tuna, soy isolate, peanut butter, chicken frankfurters, beef salami, casein and casein + methionine. In animal foods, peanut butter and soy isolate, the differences between true digestibility of crude protein and most individual amino acids were less than 5%. However, the values for true digestibility of methionine and cystine were up to 44% lower than those of crude protein in pinto beans, kidney beans, lentils, chick peas and pea concentrate. In these legumes, digestibility of crude protein was not a good predictor of digestibility of the limiting amino acids.

Amino acids in foods exist in a free form or bound in peptides, proteins, or nonpeptide bonded polymers. Naturally occurring L-amino acids are required for protein synthesis and are precursors for essential molecules, such as co-enzymes and nucleic acids. Nonprotein amino acids may also occur in animal tissues as metabolic intermediates or have other important functions. The development of bacterially derived food proteins, genetically modified foods, and new methods of food processing; the production of amino acids for food fortification; and the introduction of new plant food sources have meant that protein amino acids and amino acid enantiomers in foods can have both nutritional and safety implications for humans. There is, therefore, a need for the rapid and accurate determination of amino acids in foods. Determination of the total amino acid content of foods requires protein hydrolysis by various means that must take into account variations in stability of individual amino acids and resistance of different peptide bonds to the hydrolysis procedures. Modern methods for separation and quantitation of free amino acids either before or after protein hydrolysis include ion exchange chromatography, high performance liquid chromatography (LC), gas chromatography, and capillary electrophoresis. Chemical derivatization of amino acids may be required to change them into forms amenable to separation by the various chromatographic methods or to create derivatives with properties, such as fluorescence, that improve their detection. Official methods for hydrolysis and analysis of amino acids in foods for nutritional purposes have been established. LC is currently the most widely used analytical technique, although there is a need for collaborative testing of methods available. Newer developments in chromatographic methodology and detector technology have reduced sample and reagent requirements and improved identification, resolution, and sensitivity of amino acid analyses of food samples.

Protein and amino acid digestibility and protein quality of liquid concentrate and/or powder forms of infant formulas were studied by rat balance and growth methods. Casein plus methionine (control) and eight formulas were fed to weanling rats as the sole source of protein in diets containing 8% protein (nitrogen X 6.25). Values for true digestibility of protein, lysine, methionine, or cystine (85-92%) in liquid concentrates were up to 13% lower than those in powders. Similarly, the 2-wk relative protein-efficiency ratio values (64-85%) or the relative net protein ratio values (78-94%) of liquid concentrates were up to 25% lower than those for powders. Lower levels of bioavailable lysine and methionine plus cystine in liquid concentrates compared with powders (prepared by the same manufacturer) would suggest that inferior protein quality of liquid concentrates may be due to more heat treatment involved in their preparation.

In 1996, U.S. Food and Drug Administration regulations mandated the fortification of enriched cereal-grain products with folic acid, thereby emphasizing the need for validated methods for total folates in foods, particularly cereal products. The AOAC Official Methods (944.12, 960.46) currently used for the analysis of folate in foods for compliance purposes are microbiological methods. When the fortification regulations were finalized, no Official AOAC or Approved AACC methods for folate in cereal-grain products were in place. The AOAC Official Method (992.05) for folic acid in infant formula does not incorporate important improvements in the extraction procedure and was not considered suitable for the analysis of folates in foods in general. Amicrobiological assay protocol using a trienzyme extraction procedure was prepared and submitted for comments to 40 laboratories with recognized experience in folate analysis. On the basis of comments, the method was revised to have the conjugase (gamma-glutamyl-carboxy-peptidase) treatment follow a protease treatment, to include the use of cryoprotected inoculum, and to include the spectroscopic standardization of the standard and optional use of microtiter plates. Thirteen laboratories participated in a collaborative study of 10 required and 10 optional cereal-grain products, including flour, bread, cookies, baking mixes, and ready-to-eat breakfast cereals. The majority of the participating laboratories performed the assay by the standard test tube method; others used the microtiter plate modification for endpoint quantitation with equal success. For the required products, the relative standard deviation between laboratories (RSDR) ranged from 7.4 to 21.6% for 8 fortified (or enriched) products compared with expected (Horwitz equation-based) values of 11–20%. RSDR values were higher (22.7–52.9%) for 2 unfortified cereal-grain products. For the optional products, the RSDR ranged from 1.8 to 11.2% for 8 fortified products. RSDR values were higher (27.9–28.7%) for 2 unfortified cereal-grain products. Based on the results of the collaborative study, the microbiological assay with trienzyme extraction is recommended for adoption as Official First Action.