A microbiological method for assessing the nutritional value of proteins. 2. The measurement of 'available' methionine, leucine, isoleucine, arginine, histidine, tryptophan and valine.

Of the several characteristics that determine the nutritive quality of a manufactured protein food, its digestibility may be of prime importance. Bunyan & Price (1960) examined a series of twelve whale-meat meals which, though broadly similar in amino acid composition (Ford, 1962) showed wide differences in nutritive quality. They found in tests with rats that protein quality ratings (NPU) of the meals correlated closely with their true digestibilities of nitrogen, and also with their content of available lysine determined with fluorodinitrobenzene reagent (FDNB-lysine) as described by Carpenter (1960). Ford (1962) found that these same NPU and digestibility values also paralleled closely the contents of available leucine, methionine, arginine and tryptophan, measured microbiologically with Streptococcus zymogenes. All these findings are consistent with the view, earlier expressed by Harris & Mattill (1940) and others, that in heat-damaged proteins digestibility, and thus the biological availability of all their amino acids, might be directly determined by the extent to which the free amino groups, comprised in the main of the e-amino groups of the lysine, have combined with other reactive groups to form enzyme-resistant intramolecular linkages. But the differences in nutritive quality in the whale-meat meals cannot so simply be attributed entirely to the differences in their overall digestibility. The biological values of these meals varied widely and were roughly proportional to their digestibilities and to their content of FDNB-lysine, available methionine and available tryptophan. From the poorer meals, which contained relatively low proportions of available amino acids as judged in chemical and microbiological tests, correspondingly low percentages of the nitrogen absorbed from the gut were retained by the rat. It is a question of some fundamental interest whether this finding is explainable in terms of differences in the patterns of amino acids absorbed from the meals of different qualities or whether, even after absorption from the gut, some proportion of the amino acids might still remain locked up in indigestible peptide residues and be biologically unavailable. The present paper describes the application of Sephadex-gel filtration, combined with microbiological assays, in a study of the effects of heating on the biological availability of some amino acids in fish protein. Portions of a preparation of freezedried cod fillets were heated under a variety of controlled conditions and were then