The nutritive quality and available amino acid composition of some animal protein concentrates

The nutritive quality of protein foods is conventionally assessed in feeding trials with higher animals.These tests are cumbersome and expensive, and quite unsuited to the practical needs of the feeding-stuffs compounder who requires a quick and simple grading test.The crude protein content (total nitrogen x 6.25) gives little guide to nutritive quality and there is not necessarily any correlation between protein quality and crude protein content.In recent years various laboratory techniques for assessing protein quality have been recommended.A chemical test for available lysine (Carpenter & Ellinger, 1955a, b) has aroused much interest, especially since practical diets are largely comprised of cereals which, as a class, are deficient in lysine, making it important that the available lysine content of supplementary protein concentrates be known.The available lysine value of a food protein is a measure of the content of lysine that contains free s-amino groups, as distinct from lysine that is bound in this grouping.Bound lysine is considered nutritionally unavailable because of the resistance of these bound groups to enzymes (Eldred & Rodney, 1946; Lea & Hannan, 1950; Henry & Kon, 1950).This test involves the combination of free s-amino groups of lysine molecules in the protein with 2:4-dinitrofluorobenzene.After acid hydrolysis, the content of the dinitrophenyl-lysine complex is measured and the original available lysine can be calculated.Furthermore, the s-amino group of lysine is susceptible during processing to chemical reactions of the Maillard type, which may bind the amino acid into unavailable complexes.The available lysine value of protein supplements is, therefore, of special importance, and it has been established (cf.Boyne, Carpenter & Woodham, 1961) that, for a wide variety of protein foods, these values correlate closely with gross protein values (GPV).This term was first introduced by Heiman, Carver & Cook (1939) to supply information to the poultry industry on the supplementary value of various protein concentrates when added to a basal cereal diet.Three-day-old chicks are first maintained on the basal diet up to 14 days of age and then either continued on it for 2 weeks to form a control, or given the basal diet supplemented with the test concentrate to supply 3 % supplementary protein.The increase in weight per head of birds on the control diet is subtracted from the increase in weight per head on the supplemented diet, and the difference so obtained divided by the weight of concentrate protein consumed, to find the increase in weight per unit of concentrate protein.This value is expressed as a percentage of the value for casein as a supplement to the basal