Biological Value of Proteins in Relation to the Essential Amino Acids Which They Contain

Abstract

The methods used, and controls employed for their verification, in the analyses of fifteen high protein foods for the ten essential amino acids are described. Several of the methods available in the literature at the time this work was undertaken (September, 1943) after thorough trial, had to be discarded as inadequate. This was notably true of the electrophoretic method for separation of the three basic amino acids as well as the current colorimetric procedure for these acids singly and the oxidative method for determination of isoleucine, leucine and valine. Criticisms of these methods are given. Of the final results presented in table 3, those which apply to corn germ and beefsteak represent closely the values used in paper III of this series for making up the mixtures of the essential amino acids in imitation of these two proteins. The principal discrepancies concerned leucine, which was nearly equally too high (4–5%) and valine equally too low (1.3%) in both foods. The biological values of the mixtures in these two instances, after correction for the unnatural isomers, were brought close to those of the proteins they imitated. All methods which could be controlled by recoveries of the pure amino acids were so controlled and no procedures were adopted which did not give average recoveries of 95% or better.