A PEPSIN PANCREATIN DIGEST INDEX OF PROTEIN QUALITY EVALUATION.

A pepsin pancreatiti digest index was devised for a rapid, accurate estimation of protein quality. The index was calculated from the amino acids released by an in vitro digestion with pepsin followed by pancreatin. The amino acids were determined by automatic amino acid analysis which allowed rapid protein quality evaluations with very small samples. Using whole egg as a standard, excellent cor relation was observed between the pepsin pancreatin index values for 12 proteins and their biological values reported in the literature from feeding trials. The proteins tested were selected to cover a wide range of protein quality. The pepsin pancreatin digest index values showed better correlation with the biological values for the growing rat than did the essential amino acid index which tended to overestimate the value, or the chemical score which tended to underestimate it. Although the nutritional quality of pro teins must, in the final analysis, be estab lished with feeding trials, in vitro methods of protein evaluation are useful in screen ing new protein foods and processing methods because of their rapidity. Recent reviews of laboratory methods of protein quality evaluation have been given by Grau and Carroll (1), Campbell (2), Pearson and Darby (3), Frost (4), and Mauron (5). Two procedures widely used for screening potential protein foodstuffs, the chemical score (6-8) and the essential amino acid index (9, 10) were based on the total amino acid composition. These procedures were rapid and in many cases accurate; however, no allowance was made for variations in the digestibility and avail ability of amino acids. Sheffner et al. (Il) developed the "pepsin-digest-residue amino acid index" which combined the pattern of essential amino acids released by in vitro pepsin digestion with the amino acid pat tern in the remainder of the protein. The work involved was considerable since 10 amino acids had to be determined in an acid hydrolysate as well as in a pepsin di gest by the use of microbiological tech niques. Therefore, this method did not appear suitable for evaluation of large numbers of food proteins. The procedure to be described in the present paper gives an amino acid index based on the release of amino acids by pepsin followed by pan creatin. The labor and sample size re quired for assay was reduced by the use of automatic amino acid analysis. METHODS AND MATERIALS Pepsin digests were prepared by incu bating with shaking 500 mg dried protein (samples were dried to constant weight over P2O5at room temperature) with 12.5 mg pepsin 2inl5mlof0.lN hydrochloric acid for 24 hours at 37Â°in a water bath. Pepsin followed by pancreatin digests were prepared by incubating 100 mg protein with 1.5 mg pepsin in 15 ml of 0.1 N hydrochloric acid at 37Â°for 3 hours. After neutralization with 7.5 ml of 0.2 N sodium hydroxide and addition of 4 mg pancre atin 3in 7.5 ml of pH 8.0 phosphate buffer, the digestion mixtures were incubated for an additional 24 hours at 37Â°. Enzyme blanks were prepared by incubation under the described conditions with the protein sample omitted. Fifty parts per million merthiolate which were added to the diges tion mixture to prevent growth of micro organisms did not interfere with the diges tion and subsequent analysis. Ten milliliters of digestion mixture were added Received for publication January 13, 1964. 1 Published with the approval of the Director of the Wisconsin Agricultural Experiment Station. Sup ported in part by grants from the Herman Frasch Foundation. 2 B grade, California Corporation for Biochemical Research, Los Angeles. 3 See footnote 2. J. NUTRITION, 83: '64 257 Downloaded from https://academic.oup. com/jn/article-abstract/83/3/257/4777852