Liberation of amino acids from raw and heated casein by acid and enzyme hydrolysis.

Earlier work showed that the nutritive value of casein was decreased by dry heat and that the addition of lysine restored its nutritive value (1). The lysine was not damaged materially by the heat treatment, since analyses of acid hydrolysates of heated casein with lysine decarboxylase (2) and chemical isolation as the picrate (3) showed no decrease in the lysine content. Eldred and Rodney (2), using the lysine decarboxylase method, and Pader, Melnick, and Oser (4), using the Streptococcus fuecdis assay, found that heating casein in a dry state at 150” for a few hours decreased the quantity of lysine liberated by enzyme hydrolysis in vitro. Block, Jones, and Gersdorff (3) reported that the lysine content of casein was not affected by exposure to dry heat at a temperature at 150”, but that enzymatic liberation of the amino acid was decreased. Melnick, Oser, and Weiss (5) pointed out that factors known to increase the nutritive value of soy bean protein also increase its in vitro digestibility. In a recent report, Riesen et al. (6) showed that the degree of liberation of the ten essential amino acids from soy bean oil meal by pancreatin was increased when the meal had been autoclaved for 4 minutes at 15 pounds pressure. When the period of autoclaving was extended to 4 hours, the liberation of these amino acids was decreased below that obtained with the raw meal. The amino acid content was unaffected by the short autoclaving procedure; after prolonged heat treatment, the lysine, arginine, and tryptophan values found by microbiological assay of acid or alkaline hydrolysates were decreased. To determine whether casein was altered similarly, the effect of heat treatment on the amino acid composition and the extent of liberation of amino acids by enzymes were measured microbiologically. Since preliminary experiments indicated that this protein was much more resistant to changes in digestibility by moist heat than soy bean protein, the longer period of autoclaving was extended from 4 to 20 hours. In t.his work, the