CHYMOSIN ACTIVITY AGAINST ALPHA(S1)-CASEIN IN MODEL SYSTEMS - INFLUENCE OF WHEY PROTEINS

The influence of native or heat-denatured alpha-lactalbumin (LA) and b eta-lactoglobulin (LG) on chymosin activity against a,l-casein(CN) in buffered and simulated milk ultrafiltrate model systems was evaluated. The alpha(s1)-CN solution (2.5 mg/ml) was adjusted to pH 5.5 using gl ucono-Delta-lactone; alpha-LA or beta-LG, either native or heat-denatu red (100 degrees C for 15 min), was then added. Sufficient chymosin wa s added to hydrolyze 99% of the alpha(s1)-CN in 4 h at 20 degrees C in an uninhibited system. Fast protein liquid chromatography was used to quantify intact alpha(s1)-CN at 0, 0.5, 1, 2, 3, and 4 h and to evalu ate chymosin activity. Rate constants for each reaction were determine d. Simulated milk ultrafiltrate alone did not influence the rate of al pha(s1)-CN hydrolysis, and, in the absence of milk salts, only denatur ed beta-LG reduced the rate of alpha(s1)-CN hydrolysis significantly. When simulated milk ultrafiltrate was added to reaction mixtures, both native and heat-denatured beta-LG significantly inhibited chymosin ac tivity. Native alpha-LA did not influence hydrolysis of alpha(s1)-CN; heat-denatured alpha-LA inhibited chymosin only in the presence of sim ulated milk ultrafiltrate.