Chymosin Activity Against αs1-Casein in Model Systems: Influence of Whey Proteins

The influence of native or heat-denatured alactalbumin (LA) and b-lactoglobulin ( L G ) on chymosin activity against a s1 -casein ( C N ) in buffered and simulated milk ultrafiltrate model systems was evaluated.The a s1 -CN solution (2.5 mg/ml) was adjusted to pH 5.5 using glucono-D-lactone; a-LA or b-LG, either native or heat-denatured (100°C for 15 min), was then added.Sufficient chymosin was added to hydrolyze 99% of the a s1 -CN in 4 h at 20°C in an uninhibited system.Fast protein liquid chromatography was used to quantify intact a s1 -CN at 0, 0.5, 1, 2, 3, and 4 h and to evaluate chymosin activity.Rate constants for each reaction were determined.Simulated milk ultrafiltrate alone did not influence the rate of a s1 -CN hydrolysis, and, in the absence of milk salts, only denatured b-LG reduced the rate of a s1 -CN hydrolysis significantly.When simulated milk ultrafiltrate was added to reaction mixtures, both native and heat-denatured b-LG significantly inhibited chymosin activity.Native a-LA did not influence hydrolysis of a s1 -CN; heat-denatured a-LA inhibited chymosin only in the presence of simulated milk ultrafiltrate.( Key words: whey proteins, chymosin, a s1 -casein, inhibition)