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

Abstract

The influence of native or heat-denatured α-lactalbumin (LA) and β-lactoglobulin (LG) on chymosin activity against αs1-casein (CN) in buffered and simulated milk ultrafiltrate model systems was evaluated. The αs1-CN solution (2.5 mg/ml) was adjusted to pH 5.5 using glucono-Δ-lactone; α-LA or β-LG, either native or heat-denatured (100°C for 15 min), was then added. Sufficient chymosin was added to hydrolyze 99% of the αs1-CN in 4 h at 20°C in an uninhibited system. Fast protein liquid chromatography was used to quantify intact αs1-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 αs1-CN hydrolysis, and, in the absence of milk salts, only denatured β-LG reduced the rate of αs1-CN hydrolysis significantly. When simulated milk ultrafiltrate was added to reaction mixtures, both native and heat-denatured β-LG significantly inhibited chymosin activity. Native α-LA did not influence hydrolysis of αs1-CN; heat-denatured α-LA inhibited chymosin only in the presence of simulated milk ultrafiltrate.