Chymosin Activity Against _s1-Casein in Model Systems: Influence of Whey Proteins

¹ Department of Food Science and Nutrition, University of Minnesota, St. Paul 55108-6099

The influence of native or heat-denatured -lactalbumin (LA) and ß-lactoglobulin (LG) on chymosin activity against _s1-casein (CN) inbuffered and simulated milk ultrafiltrate model systems was evaluated. The _s1-CN solution (2.5 mg/ml) was adjusted to pH 5.5 using glucono--lactone; -LA or ß-LG, either native or heat-denatured (100°C for 15 min), was then added. Sufficient chymosin was added to hydrolyze 99% of the _s1-CN in 4 h at 20°C in an uninhibited system. Fast protein liquid chromatography was used to quantify intact _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 _s1-CN hydrolysis, and, in the absence of milk salts, only denatured ß-LG reduced the rate of _s1-CN hydrolysis significantly. When simulated milk ultrafiltrate was added to reaction mixtures, both native and heat-denatured ß-LG significantly inhibited chymosin activity. Native -LA did not influence hydrolysis of _s1-CN; heat-denatured -LA inhibited chymosin only in the presence of simulated milk ultrafiltrate.

Key Words: whey proteins • chymosin • _s1-casein • inhibition

Submitted on January 29, 1996
Accepted on September 25, 1996

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