71A-17 |
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W. OCTAVIA, L. Wang, S. E. Sombers, K. D. Hayes, and L. J. Mauer. Dept. of Food Science, Purdue Univ., 745 Agriculture Mall Dr., West Lafayette, IN 47907-2009 Plasmin is the major proteolytic enzyme in bovine milk, and plasmin activity is necessary for cheese ripening. Milk solids may be added to milk in cheese manufacturing processes. Plasmin is known to bind to casein micelles, and measurable plasmin activity is known to decrease after pasteurization. However, effects of increasing casein concentration combined with heat treatment on plasmin activity are not known. Our objective was to determine the effects of casein concentration (1%, 3%, 5%, 7% w/w) and heat treatment (none, 85 °C for 16 sec) on plasmin activity. Fresh casein micelles were isolated and reconstituted in Jenness-Koops Buffer to various casein concentrations, and a constant amount of ß-lactoglobulin (0.5% w/v) was added. Plasmin (2x10-4 %w/v) was added or not. Samples were heated and then acid precipitated to separate whey and casein fractions. Plasmin activity in both fractions was determined spectrophotometrically using a synthetic substrate (Spec PL). Our results showed that higher plasmin activity was found in casein fractions than in whey fractions, and heating led to undetectable plasmin activity in both fractions. In the absence of heat and without addition of exogenous plasmin, increasing casein concentration led to a linear increase in plasmin activity (R2> 0.95). In the absence of heat, addition of exogenous plasmin resulted in higher plasmin activity than in samples without added plasmin; however, surprisingly the maximum plasmin activity was measured at 3% casein concentration (1.75 mU/mL). Plasmin activity at 7% casein concentration was 1.35 mU/mL and was not significantly different than activity in 1% and 5% casein samples. These results suggest that there may be an optimum casein concentration for plasmin activity and that the activity assay shows no plasmin activity in heated samples. Because cheese ripening studies indicate there is plasmin activity after heating milk, another assay to measure plasmin activity is needed.
Session 71A, Dairy Foods: General
2005 IFT Annual Meeting, July 15-20 - New Orleans, Louisiana |