ENHANCEMENT OF PROTEOLYSIS IN BOVINE SKIM MILK BY HEAT AND CHEMICAL TREATMENTS

Effect of heating and of some chemical treatments on the proteolysis of skim milk caused by its major proteinase system, plasmin, was investigated. Degree of proteolysis was expressed as the increase in y-casein content of skim milk (mg/ml) after incubation with 0.02% (w/v) NaN3 at 37 °C for 20 h. Apart from reduced proteolysis in skim milk heated at > 70 °C for 10 min or at 75 °C for ~ 5 min, some enhancement (10-80%) was observed on heating at 55–63 °C for 10 min or at 60–65 °C for 15 s. Heating at pasteurization conditions (63 °C/30 min or 72 °C/15 s) had no apparent effect on the amount of proteolysis. Addition of ascorbic acid (0.2 mg/100 ml) or H2O2 (1.5 mg/100 ml) to unheated skim milk also increased the amount of proteolysis by 38 and 28%, respectively. However, the extent of this increase diminished with increasing amounts of both chemicals. Furthermore, there was a steady increase (~ 33%) in proteolysis with time of exposure to light for up to 3 h. These results suggest that oxidative conditions lead skim milk to enhanced proteolysis by inactivating an inhibitor(s) operating in the plasmin system in milk. © 1990, Proprietors of Journal of Dairy Research. All rights reserved.