The effect of raw milk storage temperature on plasmin activity and plasminogen activation in pasteurized milk

The effect of refrigerated conditions and calcium on plasminogen activation was studied in pre-pasteurized milk and in buffer. Fresh milk samples were collected and stored at 2.2, 3.3, and 4.4 °C for 24 h prior to pasteurization. Higher plasmin activity was observed along with a significant decrease in plasminogen-derived activity in samples stored at 4.4 °C. Conversely, plasminogen and serum Ca concentration were higher in samples stored at 2.2 °C. In a buffered system, Ca significantly affected the kinetic parameters of plasminogen activation. Michaelis–Menten constant (Km) significantly increased (P⩽0.05) in the presence of CaCl2 (0.2%) at both 2.2 and 4.4 °C, which led to a subsequent decrease in catalytic efficiency (kcatKm-1). Thus, the higher serum Ca observed in the milk sample stored at 2.2 °C interfered with plasminogen activation. Temperature during cold storage can have a significant influence on plasmin levels and thus contribute to the subsequent proteolysis rate in milk.