Bovine milk contains a number of indigenous proteolytic enzymes, of which plasmin is the most important (Grufferty & Fox, 1988; Bastian & Brown, 1996; Kelly & McSweeney, 2003). Plasmin (EC 3.4.21.7) is a serine proteinase with pH and temperature optima of 7·5 and 37 °C, respectively. In milk, most of the plasmin is present as its inactive precursor, plasminogen, which is converted to active plasmin by plasminogen activators (PA) present in milk, e.g., urokinase-type (u-PA) and tissue-type PA (t-PA) (Bastian & Brown, 1996). Since plasmin, plasminogen and PA are associated with casein micelles, they are incorporated into cheese curd, while plasmin inhibitors and inhibitors of PA are lost with the whey. Plasmin incorporated in cheese curd acts on its substrate, the caseins, contributing significantly to primary proteolysis during ripening (Upadhyay et al. 2004b).
