The Plasmin System in Milk and Dairy Products

Plasmin is the principal indigenous proteinase present in bovine milk, and has been the focus of many studies as it contributes in diverse ways to the quality of milk and milk-based products. The plasmin system is a complex protease-protease inhibitor system, which involves a series of interactions that ultimately result in the activation of plasminogen to active plasmin. The components of the plasmin system within bovine milk have been successfully quantified, isolated and characterised. Components of the plasmin system are affected by numerous factors such as processing conditions, environmental factors and storage conditions, which ultimately alter the rate/extent of plasminogen activation and plasmin-induced proteolysis. Factors such as pH, heat treatment, presence of whey proteins and temperature of storage can also influence the rate of plasmin-induced hydrolysis of caseins in milk and other dairy products. Plasmin-mediated hydrolysis can have both beneficial and negative effects on a wide variety of dairy products; plasmin is of great importance in the development of flavour and texture during cheese ripening for example, whereas, in ultra-high-temperature milk and high protein dairy-based beverages, plasmin-induced proteolysis can cause undesirable gelation. In this chapter, an overview of the current state of knowledge and areas that require additional research is presented.