Enzymes Indigenous to Milk: Plasmin System in Milk

Proteolysis in milk caused by the indigenous protease plasmin is one of the most important contributors to the quality of milk and its products. Plasmin-induced proteolysis can have either beneficial or detrimental effects, depending on the purpose of processing and processing conditions. In cheese, the breakdown of protein can help develop desirable flavors and texture during ripening, while in pasteurized milk and ultra-high temperature milk, proteolysis causes undesirable gelation. Plasmin is part of a complex system including its inactive form plasminogen, plasminogen activators, and plasminogen inhibitors. The components of the plasmin system interact together and with other components of milk to promote or inhibit proteolysis, mainly of the caseins, in milk and milk products depending on the thermal processing employed and on the storage conditions. Many factors such as pH, heat, mineral content, whey proteins, and storage temperature can influence the kinetics of plasmin-induced hydrolysis. The components of the plasmin system have been successfully isolated and characterized. Thermal stability of each of the components of the plasmin system as well as their interactions with caseins and whey proteins as affected by thermal processing has been investigated. A brief overview of the present state of knowledge and future research requirements is presented in this article.