Indigenous proteolytic enzymes in milk: A brief overview of the present state of knowledge

Key characteristics of dairy products, including texture, flavour and functionality, depend on the level and behaviour of milk proteins, especially the caseins. Hence, it is not surprising that the activity of indigenous proteinases in milk has been studied for many years. Milk contains two proteinase systems, both derived of from blood, one involved in dissolving blood clots (plasmin) and the other in defence against invasive micro-organisms (lysosomal proteinases of somatic cells). Both systems hydrolyse the caseins, are complex in their regulation and sensitivity to processing, and vary in activity due to factors such as stage of lactation and mastitis. While plasmin is the principal proteinase in good-quality milk, other proteinases, including cathepsins and elastase, are probably also active, particularly as the somatic cell count of milk increases. In this review, recent research on indigenous milk proteinases is discussed, and some questions raised about their level and significance.