Rennet is a complex of enzymes produced in the stomachs of ruminant mammals.  Chymosin, its key component, is a protease enzyme that curdles the casein in milk. This helps young mammals digest their mothers' milk. Rennet can also be used to separate milk into solid curds for cheese making and liquid whey. In addition to chymosin, rennet contains other important enzymes such as pepsin and a lipase.