Rennet is a natural complex of enzymes produced in any mammalian stomach to digest the mother's milk. Rennet contains a proteolytic enzyme (protease) that coagulates the milk, causing it to separate into solids (curds) and liquid (whey). The active enzyme in rennet is called rennin or chymosin but there are also other important enzymes in it, e. g., pepsin or lipase. There are non-animal sources for rennet substitutes.