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.