Rennet is a coagulating enzyme used to curdle milk. causing clumps (curds) to form and separate from the liquid (whey). Rennet is added after the cultures and is also integral to the final flavor of the cheese. Until recently, rennet was derived from the stomach lining of a calf, goat or lamb. Now, many cheesemakers use vegetable rennet (derived from plants) or microbial rennet. In addition to being less expensive than traditional animal rennet, vegetable and microbial rennet allow cheesemakers to craft cheeses suitable for vegetarains.