Rennet is a type of cheese yeast derived from calf stomach Rennet and Coagulating Enzymes What are coagulating enzymes and how do they create cheese? In order for milk to coagulate and eventually become cheese, enzymes must be added to breakdown the proteins that keep milk a liquid. Some enzymes do this better than others, but all of these enzymes are in the protein breaking subclass known as proteases. The best proteases or coagulants for making cheese are the type that break a specific protein called kappa casein. When the kappa casein is broken the milk loses its liquid infrastructure and begins to coagulate. What are Rennet, Rennin, and Chymosin? Rennet is defined in Webster's Unabridged Dictionary as "the lining membrane of the fourth stomach of the calf (and/or) a preparation or extract of the rennet membrane, used to curdle milk, as in making cheeseâ€¦." Rennet is also used broadly to describe any enzyme used for the coagulation of milk in the process of making cheese. Rennin is defined as "a coagulating enzyme occurring in the gastric juice of the calf, forming the active principal of rennet and able to curdle milk." The cheese industry uses a broader definition of the term rennin, referring to it as "any enzyme used for the controlled coagulation of milk." Chymosin, often used as another word for rennin, is the most common enzyme recovered from rennet.