Animal rennet is made from rennin, an enzyme that is secreted in the fourth stomach of calves, lambs, and goats. It is most often derived from the dried and ground stomachs of unweaned calves and is used in making cheese.

According to Dr. David B. Fankhauser of the University of Cincinnati Clermont College, the first cheese was made in ancient times when milk was stored in bags made from lamb or goat stomachs. People began salting and drying the abomasums in strips, and Germans labeled it as "rennen."

Animal rennet is a key ingredient in cheese and affects the flavor and texture of the final product.

Rennet was traditionally made from animal stomachs and the microorganism mucor meihei. Since 1990, it has been artificially secreted from genetically modified bacteria and sold commercially in liquid or tablet form.

The enzyme rennin is a single polypeptide that has an internal disulfide bridge. Liquid rennet is clear amber or dark brown and powder rennet is tan.

Rennet makes milk protein curd and the liquid portion separate into whey.