Adult cows do not have this enzyme. Chymosin is extracted by washing and drying the stomach lining, which is then cut into small pieces and macerated in a solution of boric acid or brine at 30 C for 4-5 days. Pepsin may sometimes be used instead of chymosin. This is usually derived from the abomasum of grown calves or heifers, or less commonly pigs. Pepsin may be mixed with calf rennin.
Rennet coagulates the milk, separating it into curds and whey.
This is called curdling. Chymosin breaks down the milk protein casein to paracasein which combines with calcium to form calcium paracaseinate, which separates out. Milk fat and some water also becomes incorporated into this mass, forming curds. The remaining liquid is the whey. Other substances may also be added during the cheese making process.