Rennet is a mixture of enzymes found in the stomachs of certain infant mammals. It has the specialized function of assisting in the digestion of milk from the mother. The main constituent of rennet is rennin (also called chymosin) which coagulates the milk protein, causing it to separate into solid (curds) and liquid (whey). Rennet is used commercially in cheese making and can be obtained from microbiological sources as well as by extracting from unweened animals such as calves and kids.
SOURCE: Aspergillus niger