Rennet or rennin is a natural complex of enzymes produced in any mammalian stomach to help a nursing baby digest mother’s milk. In the food industry, rennet is used as a coagulant – in cheese-making; in certain dairy products, including some yogurts; and in junket, a soft, pudding-like dessert.
Rennet is extracted from the inner mucosa of the fourth stomach chamber of young, un-weaned calves. These calf stomachs are a by-product of veal production (which is an off-shoot of dairy production).
According to the McGraw-Hill Encyclopedia of Science and Technology, “After butchering, the fourth stomach…is removed and freed of its food content.” After this the stomach goes through several steps including being dry-salted, washed, scraped to remove surface fat, stretched onto racks where moisture is removed, then finally ground and mixed with a salt solution until the rennin is extracted.
To obtain rennet using the customary method still used by many European and traditional cheese-makers, stomachs of young calves are dried and “cleaned”, then sliced into small pieces and put into an extraction solution, which will be filtered after several days. In modern production, the stomachs are deep-frozen and ground up before rennet extraction.