Nenin

Nenin, also known as neni or rennin, is a naturally occurring enzyme produced by the chief cells of the stomach in newborn mammals, particularly ruminants. Its primary function is to digest milk proteins, specifically casein. Nenin acts as a milk-clotting enzyme, causing the casein in milk to coagulate. This coagulation is crucial for allowing milk to remain in the stomach for a longer period, thereby facilitating more efficient digestion and absorption of nutrients by the young animal.

The activity of nenin is dependent on the presence of acid, as the stomach's acidic environment is necessary for its optimal function. As mammals mature and begin to consume solid foods, the production of nenin decreases significantly, and its role in digestion is taken over by other enzymes like pepsin. The milk-clotting action of nenin is similar to that of chymosin, and in many contexts, the terms are used interchangeably, especially in the commercial production of cheese. Industrial rennet, used in cheesemaking, is often derived from the stomachs of young calves, containing chymosin and other enzymes that perform a similar function to nenin.