Chullin 116 - The Rennet
The fourth stomach of the ruminants is called abomasum. Its lining contains an enzyme, called rennin, which, when added to milk, causes it to coagulate into cheese. The congealed milk, found in the abomasum of young nursing ruminants, is called rennet. Sometimes the term "rennet" is used to denote the membrane lining the stomach, and sometimes it refers to rennin.
The rennet of an animal of an idolater and of an animal that died of itself ( nevelah ) is prohibited for consumption. If one curdles milk with the meat of the abomasum and there is enough meat to impart flavor to the cheese, the cheese is prohibited. If a terefah animal suckled from a kosher animal, its rennet is permitted, because it is merely gathered in its innards.
There are three different ways to understand the above rules. (1) Both liquid and congealed rennet are not classified as milk but as refuse, thus rennet is always permitted; (2) Both liquid and congealed rennet are classified as milk, thus, it is kosher only when the milk was suckled from a kosher animal; (3) Liquid rennet is viewed as milk; congealed rennet is viewed as refuse.
Art: Julien Dupre - The Milkmaid