For the source text click/tap here: Chullin 116
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Our Mishnah discusses the laws of milk that is found in the stomach of an animal that is shechted.
Rashi and Meiri understand that the question is regarding milk that has congealed and contains the enzyme rennin, and the issue is whether it is permissible to use this to culture cheese.
The Mishnah rules that congealed milk found in the stomach of an animal of a non-Jew (even shechted) or in a neveilah is prohibited. This substance has the status of the animal in which it is found.
The rennin found in the stomach of a kosher, shechted animal may be used to process cheese, but only as long as the meat of the stomach is outnumbered and does not contribute its taste into the milk or cheese.
We explore the halachot of rennet and its history in cheese production.