QUESTION: Can one make cheese using animal rennet from a kosher slaughtered animal, or is this an issue of mixing meat and milk?
ANSWER: There are two types of kosher animal rennet. One can get kosher rennet from the digested milk that is found in the stomach of a calf. This type of rennet is kosher and dairy, but if it has already solidified, it is pareve. This is because the digested and solidified milk found in the calf’s stomach is no longer viewed as milk. This type of rennet may be used to make cheese.
There is another type of rennet that can be made from the lining of the stomach of the calf. This type of rennet is actual meat and cannot be used as is to make cheese. However, if a Jew accidentally used such rennet from a kosher slaughtered animal to make cheese, the cheese would be permitted, since the amount of meat rennet that is used is much less than one part in sixty.
Poskim discuss whether meat rennet can be used if the stomach is dried out to the point that it becomes hard like wood. The Chasam Sofer (cited in Pischei Teshuva 87:19) writes that we are no longer experts in how to properly dry out the stomach, and therefore he writes that one should not directly add dried meat rennet to milk. Instead, one should extract the rennet enzymes from the meat by soaking it in alcohol and this should be added to the milk in combination with other coagulants.
