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I actually have 2 questions which are related.

  1. Traditionally, making cheese involves using rennet, a substance derived from animal stomach. If the rennet is derived from a non-Kosher animal, it is non-Kosher (making the cheese non-Kosher as well). From my understanding, the reason for this is the rule "כל היוצא מטמא טמא" - all that is derived from an unclean (animal) is also unclean. But, even rennet from a Kosher animal is not considered meat because of the process involved in extracting it. So, why the difference? Why does the rennet lose its "meat properties" but not its "unclean properties"?
  2. Suppose we understand the differences between the meat & unclean properties, what is the difference between rennet and gelatin made from non-Kosher animal bones? I read that the opinion of the Shulchan Aruch (99) is that (completely) dry bones of non-Kosher animals are considered Kosher, and so gelatin from those bones would also be considered Kosher. Why does the rule of "כל היוצא מטמא טמא" not apply here?
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Hi elanh welcome to mi yodeya and thanks for bringing your question here. Hope to see you around the site. – user2110 Jun 4 '13 at 13:08

According to the Shulhan 'Aruch (Y"D 87:9-11) and the RaM"A (ibid), liquid milk found in the stomach of a Kosher animal (Kosher for this purpose means all of the following: "clean" species and properly slaughtered and without invalidating blemishes which would make it a Treifah), that is either salted with the stomach or left standing in the stomach for 24 hours, is prohibited to be used for making cheese, as it has absorbed taste from the meat of the stomach and is therefore considered Basar BeHalav (forbidden combination of meat and milk). According to the RaM"A (as explained by the Sha"Ch, ad loc. note 31), if the milk in the stomach has dried out it is not prohibited. The RaM"A writes further that if the stomach itself has been salted and totally dried to the point that it is like wood, it is considered to have lost its properties of meat and can be used to store milk.

With respect to rennet, which has properties of meat, since it is a catalyst for the cheesemaking process, it is not like other foods, and if it is from a non-Kosher animal, it is never nullified in the milk even in proportions of 1 part in 1,000 (note that if it is made with rennet from a Kosher animal, it still needs to become nullified to the milk; if the resultant cheese has some taste of meat, it is prohibited as meat and milk).

Permitting of dried bones, on the other hand, relies on a different principle altogether, which is that once they are dried out they no longer are considered to be food (this is possibly related to what was said above about the dried stomach, but the law is codified elsewhere: Y"D 99:1). (See this question as associated answers for more on this issue). Note, however, that there is a dispute about permitting dried bones that have been reconstituted into something edible.

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