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.

  • I rejected the edit to this answer, because it is redundant. It already stated that the rennet is the catalyst for the process. This is not the standard Halachic term identified in the edit. If readers prefer the Halachic term, I recommend editing to incorporate the plain English explanation and identify the Halachic term as such. I would also appreciate if the editor(s) use my transliteration style. – Seth J May 25 '18 at 20:23
  • Everything you write is true but it doesn't explain how one makes kosher cheese. The rennet used is not the milk found in the stomach but part of the stomach itself. And it is not the hardened stomach itself but enzymes found on its surface. I had that question myself and am sharing the results of my research in another answer. – mbloch Mar 3 at 9:54

Your first question is a strong question which I have not found answered elsewhere on MY. This other answer to your question answers the second question but doesn't address the core issue of "how can rennet be forbidden if coming from a non-kosher animal but permitted if coming from a kosher one - why does it lose its "meat properties" but not its "unclean properties"?".

In summary, the rennet from a non-kosher animal is prohibited because it is a davar maamid which is critical to the formation of the cheese and therefore never annuled. The rennet from a kosher animal is permitted because it doesn't give meat taste to the animal and is used in very small quantities.

R Chaim Jachter explains (here under Stomach Lining of a Kosher Animal) that

  • The rennet from a non-kosher animal is prohibited because it is a davar maamid, it gives texture to the cheese and is indispensable to create the cheese. Such ingredients are never annulled in larger quantities. This is seen in the Rambam MT Maachlot Assurot 3:13

The stomach skin is a very small entity when compared to the milk that it is used to solidify. Why is it not nullified because of its insignificant size? Because it is used as the catalyst to cause the cheese to curdle. Since the catalyst which causes it to curdle is forbidden, everything is forbidden, as will be explained.

  • The rennet from a kosher animal is permitted to be used if it doesn't give (meat) taste to the cheese (MT Maachlot Assurot 9:16)

It is forbidden to place the skin of a kosher animal's stomach [in milk] to serve as a catalyst for it to harden into cheese. If one used it as a catalyst, [a gentile] should taste the cheese. If it has a taste of meat, it is forbidden. If not, it is permitted. [The rationale is that] the catalyst is itself a permitted entity, for it comes from the stomach of a kosher animal. [The only question] is [whether] the prohibition against meat and milk [was violated] and that is dependent on whether the flavor was imparted.

  • the logical question at this stage is why do we not say that it is forbidden to annul a substance voluntarily (ein mevatlin issur lechatchilah). R Jachter offers two solutions to this

Rav Akiva Eiger (Teshuvot number 207, cited in the Pitchei Teshuva 87:19) explains that the prohibition of Ein M’vatlin Issur L’chatchilah does not apply if two lenient factors are in effect - the use of completely desiccated stomach linings and the fact that the stomach linings are nullified because they are less than sixty times the milk that it is placed in


The Aruch Hashulchan (Y.D. 87:43) records the common practice to produce cheese with completely desiccated stomach linings mixed together with other items. When the stomach lining is mixed together with other items to effect the curdling process, we may be lenient because this is a situation of “Zeh Vizeh Goreim,” an item that was created by two factors, one permissible and one forbidden, where we may disregard the permitted item if the permitted item could have accomplished the task even without aid of the forbidden item (see Rama Y.D. 87:11 and Shach Y.D. 87:35)

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