Q. What makes cheese kosher? What can be non-kosher about ordinary cheese?
Kosher cheese uses rennet taken from the stomach of animals slaughtered according to shechitah.
The Mishnah says that “if a person curdles milk with the skin of the stomach of an animal that was properly slaughtered and it imparts its flavour to the milk, it is forbidden” (Chullin 8:5); when rennet is used the quantity is too small to affect the flavour of the milk (Yoreh De’ah 87:11). Alternatively, it is possible to use a completely vegetarian rennet.
There is a second objection to non-kosher cheese. There is a rabbinic prohibition of “non-Jewish” cheese. This applies even when that the cheese contains only kosher ingredients. Therefore, in order to be kosher, the cheese must have been made or its manufacture supervised by a religiously reliable Jewish person.
The supervisor ensures that the machinery is completely clean, that only kosher rennet is used and, preferably, that by means of a token payment s/he is the owner of the milk.