Why is the use of rennet - the stomach lining of calves - in preparing cheese not seen as a mixture of meat and dairy according to kosher standards?

Answer 1

The simplest way to answer this is that a miniscule portion of meat in a dairy product that does not impart taste is permitted. The separation between meat and dairy need not be absolute to the level of microbes, but to the level where the taste is noticeable and the quantity is still small. When rennet is used, it is primarily the microscopic enzymes that are added to the milk. However, rennet from non-kosher animals (such as cows that are slaughtered incorrectly or pigs) makes the dish non-kosher as a more stringent measure by rabbis to prevent the consumption of non-kosher animals.

Answer 2

Some kosher-supervisions (hechsherim) permit this (for a complex reason given in the Siftei Kohen commentary, paragraph 30, in Yoreh Deah ch.87). The general Orthodox trend today, however, is to forbid it and use only non-animal-sourced rennet.Those who are lenient about animal-rennet, agree that it must come from a kosher animal and be added under rabbinic supervision.

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