In today’s world of advanced food technology, much of the rennet used is microbial, that is, artificial. Nevertheless, mainstream halachic literature posits that Chazal banned all cheese made without onsite rabbinical supervison, irrespective of the presence of animal rennet, as a precaution against the consumption of actual non-kosher animal rennetbased cheese (Rambam, Hilchot Ma’achalot Asurot 3:14 and Shulchan Aruch ibid., 115:2). Thus, cheese made from artificial rennet (as well as Portuguese hard cheese made from thistleflower rennet) is not kosher when manufactured without onsite rabbinical supervison.
It should be noted that the bulk of today’s cheese manufactured in mainland Europe does contain animal rennet. Furthermore, lipase—an enzyme added to some cheeses to hasten the breakdown of fat and endow a more powerful flavor—is almost always animal-derived (lipase is extracted from the tongues of domesticated animals), although artificial lipase substitutes are becoming more widespread. Romano cheese is usually treated with goat, lamb or kid lipase, and blue cheese often contains calf lipase.
Still, even cheese made with glatt kosher animal rennet and lipase is considered gevinat Akum when manufactured without onsite rabbinical supervison, as the sages created a general ban on such cheese.
