Does kosher animal rennet make sense?
From the OU:
Animal rennet and lipase can be kosher, however. If the kosher source animal is slaughtered, de-veined, salted and processed according to kosher law, its rennet and lipase are fine for kosher use. (There is no halachic problem with using animal-derived enzymes in cheese [mixing meat and milk] since the amounts used are miniscule. Moreover, the enzymes are not cooked with the milk, and they are flavorless. Also, the davar hama’amid principle cited earlier only applies to non-kosher substances, and the enzymes are actually kosher.) Still, even cheese made with glatt kosher animal rennet and lipase is considered gevinat Akum when manufactured by without special rabbinic supervision, as the sages created a general ban on such cheese.
https://oukosher.org/blog/consumer-kosher/say-cheese/