Are all cheeses kosher? Avodah Zarah 34-35

The rabbis legislated that cheese made by non-Jews is not kosher and one may not derive any benefit from that cheese. Since the curdling agent that comes from the stomach of any animal even a nonkosher one is not considered part of the animal and could be eaten (see the discussion starting on daf Avodah Zarah 34b), the Gemara on daf TB Avodah Zarah 35 wants to know the reason behind this legislation. The amoriam provides six different reasons. All quotes come from Sefaria.org translation

1.    “Rabbi Shimon ben Pazi says that Rabbi Yehoshua ben Levi says: It was due to the concern for puncturing, i.e., the concern that a snake might have deposited its venom in the cheese, as gentiles are not assumed to be careful about this.”

2.    “Rabbi Ḥanina says: The cheese is prohibited because it is not possible for it to have been made without containing particles of non-kosher milk”

3.    “Shmuel says: The cheese is prohibited because it is curdled with the skin of the stomach of an unslaughtered animal carcass.”

4.    “Rav Malkiyya says in the name of Rav Adda bar Ahava: The cheese is prohibited because gentiles smooth its surface with pig fat”

5.    “Rav Ḥisda says: It is because they curdle it with vinegar produced from their wine, from which it is prohibited to derive benefit.”

6.    “Rav Naḥman bar Yitzḥak says: It is because they curdle it with sap that is subject to the prohibition against consuming the fruit of a tree during the first three years after its planting [orla].”

The more different explanations why cheese made by non-Jews is prohibited leads one to believe that nobody knows the real reason. Rabbi Isaac Klein in his book Responsa and Halakhic Studies published by KTAV in 1975 wrote a long teshuva on the kashrut status of all cheeses. He concludes:

“To sum up: It is our considered opinion that commercial cheeses, all of them, including those in which rennet from any animal, kosher or nonkosher is used as the curdling agent should be permitted…

“The rennet used today cannot be considered forbidden because first, most of it is derived from dried up skins that are eitz be’alma (“mere wood”). In addition, the extraction is affected by the use of strong chemicals and acids which moves the substance from the status of a food (nifsal me-akhilat kelev). And third, the rennet goes through a number of chemical changes that transform it into a devar hadash (something brand-new-gg) And finally, the rennet is not put into the milk in a pure form but is diluted with other substances so that it is batal beshishim (annulled because the forbidden item is less than 1/60 of the total mixture-gg)… and we have decided that all the usual cheeses on the market, that lists the ingredients hard as well as soft, domestic as well as foreign, are kosher, beli shum hashash (without any doubts-gg) (pages 56-7)

I have not come to argue about the status of rennet for I completely agree with Rabbi Klein’s analysis. Nevertheless, I have discovered another ingredient that would make nonsupervisory cheeses not kosher. Once while reading the ingredients I saw the term pepsin. Since I didn’t know what it was, I looked it up and discovered that food-grade pepsin, a curdling agent for cheese, is typically derived from the glandular layer of animal stomachs, primarily pigs (porcine gastric mucosa) and doesn’t undergo the same type of transformation that rennet does. I asked one of my teachers whether this new information should reverse the Conservative Movement’s position on the kashrut of cheeses. To tell you the truth, I was disappointed with his answer. Although he did not dispute the facts on the ground, all he would say was once a leniency is issued it is difficult to retract. If the Conservative Movement believes that kashrut is an important mitzvah we should observe, shouldn’t new facts on the ground be relevant in reviewing old decisions?! Consequently, I still only eat supervise cheeses.

However, it's worth noting that some companies are now producing animal-free alternatives using methods like microbial fermentation with yeast as a host organism. All those cheeses are absolutely kosher no matter who makes them.