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- Cheese is made by acidifying the milk with bacterial cultures to produce a mixture of solid curds and liquid whey. After the whey is removed the curds are kept loose or molded into cheese. Hard cheeses have rennet added to help them set and solidify. Afterwards different cheeses are processed and aged differently. For kashrut purposes there's two main factors for cheese to be kosher. First all of the ingredients and utensils used to make it need to be kosher and secondly there is a need for a mashgiach to be watching the whole process to avoid the rabbinic enactment against non-Jewish cheese. 
- The rabbinic enactment against non-Jewish cheese (Gevinat Akum) applies even if it was made with completely kosher ingredients and even everyone in that locale uses only kosher ingredients.
- The way in which the rabbinic enactment wouldn't apply is if either: 1) a Jew owns the company even if the workers are non-Jewish, 2) a Jew puts the rennet into the milk, or 3) some say, that it is sufficient to have a Jew watching the cheese making process.
- There is a major dispute between the poskim whether or not soft cheese, such as cream cheese, cottage cheese and farmer's cheese, is subject to the prohibition of non-Jewish cheese. Some poskim hold that since soft cheese can be made without the use of rennet and is made with bacterial cultures, it shouldn't be considered a cheese for the purposes of this enactment and would be permitted without any Jewish involvement as long as all of the ingredients and utensils are kosher. However, others disagree and don't rely on that leniency.
- Since there are many rennets which are non-Kosher ingredients one shouldn't eat cheese without valid hechsher even if it was made or owned by a Jew.
- Rennet is an enzyme produced in the stomachs of ruminant mammals that is used to curdle and set milk into cheese.
- If the rennet comes from a kosher animal which was slaughtered properly by a Jew then it is kosher as long as there is 60x the milk in comparison to the rennet. In terms of actual process of how the rennet is extracted from the Kosher animal so that it should be permitted even initially is complex discussion.
- If the rennet comes from an animal which was not slaughtered properly it is forbidden and can't be nullified since it is the agent which solidifies the milk into cheese.
- Alternative sources of rennet include vegetable rennet, microbial rennet, and Fermentation-produced chymosin.
- Even if the rennet is kosher it is still possible for other ingredients added to the cheese to be non-Kosher such as lipase, an enzyme used to hasten the breakdown of fat and give it a more powerful taste.
- Rabbi Avraham Gordimer in Jewish Action (Winter 2005) and Cheese Production on Wikipedia
- Mishna (Avoda Zara 29b) says that the reason that gevinat akum is forbidden is because the non-Jews would use the curdled milk from the stomach of an Avoda Zara animal to help curdle the milk into cheese. The Gemara (Avoda Zara 35a) provides another 5 reasons as to why the cheese of non-Jews is forbidden among them is the concern that they will use the stomach lining of animal that wasn't slaughtered by a Jew as rennet. Another two reasons are that they may mix in milk from a non-Kosher animal into the cow milk being used for the cheese or that they may smear the finished cheese with pig fat. Rambam (Machalot Asurot 3:13) only cites the reason of the concern that the non-Jews will use non-Kosher rennet. The Tur YD 115:2, however, also quotes the reason of non-kosher milk being mixed in and pig fat being smeared on the cheese.
- Completely kosher ingredients: Tosfot a”z 35a s.v. chada says that some said that there’s no gezerah of gevinat akum by cheese that was made with enzymes from flowers. Meiri avoda zara 35b s.v. Hachamishi even contends that if everyone in a certain locale used rennet made from flowers there would be no gezerah of gevinat akum. However, the Rambam (Machalot Asurot 3:14) quotes some geonim who said that the cheese that the non-Jews make from vegetable rennet is forbidden even though it is made with completely kosher ingredients. Rashba (Torat Habayit 90b) agrees. The Tur and Shulchan Aruch YD 115:2 codify the opinion of the Rambam. This is also the opinion of the Shach 115:19, Kaf Hachaim 115:30, Yalkut Yosef YD 81:21, and Rav Yisrael Henkin (Edut L’Yisrael p. 176). See further in M’peninei Harav p. 153.
- Shach 115:20
- Shach 115:20
- Rambam Pirush Mishnayot a”z 2:5 says if a Jew is watching the gevinat akum is muter. Or Zaruah a”z no. 197 agrees. Rama YD 115:2 rules that if a Jew is watching the non-Jew make the cheese there is no rabbinic prohibition of gevinat akum. Even though the Shach 115:20 argues, Igrot Moshe YD 1:50 and Kaf Hachaim 115:32 are lenient.
- The Rama 115:2 adds that even if a Jew only watched the cheese making process and didn't see the milking process it is permitted after the fact since the non-Jew wouldn't use non-Kosher milk to produce cheese as milk from non-Kosher animals don't curdle. Shach 115:23 writes that this is only correct if the cheese is sold by pieces of cheese or weight but not if it is sold by volume otherwise there is a concern that the non-Jew added non-Kosher milk. Kaf Hachaim 115:34 records that such was the practice of the Jews of Yerushalayim to buy cheese produced in Sfat where Jews participated in the cheese making process even though they didn't see the milking process. Interestingly, Kaf HaChaim 115:35 writes how this practice is justified even though one time a non-Jew was caught mixing in rennet before they brought the milk to the Jews for the cheese making process.
- Rabbi Avraham Gordimer explained that the OU and other kashrut organization's policy was to rely on this leniency and to give a hechsher to cottage cheese as long as all ingredients and utensils in the factory are kosher even if a Jew isn't present all of the time.
- Igrot Moshe YD 1:50 says that we can rely on the Rama to have jews watching the cheese making process, but otherwise it is forbidden even if it all kosher ingredients. There he’s also strict on cottage cheese but says not to protest those who are lenient. In Igrot Moshe YD 2:48 he elaborates on the topic of cottage cheese and says that cottage cheese might not be considered gevinat akum since it could have been made without any rennet, so they never made a gezerat about it. He concludes that there’s a strong reason to be lenient but didn't want to establish any opinion as it being permitted or forbidden but wouldn’t protest those who are lenient. Aruch Hashulchan 115:16 implies clearly that cheese which doesn’t need rennet to become cheese is still forbidden. Chelkat Binyamin 115:60 writes that it is a unresolved question.
- Kaf Hachaim 115:36, Rabbi Avraham Gordimer in Jewish Action (Winter 2005)
- Wikipedia on Rennet
- Wikipedia on Rennet points out that using the traditional method of rennet extraction one 1 gram of rennet can be used to coagulate 2 to 4 Kg of milk and the modern extraction method after concentration yields rennet that one gram of which can be used to coagulate 15 Kg of milk. Either way, this is clearly less than the ratio of 1 in 60, as Rabbi Avraham Gordimer points out as well. Mishna Chullin 116a (according to Rashi) writes that since the rennet from the kosher animal is meat and it is being used to coagulate milk it is only permitted if there is 60 times the rennet in the milk otherwise there is a prohibition of eating milk and meat together. Shulchan Aruch YD 87:11 codifies this.
- Why is rennet from a non-Kosher animal not nullified and rennet from a kosher animal permitted if it is less than one in sixty? The Ri Migash (cited by Torat Habayit 90a) explains that this is different than rennet from a non-Kosher animal which isn't nullified because it is a coagulating agent and is intrinsically forbidden unlike kosher rennet. Rambam (Maachalot Asurot 3:13, 9:16), Rashba Chullin 116a, and Ritva Avoda Zara 35a agree. Shach 87:35 agrees. However, the Tosfot Avoda Zara 35a s.v. mipnei and Raavad (Torat Habayit 90b) argue that fundamentally rennet from a kosher animal is the same as non-kosher rennet in that they are nullified if it isn't tasted but since the issue of meat and milk is only rabbinic if it is a cold mixture it is permitted in cases of doubt. Maharshal Chullin 8:106 agrees.
- Chatom Sofer (responsa 81, cited by Pitchei Teshuva 87:19), Rabbi Akiva Eiger (responsa 1:207)
- Avoda Zara 35a says that a maamid (coagulating agent) is forbidden even if it isn’t recognizable and has no taste since its accomplishment of transforming the food is recognizable. Mishna Chullin 116a states that cheese made with rennet from an animal that lacked a properly ritual slaughter is forbidden. Rambam (Machalot Asurot 3:13) writes that the non-Kosher rennet isn't nullified since it is the agent used to coagulate the cheese. Shulchan Aruch YD 87:11 codifies this. The Maharshal Chullin 8:106 comes to the conclusion that even non kosher rennet is nullified one in sixty like the Raavad (cited by Rashba Torat Habayit 90b) unlike Rambam Maachlot Asurot 3:13. Shach YD 87:35 and Magen Avraham 442:9 reject the Maharshal. Dagul Mirvavah 87:4 supports the Maharshal from Rashi. Igrot Moshe YD 2:32 disagrees with the Dagul Mirvavah's proof. Nonetheless, Igrot Moshe is lenient for the Maharshal after the fact for a rabbinic prohibition or something that is a doubt if it is Biblical.
- Is Maamid a Biblical concern? The Bach assumes that it is. Shach 87:32 argues that for milk and meat it isn't since it is a cold mixture. Magen Avraham 442:9 writes that in general maamid is Biblical since it is like the prohibition is still intact. Chok Yakov 442:16 argues.
- Wikipedia on Rennet
- Rabbi Avraham Gordimer