Rishonim ask why the stomach lining of a N’veilah renders cheese not Kosher. Indeed, only a small amount of the lining is used and there is certainly more than sixty times of milk than stomach lining and thus the stomach lining should be nullified (Battel) by the milk. The Rambam (Hilchot Maachalot Asurot 3:13) answers that the stomach is a Davar Hamaamid (it establishes the form of the item, in this case it is the catalyst that turns the milk into cheese) and is not Battel even if it less than sixty times the product it was placed into. This approach is exceedingly logical. The reason why something is Battel is because if it is less than sixty times the product it was placed in, then it has lost its significance. The stomach lining, though, cannot be described as insignificant since it is indispensable in creating the cheese.
The Maggid Mishneh (ad. loc.) cites a different answer presented by the Ramban and the Rashba, that the fact that the taste of the stomach lining is nullified is irrelevant because Chazal enacted the prohibition of Gevinat Akum to create a social barrier between us and non-Jews and not because of a Kashrut problem. This approach is quite cogent in light of our practice to prohibit Gevinat Akum even if the reason for its enactment does not apply.
The Rambam (Hilchot Maachalot Asurot 9:16), in turn, asks that if the stomach lining is never Battel because it is a Davar Hamaamid, then cheese made from the stomach lining of animal that is slaughtered properly should also be forbidden because of the mixture between milk and meat. The Rambam answers (following the approach of his father’s Rebbe, the Ri Migash) that the rule that a Davar Hamaamid is never Battel applies only if the Davar Hamamid is prohibited already (such as the stomach lining of a non-Kosher animal). An item that is Kosher (the stomach lining from a properly slaughtered animal) cannot create a prohibited mixture of milk and meat because it is a Davar Hamaamid. See Tosafot (Avodah Zarah 35a s.v. Mipnei) for a different resolution of this problem.
One may ask then how is it permissible to make Kosher cheese if we are forbidden to intentionally nullify prohibited items (“Ein Mivatlin Issur Lichatchilah,” see Shach Y.D. 87:33). For example, we are not permitted to intentionally place a bit of meat into a glass of milk if we wish to drink the milk, even if there is at least sixty times more milk than meat. How then are we permitted to add a bit of stomach lining to milk in order to make cheese? Rav Akiva Eiger (Teshuvot number 207, cited in the Pitchei Teshuva 87:19) explains that the prohibition of Ein M’vatlin Issur L’chatchilah does not apply if two lenient factors are in effect - the use of completely desiccated stomach linings and the fact that the stomach linings are nullified because they are less than sixty times the milk that it is placed in.
The Aruch Hashulchan (Y.D. 87:43) records the common practice to produce cheese with completely desiccated stomach linings mixed together with other items. When the stomach lining is mixed together with other items to effect the curdling process, we may be lenient because this is a situation of “Zeh Vizeh Goreim,” an item that was created by two factors, one permissible and one forbidden, where we may disregard the permitted item if the permitted item could have accomplished the task even without aid of the forbidden item (see Rama Y.D. 87:11 and Shach Y.D. 87:35). For further discussion of this issue see Pitchei Teshuva (Y.D. 87:19) and Darkei Teshuva (87:138).
