QUESTION: What is Gevinas Akum?
ANSWER: Gevinas Akum refers to milk that belonged to a nochri which was made into cheese without Jewish involvement. Shulchan Aruch (YD 115:2) explains that Chazal prohibited Gevinas Akum out of concern that the cheese might be produced with non-kosher animal rennet.
Rennet is an enzyme that coagulates cheese curd and causes the water to separate from the milk solids. Animal rennet is typically extracted from the stomach lining of a calf. If the calf was not properly slaughtered, the rennet is non-kosher.
The Rishonim explain that although the amount of rennet used in cheese production is typically less than one part in sixty, the rennet is not batel because it is a davar hama’amid (an ingredient that supports a change or transformation) which is not batel even be’elef (in 1,000 parts).
Shulchan Aruch adds that all forms of Gevinas Akum are prohibited, even if the coagulant was an herb, and not animal rennet. This is based on the principle known as “lo plug” – i.e., that Chazal often enacted prohibitions across the board, without distinguishing between different situations.