Is It Kosher?
By Rabbi Eliezer Eidlitz — a comprehensive guide to the kosher status of hundreds of common foods, ingredients, and food categories.
Cheese
All of the many varieties of commercially prepared cheeses are produced by the same basic process. A type of bacteria (known as a "starter") is added to milk, souring it. Next, a curdling agent is added to coagulate part of the milk. The most common curdling agent, rennet, generally comes from the lining of the stomachs of calves. This raises clear kashrus implications.
If rennet is from a kosher species of animal ritually slaughtered under rabbinical supervision, or if it is from vegetable sources, there is no question as to the kashrus of the cheese produced under rabbinical supervision. However, most commercial cheeses are made with rennet derived from animals slaughtered by conventional non-kosher means.
Rennet cannot be rendered permissible by the halachic principle of bitul (nullification in small quantities) because it has an unmistakable coagulant effect — any substance that visibly solidifies another is always considered a substantial factor, whatever its amount.
An additional factor is an ancient decree banning the use of cheeses produced by non-Jews (gevinas akum).
As a response to the kashrus problems of cheese-making, several kosher cheese companies make use of rennet derived from exclusively kosher sources. Special kosher runs are done at standard cheese companies under rabbinical supervision. Both hard cheeses (American, Cheddar, Muenster, Swiss, etc.) and soft cheeses (cottage, cream, farmer, pot cheese) require supervision. NOTE: The general prohibition against all cheeses made by gentiles without supervision includes RENNETLESS CHEESE.