Biur Chametz says Blessed are the cheesemakers:
The rabbis were definite on this point: Non-kosher milk does not curdle, and thus cannot be used to make cheese: "Kosher milk curdles; non-kosher milk doesn't curdle" (BT Avodah Zarah 35b). In fact, the fact that milk has been turned into cheese is considered ample proof that the milk came from a kosher animal!Posted by Simon at July 20, 2004 02:11 AMIntrigued, I investigated further. Is this true? Why should milk curdle only if it's kosher? And hadn't I heard somewhere about camels' milk cheese?
Well, the main characteristic distinguishing kosher from non-kosher animals is rumination, the chewing of the cud. It turns out that the milk of ruminant animals differs from non-ruminant milk.