פירוש על עבודה זרה 70:11

If the milk was from an unclean animal (such as a camel or pig), it would have a different color from milk from sheep, goats or cows (clean animals). And if the milk was a mixture, such that it would look like kosher milk, then the milk could not be used to curdle cheese, because milk from unkosher animals cannot be used to curdle. So why should we be concerned about unkosher milk?