Why would mozzarella cheese be considered not kosher, thusly making a slice of pizza treif?
Cheese is made by putting an enzyme into raw milk, causing it to curdle. The most common source for the enzyme (also known as rennin or rennet) is from the stomach lining of cows. The stomach is dried and ground into a powder, and mixed with other chemicals and liquid – and a small amount of the new compound is placed in the raw milk. There are two common opinions about the use of such enzymes to make cheese. According to one opinion, the cheese is non-kosher because of the forbidden mixture of milk and meat. According to the other opinion, once the stomach lining becomes completely dry (“like wood,” to use the language of the Shulchan Aruch, an authoritative code of Jewish law), it is inedible and becomes a new substance, unrelated to the animal from which it came. This new substance can be considered kosher and parve, and according to this understanding, any domestic hard cheese is kosher. Imported cheeses still may make cheese the old fashioned way – using a fresh piece of stomach l
