Kosher laws are Jewish rules concerning what may or may not be eaten. These holy laws dictate which animals are clean or unclean for consumption, how animals must be slaughtered and processed, and which foods may not be eaten together. Kosher pizza is pizza that adheres to these kosher laws, making it fit to eat for those who follow this way of life.
There are many considerations that must go into the making of kosher pizza. One of the less obvious concerns is whether the dough itself conforms to kemach yashan, a law that says grain must have taken root prior to Passover to be eaten in a given year. While Jewish law mandates following this law in Israel, Jews in other countries are not required to do so. Some do, however, and for these people it is important that the pizza start with kosher dough.
All cheese used to make kosher pizza must also be kosher. By definition, any cheese not made by Jews is not kosher and therefore cannot be used on a kosher pizza. It is not necessary for a Jew to personally make the cheese, especially since in factories much of the work is done by machine, but it must be supervised according to kosher law. When rennet, an essential ingredient of cheese, is added, it must be done by a Jewish person, or the factory must be owned by Jews.
The addition of rennet is considered highly important because rennet is actually an animal-based product. This enzyme is made from the lining of an animal’s stomach, typically a calf that has never eaten anything but milk. For the rennet to be usable, the animal must have been slaughtered and processed according to kosher law. Having a Jewish person supervise the process of adding the rennet is intended to ensure that only kosher rennet is used.
Once all of the ingredients for a kosher pizza have been determined to adhere to kosher law, they can be combined to make a pizza. One of the rules of kosher states that meat and dairy cannot be combined, so a kosher pizza will have cheese and sauce on it, but no meat of any kind. (Rennet is added to milk in a minuscule amount and does not break kosher laws.) Instead, kosher pizza is made from several different kinds of cheese with added herbs and may include chopped vegetables such as mushrooms and tomatoes.