This is something I get asked about more than you would think. People are baffled confused as to whether cheese is vegetarian, and the simple answer is, well, sometimes.
Very simply, cheese is made by the separation of milk into curds and whey. The curds (the lumpy bits) are separated from the whey (the watery bit) and squashed together to form cheese. Traditionally, animal rennet is used for this process.
Animal rennet is the name given to a collection of enzymes which exist in the stomachs of very young mammals to break down their mothers’ milk. For cheese production, very young cows are killed to extract these enzymes from their stomach linings, hence making cheeses produced this way a “product of slaughter” and therefore not suitable for a vegetarian diet.
There are plant-based alternatives that can be used to coagulate milk into curds and whey – even an acid such as lemon juice or vinegar will do the job, and this is what seems to be used in most cheese based production these days.
So what about Parmesan?
Parmesan, or Parmigiano-Reggiano has a Protected Designation of Origin status, meaning that where, and importantly here, how it is produced has to meet strict criteria (detailed here), else it is illegal to use the name “Parmigiano-Reggiano”, or it’s translation, “Parmesan”. One such criterion is that the milk coagulation must take place “exclusively by the use of calf rennet”. Therefore vegetarian Parmesan is technically, and legally impossible.
Incidentally, the leftover whey is usually dried into a powder and used in the production of many other foods, including chocolate and protein shakes, which also explains why some chocolate is not vegetarian either.