I know, I'm shook, too
The other day, BuzzFeed writer Delaney Strunk had a revelation that shook her vegetarian, cheese-loving soul to the core. It turns out that Parmesan cheese isn't vegetarian. Strunk came to this realization while watching a Tasty cooking video that called for vegetarian Parmesan. That led her to a Google search that showed her that Parmesan cheese is not vegetarian, when made in the traditional method. That's because, she explains, "to earn the authentic legal title of Parmigiano Reggiano, the cheese must be created using calf rennet," which is a fancy term for the enzymes in the stomach of a cow.
It might sound gross to use enzymes from an animal's stomach to make cheese, but using rennet is a long-standing tradition that dates back thousands of years, especially among European cheesemakers. As Harold McGee explains in On Food and Cooking, "At least 2,500 years ago, shepherds began to use pieces of the first stomach of a young calf, lamb, or goat to curdle milk for cheese; and sometime later they began to make a brine extract from the stomach."
Sadly for those cheese-loving vegetarians—including BuzzFeed's Strunk—Parmesan isn't the only cheese made with rennet. She notes that vegetarians also can't eat Gruyère, Manchego, Pecorino Romano, or Gorgonzola because it's got those sweet, sweet stomach enzymes.
Fortunately for people who don't want to give up either vegetarianism or Parmesan, there is such a thing as vegetarian Parmesan, even if it's not technically Parmigiano Reggiano. In those cases, the animal rennet is replaced with engineered, plant-based rennets. In fact, according to McGee, "most cheese in the United States is made with these engineered 'vegetable rennets,'" which is probably good news for those who might be a little squeamish about the whole cow stomach thing.
So, the lesson here is that if you're just picking up a block of cheese, you can't assume it's vegetarian. And if you really want to make sure what you're eating contains no animal products, just make vegan parmesan cheese at home with nutritional yeast, cashews, garlic powder, salt, and a high-powered blender.