Is Pesto Vegetarian?

Short answer
Traditional basil pesto is not vegetarian because animal rennet is used to produce the cheeses it contains. However, creating vegetarian pesto using cheese substitutes such as tofu or nutritional yeast is perfectly possible.

Long answer
People often ask us whether pesto is vegetarian, and they tend to be very surprised when we say it's not. The reason it's not is because classic basil pesto contains Pecorino and Parmigiano-Reggiano, both made from milk curdled in animal rennet. That means most pesto sauces on supermarket shelves are not vegetarian.

Animal rennet is used in Italian hard cheese production because this complex set of enzymes can quickly separate milk into curds and whey. The rennet itself comes from the lining of a calf's stomach before being washed, brined, salted, and used.

You may wonder why no one has been clever enough to make a non-animal rennet. Well, some have, but EU law dictates that Parmigiano-Reggiano producers must continue to use animal rennet out of respect for tradition.

All is not lost, though. If you are making pesto yourself, you can easily swap the traditional cheese for a vegetarian alternative. Nutritional yeast is the magic ingredient preferred by producers of vegetarian pestos because it's cheaper than cheese but still has the same cheesy flavour. Better still, it doesn't just make the sauce vegetarian; it makes pesto vegan, too.