Is Pesto Vegetarian?

Parmesan cheese being grated.

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

Long answer
We're often asked whether pesto is vegetarian, and people are surprised when we say it's not. The reason is because classic basil pesto contains pecorino and Parmigiano-Reggiano, which are both made from milk curdled in animal rennet. That means that most of the pesto sauces you see 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 stomachs before being washed, brined, salted, and used.

You may wonder why no one has been clever enough to make a non-animal rennet. Well, they have, but Italian producers of Parmigiano-Reggiano not only continue to use animal rennet out of respect for tradition but also because EU law dictates that the cheese cannot be sold under that name if it doesn't contain it.

All is not lost, though. If you are making pesto yourself, you can easily switch 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; but it makes pesto vegan too.