Question: What is Vegetarian Rennet?
Rennet used in cheesemaking traditionally comes from animals. Specifically, the stomach lining of young animals that have been butchered. This can be a problem if you are a vegetarian who eats cheese.
Answer: If you're vegetarian, buy cheese made with microbial rennet or rennet derived from plants. Typically, plant-based rennet for cheese comes from thistle. Microbial rennet is usually fermented from bacteria but can also be genetically modified.
The FDA does not require cheesemakers to specify what type of rennet they use. However, rennet made from animal chymosin is usually called “traditional rennet” on a cheese label and most cheesmakers who use plant-based rennet will specify "thistle rennet" or "plant rennet" or "vegetarian rennet" on the label.
Your best bet is buying cheese from a cheese shop with knowledgeable cheesemongers. They should be able to point out all the cheese in the store that is not made from animal rennet. For more guidance, consult this partial list of artisanal cheeses made with vegetarian rennet.
Make Cheese at Home Without Rennet
There are several types of simple, fresh cheeses that you can make at home without rennet.