Understanding rennet — what it is, how it works in cheesemaking, and the difference between animal and vegetarian (microbial and FPC) rennet.
Rennet is one of cheesemaking's key ingredients, used to coagulate milk into curds. But there are different types — animal and vegetarian — with implications for vegetarians. Here's a guide to understanding rennet.
What Rennet Is
Rennet is a substance (containing enzymes, chiefly chymosin) used in cheesemaking to coagulate milk — that is, to set it into solid curds, separating the curds from the liquid whey. This coagulation is a fundamental step in making most cheeses (especially firm and aged ones), allowing the curds to be cut, drained, and formed into cheese. Rennet works by acting on the milk's proteins (casein), causing them to bond and set into a gel. So rennet is the coagulant that turns milk into curds, a key cheesemaking ingredient. Understanding rennet helps you understand cheesemaking and choose vegetarian cheeses.
How Rennet Works
Rennet's enzymes (mainly chymosin) act on the casein proteins in milk, cleaving a specific part of them, which destabilizes the proteins and causes them to clump together and form a gel — coagulating the milk into curds. This is different from acid coagulation (used in cheeses like ricotta and paneer), which curdles milk via acidity; rennet creates a firmer, more elastic curd suited to many cheeses, especially firm and aged ones. So rennet works by enzymatically setting the milk's proteins into curds, producing a firm, workable curd. This rennet-set curd is the basis of most rennet-coagulated cheeses, from cheddar to brie.
Traditional Animal Rennet
Traditionally, rennet comes from animals — specifically the stomachs of young ruminants (like calves), which naturally contain chymosin (the enzyme that helps young animals digest milk). This animal rennet has been used for millennia and is still used for some traditional cheeses. However, because it's derived from animal stomachs (requiring the slaughter of young animals), animal rennet makes such cheeses unsuitable for vegetarians. So traditional animal rennet, from young animals' stomachs, is the original coagulant but isn't vegetarian. Many traditional and some protected cheeses (like authentic Parmigiano-Reggiano) use animal rennet.
Vegetarian Rennet Options
For vegetarian cheese, non-animal rennet alternatives are used. The two main types: microbial rennet, produced by certain molds or fungi (a non-animal source of coagulating enzymes), and fermentation-produced chymosin (FPC), made by genetically modified microbes that produce chymosin identical to the animal enzyme (FPC is widely used and very effective). There are also some plant-based coagulants (like cardoon thistle, used in certain Portuguese and Spanish cheeses). So vegetarian rennet — microbial rennet, FPC, or plant coagulants — allows cheese to be made without animal rennet, suitable for vegetarians. Much modern cheese uses these vegetarian alternatives.
Rennet and Choosing Vegetarian Cheese
The type of rennet matters for vegetarians, as animal-rennet cheeses aren't vegetarian. To choose vegetarian cheese, look for labels saying "suitable for vegetarians," "microbial rennet," "vegetable rennet," or "vegetarian rennet," or choose acid-set cheeses (like paneer, ricotta) that use no rennet. Note that the same cheese type (e.g. cheddar) may use animal or vegetarian rennet depending on the producer, and some traditional/protected cheeses use animal rennet. So check the rennet type if you're vegetarian. Many cheeses are now made with vegetarian rennet, but not all, so reading labels (or asking) is key for vegetarians.
Frequently Asked Questions
What is rennet?
Rennet is a substance containing enzymes (chiefly chymosin) used in cheesemaking to coagulate milk into curds, a key step in making most cheeses. It works by setting the milk's proteins into a gel.
What's the difference between animal and vegetarian rennet?
Animal rennet comes from young animals' stomachs (not vegetarian); vegetarian rennet includes microbial rennet (from molds/fungi), fermentation-produced chymosin (FPC, from microbes), and plant coagulants — all non-animal.
How do I know if cheese has vegetarian rennet?
Look for labels saying "suitable for vegetarians," "microbial/vegetable rennet," or "vegetarian rennet," or choose acid-set cheeses (like paneer, ricotta) that use no rennet. Check, as it varies by producer.