What Is Rennet? A Guide to Every Type

Rennet is the ingredient that turns liquid milk into solid curd. Without it, there is no cheese. But not all rennet is the same — and the type used has a real impact on flavor, texture, and who can eat the final cheese.

Here's how it works: milk is mostly water, fat, and protein. The key protein is casein. Rennet contains an enzyme called chymosin that causes casein molecules to clump together, separating into curds (solid) and whey (liquid). The cheesemaker then drains, presses, and ages those curds into cheese.

Animal Rennet

The original. Traditional rennet is extracted from the stomach lining of young ruminants — typically calves, but also lambs and kids. The fourth stomach chamber (the abomasum) naturally produces chymosin to help the animal digest its mother's milk. Cheesemakers have been harvesting this enzyme for thousands of years.

Animal rennet produces the most complex, nuanced flavors, especially in long-aged cheeses. It's still mandatory in many PDO-protected cheeses. Parmigiano-Reggiano is the classic example — by law, it must use animal rennet.

Thistle Rennet

A remarkable outlier. Some traditional cheesemaking cultures, particularly in Portugal and parts of Spain, coagulate milk using the dried stamens of the cardoon thistle (Cynara cardunculus). The plant enzymes behave differently than chymosin — they break down protein more aggressively, producing a silkier, more intensely flavored paste with a characteristic slight bitterness.

The result is unmistakable. Queijo Serra da Estrela, Portugal's most celebrated cheese, is made exclusively with thistle rennet — runny, oozing, and deeply savory.

Microbial Rennet

Most industrial cheese today uses microbial rennet, derived from cultivated molds — primarily Rhizomucor miehei. It's more affordable and scalable than animal rennet, making it the workhorse of commercial cheesemaking.

A more refined version, fermentation-produced chymosin (FPC), uses yeast or fungi to produce the exact same chymosin enzyme as a calf stomach — just grown in a lab. FPC now accounts for the majority of global rennet use. Most supermarket cheddar is made with it.

Microbial rennet is suitable for vegetarians, though its vegan status is debated since it replicates an animal enzyme.

Vegan Cheese — No Rennet Required

Vegan cheese is made from plant milks — cashew, coconut, oat, almond. Because these contain no casein, there is nothing for rennet to coagulate. Instead, vegan cheesemakers use acids, agar, tapioca starch, or carrageenan to set their cheeses. Miyoko's Creamery makes some of the most sophisticated examples, using cashew milk and live cultures.

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Rennet at a Glance

The rennet is invisible in the final cheese, but its fingerprints are everywhere — in the texture, the depth of flavor, and the story behind each wheel.