Rennet is a set of enzymes used to coagulate milk into solid curds during cheesemaking. It works by breaking down a specific protein on the surface of milk particles, causing them to clump together and form the semi-solid mass that eventually becomes cheese. Nearly every cheese you’ve eaten was made with some form of rennet, though the source of that rennet varies widely, from calf stomachs to genetically engineered bacteria to thistle flowers.
How Rennet Turns Liquid Milk Into Cheese
Milk contains tiny protein clusters called casein micelles. These micelles stay suspended in liquid because they’re coated with a protective protein called kappa-casein, which keeps them from sticking to each other. Rennet’s key enzyme, chymosin, snips away that protective coating. Once enough of it is removed, the now-exposed micelles begin clumping together in a process that follows predictable physics: the rate of clumping depends on both the amount of enzyme present and how much milk protein is available.
The result is a gel-like curd that traps fat and other milk solids, while the liquid whey drains away. This separation is the foundational step in making virtually all cheese, from soft fresh mozzarella to aged cheddar. Cheesemakers typically add rennet to milk warmed to around 30 to 35°C (86 to 95°F), though the enzymes themselves reach peak activity closer to 45°C at a slightly acidic pH of about 5.2.
Traditional Animal Rennet
The original source of rennet is the stomach lining of young ruminant animals, usually calves. Specifically, it comes from the fourth stomach, called the abomasum. Calves naturally produce chymosin there because the enzyme helps them digest their mother’s milk. For centuries, cheesemakers extracted these enzymes by soaking strips of dried calf stomach in a salt solution, then adding that liquid to warm milk.
Despite its long history, animal rennet has become rare. Less than 5% of the world’s cheese is now made with it. The decline is driven by cost, limited supply of calf stomachs, and growing demand for cheese that fits vegetarian, halal, or kosher diets. You’ll still find animal rennet in certain traditional European cheeses where its use is protected by regional regulations, such as Parmigiano-Reggiano.
Fermentation-Produced Chymosin (FPC)
The most common rennet in commercial cheese today is made through genetic engineering. Scientists take the gene responsible for producing chymosin in calves and insert it into a microorganism, commonly a strain of E. coli bacteria called K-12. That microorganism is then grown in fermentation tanks, where it produces chymosin identical to what a calf’s stomach would make. The enzyme is harvested, purified, and sold to cheesemakers.
This product is called fermentation-produced chymosin, or FPC. It’s widely accepted in the cheese industry because it produces consistent results, costs less than animal rennet, and doesn’t require slaughtering calves. FPC is generally considered suitable for vegetarian diets, since no animal tissue ends up in the final product. Its regulatory status for kosher and halal certification varies by certifying body, so labels are worth checking if that matters to you.
Microbial Rennet
A separate category from FPC, microbial rennet comes from fungi that naturally produce milk-clotting enzymes without any genetic modification. The most widely used species is a mold called Rhizomucor miehei, which produces an enzyme that functions similarly to chymosin. Companies like Takabio and DSM manufacture these enzymes at industrial scale, and they’ve undergone safety evaluation by agencies including the European Food Safety Authority.
Microbial rennet is vegetarian-friendly and typically more affordable than animal rennet. However, some cheesemakers note that fungal enzymes can behave slightly differently during aging, occasionally producing bitter flavors in cheeses that are aged for long periods. For fresh and short-aged cheeses, the results are largely indistinguishable.
Plant-Based Rennet
Certain plants produce enzymes that can coagulate milk, and they’ve been used in cheesemaking for centuries, particularly in Portugal and Spain. The most common botanical sources are cardoon (a wild relative of the artichoke) and the artichoke plant itself. Their flowers and leaves contain protease enzymes that break down casein proteins in a way that mimics animal rennet, though not identically.
Cheeses made with thistle or cardoon rennet tend to have a distinctive creamy, slightly tangy flavor profile. Traditional Iberian cheeses like Serra da Estrela and Torta del Casar are defined by this ingredient. Plant rennet is suitable for vegetarian diets, but it remains a niche product globally because the enzymes are harder to standardize and can produce a softer, less predictable curd compared to chymosin.
How to Tell What’s in Your Cheese
In the United States, labeling rules allow cheesemakers to be vague. Under FDA regulations for standardized cheeses (cheddar, blue cheese, brick cheese, and many others), enzymes of animal, plant, or microbial origin can all be listed simply as “enzymes” on the ingredient label. That single word tells you nothing about whether the rennet came from a calf, a fungus, or a genetically engineered bacterium.
Some brands voluntarily specify “vegetable rennet,” “microbial rennet,” or “animal rennet” on their packaging, especially those targeting vegetarian consumers. If the label just says “enzymes,” your best option is to check the manufacturer’s website or contact them directly. Cheeses labeled as vegetarian-friendly or bearing a vegetarian certification mark will have avoided animal rennet. Organic cheeses in the U.S. may still use microbial or fermentation-produced chymosin, so “organic” alone doesn’t clarify the rennet source.
Why the Type of Rennet Matters
For many people, the type of rennet is a practical concern tied to dietary restrictions. Strict vegetarians need to avoid animal rennet. Observant Muslims and Jews may need specific certifications depending on whether the rennet is from a properly slaughtered animal or a microbial source. Vegans avoid all cheese made with animal milk regardless of rennet type, so the question is moot for them unless they’re choosing plant-based cheese alternatives.
From a flavor and texture standpoint, most people can’t taste the difference in everyday cheeses. The distinctions become more noticeable in aged or artisanal varieties. Animal rennet is sometimes preferred by traditional cheesemakers for its complex enzymatic profile during long aging. Plant rennets create their own characteristic textures. For the bulk of commercially produced cheese, FPC and microbial rennet deliver reliable, neutral results that let the milk and the aging process define the flavor.