Rennet is a set of enzymes that curdles milk into solid curds and liquid whey, the essential first step in making nearly every type of cheese. The key enzyme in rennet, called chymosin, works by cutting a specific protein on the surface of milk particles, causing them to clump together into a gel. Rennet comes in several forms today: traditional animal-derived, microbial, plant-based, and a bioengineered version that dominates commercial production.
How Rennet Turns Liquid Milk Into Curds
Milk contains tiny protein clusters called casein micelles. These micelles float freely because their outer layer carries a negative electrical charge that makes them repel each other, keeping the milk liquid. Chymosin works by snipping away this protective outer layer, specifically a protein segment that acts like a shield on the micelle’s surface.
Once about 65 to 90% of this protective protein has been cut away, the micelles lose their ability to repel one another. Positively charged spots on the micelle surface, previously hidden, become exposed. Calcium ions dissolved in the milk then act as bridges between neighboring micelles, pulling them together. The micelles first link into small chains rather than random lumps, and these chains gradually knit into a three-dimensional gel network: the curd. The liquid that drains out is whey.
This process is temperature-sensitive. Chymosin works best between 35°C and 40°C (roughly 95°F to 104°F). Below that range, the enzyme still cuts the protective protein but the micelles aggregate much more slowly. Above it, the enzyme begins to lose its effectiveness. Most cheesemakers add rennet to milk held at around 30 to 32°C, a slightly lower target that gives them more time to work with the curd before it firms up.
How Rennet Differs From Acid Coagulation
Not all cheese uses rennet. Fresh cheeses like ricotta, paneer, and cream cheese are often made by adding acid (vinegar, lemon juice, or bacterial cultures) to milk instead. The two methods produce fundamentally different curds. Rennet-set curds are firmer, with greater hardness and chewiness, and they trap more fat and protein. Acid-set curds hold more moisture and are softer, more brittle, and lower in fat content. This is why rennet-set cheeses can be aged into hard varieties like cheddar or Parmesan, while acid-set cheeses are almost always eaten fresh and soft.
Animal Rennet
Traditional rennet comes from the stomach lining of young calves, kids, or lambs. These animals naturally produce chymosin to digest their mother’s milk, and for centuries, cheesemakers harvested this enzyme by soaking dried stomach tissue in a salt solution. Animal rennet is still considered the gold standard for many protected-origin cheeses in Europe, where regulations may require its use. It produces a clean-flavored curd with predictable results during long aging.
Fermentation-Produced Chymosin
The most widely used form of rennet today isn’t extracted from animals at all. About 90% of commercial cheese in the United States is made with fermentation-produced chymosin, or FPC. This version is made by taking the gene responsible for chymosin production in calves and inserting it into a microorganism, typically a fungus or yeast, which then produces identical chymosin through fermentation. The enzyme is isolated and purified from the fermentation broth.
Pfizer developed the original technique using a strain of E. coli bacteria. Today, the major manufacturers use different organisms. Chr. Hansen produces its version using the fungus Aspergillus niger, while DSM uses the yeast Kluyveromyces lactis. Both have been granted “generally recognized as safe” status by the FDA.
FPC sits in an unusual spot for labeling purposes. The final enzyme is chemically identical to calf chymosin, and no animal was harmed to produce it, so many vegetarians consider it acceptable. However, because the production organism is genetically modified, FPC-made cheese cannot carry the Vegetarian Society’s approved trademark in the UK, which requires ingredients to be GMO-free. This means a cheese can be technically vegetarian (no animal slaughter involved) but still fail to qualify for certain vegetarian certifications.
Microbial Rennet
Microbial rennet comes from fungi that naturally produce milk-clotting enzymes, without any genetic modification. The most common source is the fungus Rhizomucor miehei, which produces an enzyme that cuts the same bond in milk protein that chymosin does. Chr. Hansen manufactures a widely used version from this species.
Early microbial rennets had a reputation for producing bitter flavors in aged cheeses, because the fungal enzymes were less specific than chymosin and broke down other proteins during ripening. Modern formulations have largely addressed this. Research comparing white cheese made with calf rennet versus Rhizomucor miehei rennet over a 90-day ripening period found that the levels of bitter-tasting amino acids were essentially the same regardless of rennet type. The proteolytic action of the microbial enzyme was nearly identical to that of calf rennet.
Plant-Based Rennet
Several plants produce enzymes capable of curdling milk, and some have been used in cheesemaking for centuries. The most famous is the cardoon thistle (Cynara cardunculus), whose dried flower stamens are soaked in water to create a coagulant. Shepherds in Extremadura, Spain, originally developed this method to avoid sacrificing young lambs for their stomach enzymes. It became central to iconic cheeses like Torta del Casar and La Serena, both made from sheep’s milk.
Plant rennets contain different enzymes than animal chymosin, and they behave differently depending on the type of milk. Thistle rennet works well with sheep’s milk and goat’s milk but tends to produce bitter flavors in cow’s milk cheese. This is why its traditional use has remained linked almost exclusively to sheep’s milk varieties. Other plant sources, including fig sap and certain types of nettles, have been used regionally but none have achieved wide commercial adoption.
Rennet Strength and Dosage
Rennet potency is measured in International Milk Clotting Units, or IMCU, a standardized measure defined by ISO. One unit represents the amount of enzyme needed to coagulate 10 mL of reconstituted skim milk at 30°C in 100 seconds. Commercial liquid rennet products typically range from about 200 to 280 IMCU per milliliter, though the concentration cheesemakers target in milk is much lower, usually 30 to 60 IMCU per liter of milk.
To calculate how much rennet you need, multiply the IMCU value of your rennet by 0.01. That gives you the number of liters of milk that one milliliter of rennet will set. So if your rennet is 200 IMCU/mL, one milliliter will set 2 liters of milk. For a 10-liter batch, you’d use 5 mL. A stronger rennet at 280 IMCU/mL would only require about 3.6 mL for the same batch. “Single strength” rennet is the concentration where 200 mL will set 1,000 kg of milk in 30 to 40 minutes at 30 to 32°C.
Getting the dose right matters. Too much rennet produces a tough, rubbery curd that’s difficult to work with and can develop off-flavors during aging. Too little results in a fragile curd that loses fat and protein into the whey, reducing your yield and producing a weaker-textured cheese. Most recipes specify both the amount and the expected setting time, typically 30 to 45 minutes, so you can adjust if your milk firms up too quickly or too slowly.
Which Type to Choose
For home cheesemakers, the choice of rennet depends on what you’re making and your dietary preferences. Animal rennet remains the traditional choice for aged, European-style cheeses and is required for authentic production of many protected varieties. FPC is the practical default for most applications, offering consistent results, wide availability, and no animal slaughter, though it does involve genetic modification. Microbial rennet is the go-to for cheesemakers who want both a vegetarian and non-GMO option, and modern versions perform comparably to animal rennet in most cheese styles. Plant-based thistle rennet is best reserved for sheep’s or goat’s milk cheeses where you want the distinctive, slightly floral character it brings to the final product.
All four types are available as liquids, and some come in tablet or powder form. Liquid rennet is easier to measure precisely, while tablets have a longer shelf life. Regardless of form, rennet should be stored in the refrigerator and used before its expiration date, as enzyme activity declines over time.