What Is Rennet?

Rennet on a plate with cheese

Verywell / Alexandra Shytsman

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What Is Rennet?

Rennet is an enzyme, usually derived from animals, that is used in the cheesemaking process. Parmesan, gorgonzola, pecorino romano, Camembert, Emmenthaler, manchego, Gruyère, and other artisanal cheese varieties are all traditionally made with animal rennet. Vegetarian rennet and microbial rennet are available but are not as widely used. If you follow a vegetarian diet or choose to avoid animal rennet, it's important to read labels carefully before choosing the right cheese for your eating plan.

Rennet is derived from the cleaned, frozen, salted, or dried fourth stomachs (abomasa) of calves, lambs, or goats. Bovine rennet is derived from adult cows, sheep, or goats. Calf rennet is the most widely used. Animal rennet is sometimes called "stomach-based rennet."

Rennet and bovine rennet contain an active enzyme called rennin (also called chymosin). In a young unweaned animal, this enzyme helps to coagulate milk so it remains in the stomach longer. Rennin converts liquid milk to a semisolid similar to cottage cheese, so that it stays in the stomach long enough for proteins to be digested properly.

Outside of the animal, the enzyme can also be used to coagulate milk in commercial cheese processing. Rennet is the commercial form of rennin and it is sold as a clear amber to dark brown liquid preparation or a white to tan powder or paste. It is used to coagulate milk, create curds, and thicken cheese during the manufacturing process. Simply put, it sets the cow's milk, sheep's milk, or goat's milk to a semisolid mass.

Many people wonder if animals are killed just for rennet. In most cases, animals (usually young calves) are slaughtered for their meat (often veal) and the fourth stomach which supplies the rennin is considered a byproduct.


Because rennet is an additive, not a food, the product has no nutritional value. It contains no calories, fat, protein, or carbohydrate and supplies no significant vitamins or minerals. There is some salt added to rennet as a preservative, but it usually runs off during the cheesemaking process.

Allergies and Food Safety

Rennet has been affirmed by the U.S. Food and Drug Administration as GRAS or "generally recognized as safe." Food additives with this designation have been reviewed by qualified experts and have been adequately shown to be safe under the conditions of its intended use.

The American Academy of Allergy, Asthma & Immunology has postulated that a rennet allergy is possible when consumers eat cheese. For example, if someone has a reaction to cheese and cow’s milk has been eliminated as an allergen, it is possible that rennet or another additive is the culprit.

In fact, research has identified animal rennet as a potent allergen, but generally in settings where exposure is substantial, such as in rennet production facilities. Other studies have suggested that both microbial rennet (derived from mold) and animal rennet may produce allergic reactions.

Rennet and Organic Cheese

There has been some concern that using microbial or animal rennet can mean that a cheesemaker is not able to use a USDA certified organic label on their product. Some food bloggers have noted that very small quantities of processing aids or preservatives used to make rennet are not organic and therefore make it harder to identify cheese that is organic. But the USDA says that "rennet is an example of a non-agricultural substance that is allowed in organic food products."

Animal Rennet Alternatives

Through the 1970s, animal rennet was the coagulant of choice among many cheesemakers in the United States and Europe. The product was widely available and it was a cost-effective additive for most cheesemakers. But as veal production began to taper off in the 1980s and 1990s (especially in the U.S.) rennet pricing and supplies became less consistent. This encouraged cheesemakers to come up with rennet alternatives.

Vegetable Rennet

Vegetable rennet or plant rennet is one alternative that has gained some traction, Vegetable rennet is made from certain vegetables that have coagulation properties. These plants (such as artichokes, nettles, or cardoon thistle) do not contain chymosin, but they have other enzymes that can coagulate milk.

There are certain parts of the world where the use of plant rennet (sometimes also called thistle rennet when it is made from thistle) is more common. For example, in Spain and Portugal cheesemakers use it to make artisanal products. However, many cheesemakers complain that vegetable rennet is inconsistent in its ability to thicken cheese. It can also affect the flavor of the cheese.

Microbial Rennet

Microbial rennet is made from specific molds that contain enzymes similar to chymosin. Even through this rennet is made from mold, there is no mold in the final product. There are also microbial rennets that are made from cells extracted from an animal's stomach. The cells are implanted into yeast cultures and then the yeast is genetically modified to be useful for cheesemaking.

Microbial rennet is known to yield inconsistent results. It may also impart a bitter flavor and is not favored by many traditional cheesemakers.

Homemade Rennet

Some people who make cheese also make their own rennet at home. There are online resources that provide instructions for making animal rennet, but this would obviously only be doable for those who live on a farm with livestock. The fourth stomach must be extracted shortly after the animal is slaughtered, so even getting this byproduct from a butcher would not work.

It is far more likely that home artisans would make rennet from plants, particularly nettle. You may be able to find nettle in your local natural foods store. You can also harvest nettle yourself, but there are safety implications. Some varieties can cause a skin rash from exposure to tiny hairs on the stems and leaves. It can also be toxic to dogs.

If you do choose to make your own nettle rennet, you'll boil nettle leaves with a small amount of salt, let them soak, and then drain the mixture. This nettle rennet is likely to be useable for a week or two when stored in the refrigerator. Homemade rennet is likely to work better with softer younger cheeses and less so with harder or aged cheese.

Where to Find Rennet

Some consumers may look for rennet to use in their own cheesemaking production, while other consumers may look to identify animal rennet to avoid it.

Finding Rennet in Cheese

When trying to find rennet in cheese, you can start by checking the product label, but manufacturers are not required to disclose whether or not they use animal rennet in the production of their cheese. In fact, if they use any type of rennet, they are not required to disclose what type of rennet it is. A product label might just say "enzymes" but some may disclose "animal enzymes."

Your best bet when looking for animal rennet-free cheese is to visit a local cheesemonger. An expert will be able to identify which cheeses (usually aged cheeses) are more likely to use animal rennet and which might use plant or microbial rennet. Softer cheeses are less likely to use any type of rennet. Also, vegan cheese alternatives do not use milk and do not require rennet for coagulation.

Cheese With No Animal Rennet

There are some cheese brands that advertise that they do not use animal rennet in the production of their cheese. If you don't have access to a cheesemonger, you can also do some detective work online to find out which companies use animal rennet and which ones do not. These are just some of the companies that provide information about the type of rennet they use.

  • Tillamook. This company has long used a vegetarian-friendly rennet in the production of many of its products. The rennet is also kosher and Halal certification. However, they traditionally used animal rennet for some of its aged cheeses. Recently they switched to a fermentation-produced, vegetarian-friendly rennet. But some of the older cheese is still in circulation. If you want animal rennet-free Tillamook cheese, look for "contains no animal rennet" on the label.
  • Organic Valley. This company makes cheese sticks, sliced cheese, and block cheese found in stores throughout the country. Organic Valley uses plant-based, microbial enzymes in the production of most of their cheeses. However, an animal-derived enzyme is used in their blue cheese.
  • Cyprus Grove. This California-based company clearly states on their website that they use only microbial (vegetarian) rennets in their cheesemaking process. Cyprus Grove cheeses are found in supermarkets around the country and include Humboldt Fog soft-ripened goat cheeses, aged cheeses, and other types of fresh cheese.
  • Cabot Creamery. According to the company's website, Cabot uses a microbial-based enzyme to manufacture all of their cheeses with the exception of their American cheese slices and shredded Mexican blend cheese. Cabot is widely known for its cheddar cheese, but they also make other varieties.
  • Point Reyes Farmstead Cheese. This women-owned company makes a well-known blue cheese and several other table cheeses. Search on their website for production information about different varieties. You'll see information about the type of rennet used in each one.

Finding Rennet for Cheesemaking

Many online vendors sell animal, plant, and thistle rennet. Kosher and Halal rennet varieties are also available. Cheesemaking supply companies can also answer questions about the best product for your cheesemaking needs. Many cheesemakers find that liquid and paste rennet are the easiest types to work with because they are easier to measure. Experts also recommend that you test and dilute rennet before using it.

Rennet should be stored in the refrigerator. Various types of rennet have different "best by" dates. Rennet doesn't go bad but will lose potency over time.

8 Sources
Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Chymosin (Rennin) and the Coagulation of Milk. VIVO Pathophysiology. Colorado State University.

  3. Generally Recognized as Safe (GRAS). U.S. Food and Drug Administration.

  4. Aged Cheese Allergy. American Academy of Allergy, Asthma & Immunology.

  5. Jensen A, Dahl S, Sherson D, Sommer B. Respiratory complaints and high sensitization rate at a rennet-producing plant. Am J Ind Med. 2006 Oct;49(10):858-61. doi:10.1002/ajim.20378

  6. van Kampen V, Lessmann H, Brüning T, Merget R. Berufliche Occupational allergies against pepsin, chymosin and microbial rennet. Pneumologie. 2013 May;67(5):260-4. German. doi:10.1055/s-0032-1326407

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By Malia Frey, M.A., ACE-CHC, CPT
 Malia Frey is a weight loss expert, certified health coach, weight management specialist, personal trainer​, and fitness nutrition specialist.