Natural calf rennet is the traditional and original source of rennet, widely prized for its high-quality enzyme content and ability to produce premium cheeses with superior texture and flavor. It is derived from the stomachs of unweaned calves, which naturally produce the enzyme chymosin, essential for milk coagulation.
What is Calf Rennet?
Calf rennet is an extract obtained from the fourth stomach (abomasum) of young, milk-fed calves. The key enzyme, chymosin, specifically targets kappa-casein in milk, causing it to coagulate into curds. This process is the cornerstone of traditional cheesemaking and is responsible for the firm yet smooth texture of many hard and semi-hard cheeses.
Specialty cheeses: Artisan varieties requiring precise coagulation and aging.
Fun Fact
The use of calf rennet dates back thousands of years, with ancient cheesemakers discovering that the stomach lining of milk-fed calves was the most effective natural coagulant for transforming milk into long-lasting cheese.
✅ Key Takeaway: Natural calf rennet remains the gold standard for cheesemakers seeking authentic texture, flavor, and traditional quality. Its careful production—from calf selection to enzyme extraction—ensures that every wheel of cheese meets the highest culinary standards.
OTHER SOURCES
Natural calf rennet is extracted from the inner mucosa of the fourth stomach chamber (the abomasum) of young calves. These stomachs are a by-product of veal production.
If rennet is extracted from older calves (grass-fed or grain-fed) the rennet contains less or no chymosin but a high level of pepsin and can only be used for special types of milk and cheeses.
As each ruminant produces a special kind of rennet to digest the milk of its own mother, there are milk-specific rennets available, such as kid goat rennet especially for goat's milk and lamb rennet for sheeps milk.
Rennet or digestion enzymes from other animals, like swine-pepsin, are not used in cheese production.
Rennet is a complex set of enzymes crucial in the production of cheese. Its primary function is to coagulate milk, turning it from a liquid into curds and whey, which forms the foundation of countless cheese varieties around the world.
What is Rennet?
Rennet is traditionally sourced from the stomach lining of ruminant animals, such as calves, lambs, or goats. The key active enzyme in rennet is chymosin (also called rennin), which acts on milk proteins—mainly casein—causing them to thicken and form curds.
Rennet isn’t just about coagulation; it also influences:
Texture: Soft, semi-hard, or hard cheeses.
Flavor: Proper enzyme activity allows cheese to develop rich, complex flavors during aging.
Aging potential: Cheeses coagulated with rennet often age better than acid-coagulated cheeses.
Fun Fact
Some cheeses, like Mozzarella and Paneer, can be made without rennet, relying instead on acid (like lemon juice or vinegar) for curd formation. However, rennet-based cheeses usually offer a more elastic, meltable texture.
✅ Key Takeaway: Rennet is the silent hero of cheesemaking, transforming milk into the diverse, delicious world of cheeses. From traditional Parmigiano-Reggiano to artisan vegetarian varieties, understanding rennet is essential for both chefs and home cooks aiming to perfect their craft.
OTHER SOURCES
Rennet is a natural complex of enzymes produced in any mammalian stomach to digest the mother's milk, and is often used in the production of cheese.
Rennet contains many enzymes, including a proteolytic enzyme (protease) that coagulates the milk, causing it to separate into solids (curds) and liquid (whey).
The active enzyme in rennet is called chymosin or rennin (EC 3.4.23.4) but there are also other important enzymes in it, e.g., pepsin or lipase.
There are non-animal sources for rennet that are suitable for vegetarian consumption.
Cheese Rennet...
Rennet is one of the foundational ingredients of the cheesemaking process. Its function may seem simple—coagulating milk—but the type of rennet you choose influences every stage of your cheese’s development, from curd formation to final flavor. Understanding rennet deeply is essential for both home cheesemakers and professional affineurs, as it determines consistency, quality, and the identity of the cheese itself.
This extended guide explores why rennet matters, how different types behave, and how your choices shape the outcome of your cheese.
1. Rennet Is the Architect of Cheese Texture
Curd texture is the backbone of every cheese style. Rennet’s enzymatic action determines:
How quickly curds form
How firm the curds become
How much moisture the curd retains
The elasticity and stretchability of the final cheese
Animal Rennet: Superior Elasticity and Firmness
Animal rennet contains high levels of chymosin, an enzyme that produces tight, firm, well-structured curds. This is why traditional aged cheeses require animal rennet—it builds a curd that can withstand months or years of aging without collapsing or breaking down.
Microbial rennet produces consistent coagulation but slightly weaker curds.
Vegetable rennet often produces softer curds and may behave unpredictably depending on the plant source.
Why this matters:
Soft, weak curds release moisture slowly and inconsistently, altering yield and final texture.
2. Rennet Shapes Flavor—Even Long After Curdling
Many cheesemakers underestimate the lasting influence of rennet. After coagulation, rennet enzymes continue breaking down milk proteins throughout aging—a process called proteolysis.
Rennet Choice = Flavor Direction
Animal rennet enhances:
Nutty notes
Butteriness
Meaty and savory umami
Deep complexity during long aging
Microbial rennet can introduce:
Mild bitterness over long aging
Slight “yeasty” or “fermented” notes in some batches
Vegetable rennet contributes:
Herbaceous, grassy, or earthy notes (from thistle/cardoon)
Potential bitterness if aged too long
Why this matters:
A cheese aged for six months to two years depends heavily on rennet type for its final flavor development. Choosing the wrong one can produce unexpected off-flavors.
3. Rennet Dictates a Cheese’s Aging Potential
A cheese designed to age needs a strong, resilient protein structure. Rennet helps create this framework.
In Long-Aged Cheeses:
Curds need to be firm, uniform, and cohesive.
The enzyme must not break down proteins too aggressively or unevenly.
Flavor should develop cleanly without bitterness.
Animal rennet or FPC (fermentation-produced chymosin) is ideal because both remain stable and predictable even over long ages.
In Fresh Cheeses:
Curds are not aged long, so rennet choice is more flexible.
Microbial or vegetable rennet works well for:
Rennet and cultures work together during curd formation.
Some cultures produce acids that speed up rennet action
→ Good for farmhouse cheddar, feta, and blue cheese.
Some cultures produce enzymes that compete with rennet
→ Can weaken the curd during long aging if paired with a weak rennet.
Why this matters:
Rennet must be chosen to harmonize with the target bacteria and the desired speed of coagulation.
10. Rennet Impacts the Entire Cheesemaking Workflow
Choosing the right rennet affects:
Setting time
Cutting time
Stirring intensity
Cooking temperature
Molding pressure
Salt absorption
Aging strategy
Every step of the cheesemaking process is influenced by how the curd was formed. A well-chosen rennet makes the entire process smoother and more predictable.
⭐ Conclusion: Why Knowing Your Rennet Truly Matters
Rennet is not a simple additive—it’s a complex enzyme that determines:
Texture
Flavor
Aroma
Aging potential
Shelf life
Consistency
Yield
Traditional authenticity
Consumer suitability (vegetarian, kosher, halal)
When you understand your rennet, you gain control over the entire cheesemaking process. Knowing your rennet transforms cheesemaking from guesswork into craft, and from craft into mastery.
OTHERS SOURCES
A coagulant is anything that curdles milk. Rennet is a generally used as a generic term used to describe a an animal dervived coagulant that includes the enzyme rennin or chymosin (the two terms refer to the same thing—-rennin is an older term while chymosin is the more chemically specific term for the same enzyme). But rennet is only one of several types of coagulating agents.
Essentially there are 5 types of coagulants used to make cheese:
1.Animal Derived Rennet:
This the rennet used in most traditional cheese making plants. The rennet or rennin refers to collection of enzymes that comes from the fourth stomach of ruminant animals (kid, calf or lamb). If we look at rennet through a chemist’s eyes, rennet is chiefly made up of two enzymes that break down protein chains in milk: chymosin and pepsin.
The enzyme mostly responsible for coagulating milk is chymosin as it works to breakdown casein—the primary protein of concern to cheesemakers. As chymosin is the chief coagulating agent, today we see highly clarified versions of animal rennets which contain up to 97% chymosin—an outcome considered to be desirable by some cheese makers hoping to achieve a very clean taste profile and quick set. In addition, there are now also organic animal rennets on the market.
Animal rennet in any form is still the most expensive coagulant (Up to 2x more expensive than alternatives) on the market. It’s price is a function of supply which in turn is tied to events on the global market for cow, goat and lamb meat products. Some perceive the use of animal stomachs to produce rennet naturally as somewhat primitive. Others perceive it as an example of how to ensure we make use of all parts of an animal.
2.Microbial Rennet:
Microbial rennet is term used to describe a coagulating agent produced by a specific type of mold, fungus or yeast organism grown and fermented in a lab setting. This coagulant is considered vegetarian friendly as the enzyme produced by the organism is not derived from an animal.
While this type of rennet is appropriate for vegetarians, cheese makers agree that cheeses made with this type of microbial rennet tends to result in bitterness in the flavor profile especially when the cheese is aged. This coagulant is less expensive than animal rennet. But true microbial rennet is now hard to find. Its use has been replace d by FPC Rennet.
3.FPC-Fermentation Produced Chymosin Rennet:
This is fairly new type of microbial rennet (1990). This version of microbial rennet is made by taking the rennin producing gene out of the animal cell’s DNA string and then inserting into the a bacteria, yeast or mold host cell’s DNA string.
Once inserted, the newly placed gene initiates the production of the chymosin enzyme within the host. The host culture is cultivated and fermented. The result. An inexpensive harvest of real chymosin enzymes. This is seen to be an improvement on the original microbial rennet as it is real chymosin and not a mold or yeast based substitute.
Moreover, it can be more economically produced in unlimited supply and addresses some of the concerns with pure microbial rennet regarding the bitter flavor in aged cheeses. The procedure itself has been around for some time and is similar to the procedure used to make many vaccines. But, there is more to consider.
FPC rennet is a GMO product. And, according to the culture companies, 90% of all cheeses produced in North America is made with FPC rennet. However, ingredient labels do not distinquish between this type of microbial rennet or the original non-GMO based type. And the fact that use of FPC type microbial rennet is not labeled a GMO product leaves those who oppose the use of GMOs in the dark when it comes to choosing their cheeses.
In addition, further confusion and debate arises over the general differences between GMO products versus “genetically engineered” products as the latter elicits deeper concerns from those opposed to this type of science.
While FPC rennet is GMO, it is not a genetically engineered product, technically speaking, because, the DNA taken from the animal cell and inserted into the DNA string of a bacterium or mold is not changed. Genetically engineered foods actually goes as far as to modify the specific gene responsible for a particular function in order to improve its function. In other words, it takes messing with genes to another, deeper level. Its like playing with the shape of the lego block its self, not just with the order of the lego blocks.
In the end, what this means is that most cheese in North America is made from vegetarian friendly but still animal originated, GMO derived FPC rennet. And that while use of this type of rennet is banned in GMO-free European countries doesn’t mean the cheese we buy from those countries are necessarily FPC free.
To quote one Danish expert “We can’t use FPC rennet in Denmark for our own domestic cheese or cheese make for other European countries which have banned its use. We only use it for cheese we export to North America. .”Why? Because again, it is a cheaper and more consistently available form of rennet.
4.Vegetable Rennet:
True vegetable (vrs vegetarian rennet term which is used interchangeably with microbial rennet) rennet comes from plants which produce certain enzymes that have coagulating properties. Some examples include cardoon thistle, fig tree bark or nettles.
These are “real” vegetable rennets. However, they often also have undesirable effects on cheese flavor (bitterness) and are a little more unpredictable when used in cheeses not traditionally made with vegetable rennet. Still, some traditional Portuguese cheeses are still made with vegetable rennet as are cheeses in other countries where killing lets say, a calf, would not be allowed or economically wise.
In North America, commercially produced vegetable rennet is hard if not impossible to source. However, some artisan cheese makers (e.g. in Maine) are experimenting with it once again and word has it that a company in Edmonton, Alberta will soon be commercially producing a true vegetable rennet.
5.Citric Acid or Vinegar:
Finally, some cheeses like Ricotta are coagulated using simple lemon juice or vinegar. However, this coagulant is mostly used when making a heat precipitated curd. These coagulants are decidedly vegetarian. But this coagulant has a very limited use due to its limitations and noticeable taste profile.
Rennet is a coagulating enzyme used in cheesemaking to turn liquid milk into a firm curd. It works by breaking down kappa-casein, allowing milk proteins to clump together and form a solid mass.
⭐ Types of Rennet
1. Animal Rennet (Traditional)
Made from the abomasum (fourth stomach) of young calves, lambs, or kids.
Online retailers (Amazon, New England Cheesemaking Supply, etc.)
Some specialty grocery stores
OTHER SOURCES
What is rennet?
Traditional animal rennet is an enzyme derived from the stomachs of calves, lambs or goats before they consume anything but milk. (Ours is all from calves.) It is about 90% pure chymosin.
Vegetable rennet is obtained from a type of mold (Mucur Miehei). However, even though it is derived from mold, there is no mold contained in the final product. It is an equivalent chymosin product which works equally well but is not animal derived.
We have recently added organic vegetable rennet to our catalog. Rennet thrives at temperatures in the 85-105F range, but it won't be deactivated completely until it reaches the 140F's. Rennet continues working to set the milk as long as it has the right conditions. So, when a recipe calls for cutting the curds after a certain time period, it is important to follow the directions. Otherwise, your curds may be too firm for the cheese you are trying to make.
How do I choose which rennet to use?
Rennet is standardized, so all the different kinds of rennet (liquid, tablet or powder) work the same to set milk. Liquid is the easiest to work with because you can measure it very precisely. However, the powders and tablets will keep better under more adverse conditions.
Calf rennet is considered to be the best choice for longer aged cheeses because some of its residual components help to complete the breakdown of proteins. Some of the more complex proteins in the vegetable rennet can have a slightly bitter taste after 6 months of aging. The liquid vegetable rennet is Kosher, but it has been re-packaged without Kosher supervision.
How much salt is in rennet?
The amount of salt in rennet is miniscule. It is there as a preservative. Considering that you add 1/4 teaspoon of rennet to a gallon of milk and that much of the rennet runs off with the whey during draining, the amount of salt left in the cheese is virtually impossible to measure.
If you are interested in making cheese with no salt, the fresh cheeses, Mozzarella and Ricotta are best suited for this. The aged cheeses require a slight amount of salt to sow bacterial activity.
Rennet coagulation is the enzymatic process that transforms milk from a liquid into a gel-like curd by breaking down specific milk proteins.
This step is fundamental for producing most cheeses, especially firm and aged types.
🔬 How Rennet Coagulation Works
Rennet coagulation happens in two main phases: 1. Enzymatic Phase (Primary Phase) 2. Aggregation/Gelation Phase (Secondary Phase)
1. Enzymatic Phase (Chymosin Action)
The key enzyme in rennet, chymosin, targets the milk protein κ-casein, which normally stabilizes casein micelles and keeps milk liquid.
* This extract contains a number of enzymes which are designed to help these animals digest their mother's milk, and when added to milk, rennet will cause the milk to coagulate, forming the curds and whey which are so essential in the cheesemaking process.
* Junket is a dessert which is made with sweetened, flavored milk, mixed with rennet to coagulate it.
The coagulation causes the junket to set into a soft pudding, which is typically served chilled, often topped with fruit or spices. Cheese Rennet VIDEO:
Rennet is a collection of enzymes—mainly chymosin, but also pepsin and lipase—used to coagulate milk.
Its primary function is to turn liquid milk into curds (solids) and whey (liquid), which is the first essential step in making cheese.
⭐ How Rennet Works
Rennet specifically acts on κ-casein, a protein that keeps milk in a liquid state.
When rennet breaks this protein down:
* Some cheesemakers continue to make and use rennet in this way; the vast majority use commercially processed rennet, which is made by creating a slurry and then subjecting it to a compound which will cause the enzymes to precipitate out.
The main enzyme in rennet is rennin, although there are a few other enzymes as well, and the precise content depends on the animal the rennet comes from; sheep rennet, for example, is different from cow rennet.
* Before the rennet can be added to a recipe, it must be dissolved in cool water; dissolution is critical for the rennet to work.
One important thing to know when working with rennet tablets is that high heat will deactivate the enzymes, which is why cool water is required for dissolution, and why temperature control of the milk used for cheese, junket, and other foods which call for rennet is very important. Cheese Rennet VIDEO
