Rennet – What is it and Where Does it Come From?

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Last updated on August 23, 2021

Rennet containing cheeses

What is Rennet?  The word “rennet” refers to a group of structurally similar chemicals from different sources.

All rennet is a type of enzyme, a particular kind of protein that facilitates chemical reactions. In the case of rennet, they act on other milk proteins to help them stick together so that liquid milk can turn into solid cheese!

Uses for Rennet

Hard cheeses using Rennet

Rennet is a crucial part of the production of most cheeses, especially harder, aged cheese such as cheddar. However, some very fresh, spreadable cheeses (like cream cheese) require a soft curd, so the rennet is left out. 

Where does Rennet come from?

Animal rennet:

For most of human history, all of the rennet used to make cheese has come from isolating the stomach of young ruminants. It allows them to coagulate mothers’ milk in their stomachs, retaining it longer to extract more nutrients.  

Humans learned to accomplish the same trick first using the slaughtered animals’ stomach lining and later the enzymes isolated from it. 

Plant-based rennet:

Similar enzymes exist in plants. Nettles, artichokes, and some other thistles contain rennet-like enzymes that also thicken milk (other less common sources include caper leaves, mallow, and paneer booti).  

Some cheeses are traditionally made using these plants; for example, Spanish Torta del Casar is a creamy ewe’s milk cheese coagulated with cardoon thistle flower.

Although vegetarians appreciate plant-based rennets, these enzymes are often weaker and associated with a bitter aftertaste. Cheeses made with vegetable rennets typically last no more than six months.

Microbial Rennet:

Microbial rennet refers to an enzyme that has been grown in, then isolated from, bacteria or yeast.  

Also known as recombinant rennet, this option is contrived through a similar process to the one used to produce many pharmaceuticals currently on the market. It involves inserting rennet-producing genes into bacteria or yeast, then propagating those organisms in a nutrient broth, allowing them to yield their metabolic byproducts.

Eventually, the broth is treated to kill the organisms, and then the rennet is separated and purified. The result is an enzyme that is the same as the one inside a calf’s stomach, but without killing any animals! 

Types of Rennet

Rennet used in a variety of cheeses

You can find rennet in liquid, paste, powder, or tablet form. Liquid or paste works better for the home cheesemaker since they are more diluted and easier to use with small volumes of milk. Rennet tablets are concentrated and more appropriate for large volumes (20 gallons or more). The powdered form is usually only sold to commercial cheesemakers.

Where to Buy Rennet

Home cheesemakers can purchase supplies online from specialty suppliers like,, or large retailers like The source of rennet (animal, vegetable, or microbial) should always be clearly labeled! If your town has a brewing supply store, it is also worth checking with them; many brew stores stock a few cheesemaking items. 

Rennet Cheese Recipe Resources

The same websites that facilitate the purchase of rennet are also excellent resources for recipes using rennet. For example, Cheesemaking and fermentation sites,, provide a huge cache of recipes as well as any tool or ingredient you might need to produce them.

How to Make Rennet

While it is possible to make vegetable rennet if nettles or cardoon thistles grow in your area, be advised that homemade rennet is rarely of consistent concentration! Cheese made with homemade rennet will require some experimentation before you can expect success. Cultures for Health offers a helpful guide for the preparation of thistle rennet.

How to Store Rennet

Because rennet is an enzyme, it is best to keep it cold. When enzymes get warm, they degrade and lose chemical activity. To preserve your rennet at its most potent, store it in the freezer.


What is a rennet tablet?

A rennet tablet is a concentrated form of rennet that is easy to store long-term. It is best to use with large volumes of milk.

Can you make cheese without rennet? Is rennet necessary to make cheese?

You may choose to make some soft cheeses without rennet. Rennet makes a curd stronger and stiffer, so when a soft curd is desired, rennet is unnecessary. Examples of cheeses that you can make with or without rennet are ricotta, feta, provolone, and cream cheese.

Are calves killed for rennet? 

Most rennet (probably up to 90%) used in the world today is microbial rennet, meaning that it did not come from an animal at all.

However, if your bottle or package claims that it is “calf rennet” or “lamb rennet,” then it was indeed isolated from an animal. 

In many--but not all--cases, animal rennets are a byproduct of animal slaughter. Animals butchered for meat produce many pounds of waste in parts that will not be sold for food.  

Among these parts is the stomach lining, from which rennet is isolated; usually, the rennet is taken as a secondary product from these meat animals.

Is rennet still used to make cheese?

Yes. Most cheese in the world is made using rennet. Today, much of it is microbially-produced rennet.  

What is a substitute for rennet?

Rennet is a milk-coagulating enzyme. Several other coagulating enzymes behave in a similar manner. The molds Rhizomucor pusillus and Rhizomuchor miehie, along with the chestnut blight fungus Cryphonectria parasitica, all produce a type of coagulant. In all cases, the enzyme must be isolated from the killed organism much like microbial rennet.

Why is rennet salted?

Salt is often added to rennet as a preservative, but it also aids in the chemistry of curd formation. When the rennet chops off the peptide “hairs” of the casein micelles, the micelles come together through the formation of ionic bridges. 

A small amount of salt strengthens these bridges and helps hold the curd network together.

Is gelatin the same as rennet? 

No, although both are used to form a gel, and both originally came from animals. Gelatin is a form of animal collagen (a protein in animal connective tissues) that has been hydrolyzed or broken into multiple long, thin pieces.  

These pieces have a type of chemistry that makes them very eager to soak up water, so gelatin becomes thick when water is added. The pieces also “grab” one another in such a way that each piece is holding on to several others in a complicated 3D structure. 

On the other hand, rennet is an enzyme, meaning that it catalyzes chemical reactions. Rennet’s job is to neutralize the charge of small protein micelles so that they no longer repel one another. When those micelles come close together, they grab each other in a similar (but weaker) way to how the collagen strands bond in a piece of gelatin.


If you didn't know about the complexities of this special enzyme, we sure hope you understand a lot about rennet after reading this article. Where would we be without this chemical reaction to help make all the delicious cheese that we love?

If you have any questions or comments, please don't hesitate to contact us! 



About the author, Caitlin Clark

Caitlin is a Ph.D student and chocolate researcher at Colorado State University. Her research in the Food Science program focuses on chocolate fermentation (that’s right, it’s a fermented food!) and small-batch post-harvest processing techniques. When she is not acting in her capacity as resident chocolate guru, she researches other fermented foods and beverages like beer, sausage, and natto. Caitlin was drawn to fermented foods while living in rural Spain for six years, where she was exposed to traditional, time-honored practices of food preservation. At home, she practices Bollywood dance for fun and is followed everywhere by two small pet rabbits.