A Short Discussion on Rennet and Other Popular Coagulants

Coagulant is a very important ingredient when making cheese, and there are so many different kinds and forms, it can get confusing at times. The most common coagulant throughout history and into the present day is rennet, or rennin, the enzyme found in the stomach of young ruminants that have not been weaned from their mother’s milk. Rennet is essentially an enzyme used to convert milk sugars (lactose) into lactic acid. The lactic acid then acts upon the milk’s proteins, getting them to clump together as solids (curds), and separate from the remaining liquid (whey). Of course, there are bacteria already present within the milk itself that will convert any milk into these two forms if the milk is allowed to sit for long enough, but the rennet allows for a faster, more controlled coagulation.

There are many different kinds of coagulant, but here are a few of the most popular forms:


Animal coagulant is almost always calf rennet since it is generally accepted that calf rennet produces better aged cheeses. Calf rennet in the old days was produced by killing a calf in the first months of its life and soaking the cleaned stomach in water for a time, after which the surrounding water was used as a weaker form of today’s liquid rennet. Some people cleaned the stomach out, filled it with milk and put little bits of old cheese into it, thereby creating rennet that was a culture and a coagulant. There were also ways of scraping the lining of the stomach or drying the stomach and cutting pieces off to produce stronger doses of coagulant.

Today, rennet is made in a much more industrialized process, but still starting with a stomach of a ruminant.

Deep frozen calf (or, in some cases, adult cow or pig) stomachs are placed into large vats of an enzyme-extracting solution. This makes the surrounding solution into a crude version of rennet extract. The rennet is then activated by adding acid that is similar to stomach acid. Up to this point, the extracted stomach enzymes are in an inactive form and the acid addition “wakes them up,” so to speak. Then, the acid is neutralized and the liquid rennet is filtered several times, then goes through a concentration process until it reaches a very high potency: usually about 1:15,000. A filler such as saltwater is added, so that the rennet may be used in familiar measurements such as teaspoons, and sodium benzoate or a similar preservative is added as well to prolong shelf life. And, rennet is usually diluted even further before you add it to your milk, for most recipes tell you to put it into water before you add it.

Calf rennet comes in liquid, powdered, and tablet form. Liquid rennet may be stored in the refrigerator and is generally the most strong, reliable form. (It is often double strength.) It should also be kept in a darkened environment, because prolonged exposure to light will cause the rennet enzymes to break down. Tablet and powdered rennet must be stored in the freezer and it usually takes a larger volume of dry rennet than liquid rennet to set the same amount of milk. But, tablet or powdered rennet has a longer lifetime and keeps strength longer.

If you buy only calf rennet, tablet or liquid, you will have all you need in the terms of coagulants, since most all recipes call for it.

To use rennet, check the label of the type you have chosen. Rennet that is in a tablet form must be crushed and added to the same amount of water called for in the recipe. Powdered rennet is usually (but not always) dissolved before adding to the milk.


This includes, but is not limited to:

Bark of the fig tree
Our Lady’s Bedstraw

The fig bark must be made into an extract; an infusion made of Our Lady’s Bedstraw or nettle. The rest can generally be used by crushing and straining the leaves to produce a liquid and then that liquid is the coagulant. You would have to check on the specific type of plant you will be using to find out the usable ratio and strength of whichever one you decide upon. Some say that cheese made with vegetable coagulant and then aged for a long time may have a slightly bitter taste.

If you are purchasing liquid vegetable coagulant from a cheesemaking supply house, generally it will be an enzyme derived from the mold calledmucur meihei, and it is usually kosher, or appropriate for use in kosher cheeses. (These are usually listed on the ingredient labels of commercial cheeses as “microbial enzymes.”)

Legend has it that some women in Northern Europe claimed that if you fed butterwort to your cows just before milking time, your milk would coagulate within three hours of being outside of the cow. Who knows, it might work!


You can also coagulate milk with ingredients you can find in your kitchen. Lemon juice will curdle milk, as will vinegar, but neither will produce the clean, firm curd you need for quality cheesemaking, nor are they very strong or effective. These coagulants are generally used in soft cheesemaking with long ripening periods.


The most popular chemical coagulant is called chymosin and is used most frequently for vegetarian cheeses. It is very popular among industrial large-scale cheesemakers. It is considered high-quality and reliable. Chymosin is manufactured under a number of brand names, such as Chymostar Classic, Chy-Max, and Chymogen. Chymosin is made in a laboratory. The genes that produce natural chymosin found in calf rennet are transferred from the calf cells to prepared bacterial cells. When these bacterial cells reproduce, the daughter cells begin producing a chymosin which is found to be identical to calf chymosin. These cells are now the base for chemical rennet, directly untouched by animal cells.


Junket is just a very weak form of rennet, traditionally used to set custards. It is possible to set milk with junket, but it should only really be used for soft cheeses because it just isn’t strong enough to set a firm curd. Cheese rennet tablets are generally somewhere around five times stronger than Junket tablets.

A Note about Water used to dilute Coagulant:

Also, as a reminder, any water you use to dilute or dissolve your rennet or any other coagulant must be unchlorinated, because the chlorine will kill the enzymes in the rennet. If your tap water is chlorinated or you aren’t sure about it, you can use bottled water or distilled water. This rule applies to any additive that must be diluted and added to your milk during cheesemaking.

*This article is one I wrote for the Cultures for Health website. It can be found, along with a ton of other articles and recipes for cheesemaking, here.

-Tracy M. B.


  1. I didn't know that about chlorinated water. We have a well, but that is good to know. Thx!

  2. I was interested to learn about the herbs and other plants that can be used as coagulants. It might come in handy some day if there is no ready supply of rennet.