Rennet 101

Rennet and other coagulation enzymes are responsible for the coagulation of many different types of cheese.


Rennet is one of those interesting words that draws much of its meaning from the context in which it’s used. Colloquially, rennet refers to any enzyme used to coagulate/clot milk. A more technical/stringent definition would be enzymes used to coagulate/clot milk derived from the fourth stomach of ruminants. For the sake of brevity, we’ll be using the former definition. In fact, all the posts on Cheese Science Toolkit use the colloquial definition. The vast of majority people who work with cheese use the colloquial definition. We’ll dive into the semantic subtleties in the following sections. This post isn’t going to be an exhaustive list of rennet or coagulant types. The most popular will be mentioned.

Coagulation Review

We’ve already been through the rigmarole of rennet-mediated coagulation in a previous post so we won’t spend too much time on it here. Just a quick reminder that rennet acts on casein proteins and initiates the coagulation process. Since rennet degrades proteins (i.e. clips the κ-casein hairs), we call it a protease. For the chemistry geeks out there, rennet is an aspartic protease.

Simple rennet coagulation

Rennet clips off the hairy layer and allows the casein micelles to attach


Calf Rennet

Calf rennet is a mixture of milk clotting enzymes extracted from the abomasum, or 4th stomach, of baby calves. You may be saying “Hey, I thought rennet was the enzyme. Rennet’s a mixture of enzymes?” This discrepancy stems from the discussion of definitions in the opening paragraph of this post. Calf rennet contains two main coagulation enzymes: chymosin (aka rennin) and pepsin. Calf rennet is usually ~90% chymosin and ~10% pepsin. These are both proteases that will clot milk. This mixture of enzymes is one factor that leads to the reported flavor superiority of cheese made from calf rennet. The small amount of pepsin will break down casein in a slightly different way compared to just chymosin alone and potentially produce different flavor later down the ripening road.

The structure of chymosin. Visualized using 3Dmol.js
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Microbial Coagulant

These coagulants are produced by fungi. (fungi are microorganisms, thus the name) Two common fungi that produce microbial coagulants are: Cryphonectria parasitica and some Rhizomucor fungi. You may hear these referred familiarly as “Endothia” and “Mucor”, respectively. For the arboreal-minded folks who are reading, Cryphonectria parasitica not only produces a microbial rennet, but also causes Chestnut blight.


Fermentation Produced Chymosin

This type of coagulant is produced by genetically engineering microbes. The gene responsible for chymosin production is taken from the calf and embedded in a microbe. The microbe now produces 100% pure chymosin. This coagulant came about due to the shortage of calf rennet and the need for a consistent source of coagulant.


Vegetable Coagulants

These coagulants are also known as thistle or cardoon “rennets” and encompass plants that possess coagulation enzymes. Cynara cardunculus is an example of such a plant. These often are very proteolytic and can breakdown the casein proteins to form bitter peptides.


An example of a thistle plant. Source



Type Source Enzymes
Calf rennet Calf stomach (abomasum) Chymosin and pepsin
Microbial Cryphonectria parasitica and some Rhizomucor fungi Various proteases
Fermentation Produced Chymosin Genetically modified microbes Pure chymosin
Vegetable (thistle) Cynara cardunculus Cyprosin, cardosin


For More Information

Rennet 101

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