Rennet: let’s find out together what it is

A brief history of rennet

The origins of animal rennet date back about 7,000 years.

Animal rennet was discovered by accident many years ago, thanks to the transport of milk in the wineskins made from the stomachs of ruminants, which, with their natural enzyme content, gave rise to the coagulum now referred to as “cheese.”

This physicochemical process is based on the action of chymosins, which, by removing the water-soluble parts of caseins, break the equilibrium causing the suspended mass to collapse, which will then aggregate, forming a protein lattice commonly called “curd.”

This series of physicochemical reactions induced in milk by animal rennet is called “presamic coagulation.”

But what is animal rennet made from?

Rennet is obtained by a process of extracting enzyme complexes found in the fourth stomach of unweaned ruminants such as calf, lamb, kid and buffalo calf.

Within this group of enzymes, chymosin and pepsin are the ones of most interest from a dairy perspective.

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An important clarification

Animals are slaughtered for their meat and not to make rennet, and stomachs are considered a “waste material.” So why not turn them into rennet?

With a strong focus on sustainability from a Circular Economy perspective, the Caglificio Clerici recovers stomachs in selected and certified slaughterhouses, reducing pollution and food waste and resulting in a key product for the dairy sector and cheese production.

But what, then, is rennet?

Rennet is a natural ingredient of animal origin rich in enzymes such as chymosin and pepsin, which is essential for cheese production.

As scientists would say: it is a “natural enzyme compound that coagulates milk” whose main enzymes are chymosin (or rennin) and pepsin, which vary in proportions depending on the age of the animal and the type of feed. The younger the animal and the greater the role of milk in its diet, the higher the percentages of chymosin. Similarly, the fourth stomach-also called the abomasum-of an adult cattle will contain almost exclusively pepsin.

The use of rennet as a coagulant in the dairy industry is very old, but perhaps not everyone is aware that there are different types of coagulants, which differ in their origin and fields of use.

Various animal curds are commercially available: bovine, goat, sheep, and buffalo, each of which can be in powder, liquid, or paste form depending on the use and type of cheese desired.

How is it used?

In the case of a grana-type cheese, the milk that arrives at the dairy is poured into special inverted bell boilers, whey starter (a natural population of lactic acid bacteria)obtained from the residual whey from the previous day’s processing, and in the process the milk is heated to the optimum temperature for the addition of rennet.


With varying times depending on the type and amount of rennet used, the milk is expected to coagulate to form a gel (curd), which is then broken down to the size of a grain of rice and all cooked to a temperature of 56 °C. When the temperature is reached, the cooking phase is interrupted to move on to extraction and placing in fascere, before moving to the salting phase to then season a minimum of 9-11 months.

But what cheeses are made with rennet? We learn to distinguish them…

The Parmesan cheese and Grana Padano for example are typically made with rennet powder. For the production of classic mozzarella as well as the delicious Mozzarella di Bufala DOP, classic liquid rennet and liquid rennet of buffalo origin are used, respectively.

We use rennet powder for fontina cheese in keeping with tradition. For provolone, we use liquid rennet for the mild version and paste rennet for the spicy version. In the case of theEmmental rennet powder. In Gorgonzola we use liquid rennet or calf paste rennet for traditional depending on the specification, and in the case of pecorino all three types of rennet can be used depending on the type and the respective specification.

In short, rennet is as crucial to cheese making as milk, and there are types for every need and every cheese variety.

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