Rennet: what it is, what it is used for and how it is used in cheese

Rennet is a coagulant used in cheesemaking. Its main function is to promote milk coagulation, transforming milk from a liquid into a more compact structure from which the curd is formed.

This happens thanks to the action of specific enzymes found in rennet, particularly chymosin, which act on milk proteins and allow them to aggregate. There is no need to go into the biochemistry to understand its role: in practical terms, rennet is one of the elements that enables the cheesemaker to begin the actual transformation of milk into cheese.

The type of rennet or coagulant used varies depending on the cheese and the rules governing its production process.

Rennet is mainly used to turn milk into curd.

In simple terms, the process follows this sequence:

1. milk
2. addition of rennet
3. coagulation
4. curd formation
5. curd processing
6. cheese

After rennet is added, the milk gradually loses its uniform liquid consistency and forms a gel-like mass. This mass is known as the curd.

Depending on the type of cheese being produced, the curd is then cut, processed, sometimes cooked, separated from the whey, moulded, salted and finally taken through the subsequent maturation or aging stages.

Rennet therefore acts at an early but crucial stage of cheesemaking: without proper milk coagulation, the structure needed for the subsequent processing stages could not be formed.

The rennet traditionally used in many cheesemaking processes is of animal origin.

Calf rennet, for example, is obtained from the abomasum, a part of the stomach of a young calf. It is naturally rich in the enzymes needed to coagulate milk.

The Grana Padano Consortium specifically states that the rennet used in the production of Grana Padano PDO is extracted from the calf’s abomasum.

Alongside animal rennet, there are also milk coagulants obtained from other sources, including microorganisms, plants and fermentation processes.

In everyday language, these are often all referred to as “types of rennet”. From a technical point of view, however, it is more accurate to distinguish animal rennet from other milk coagulants.

In cheesemaking, different categories of rennet or milk coagulants can be used.

Not all of these coagulants can be freely used in every type of cheese. For PDO products, the relevant product specification is always the reference, as it defines the permitted raw materials and production methods.

To produce Parmigiano Reggiano PDO, only calf rennet is used.

The official Product Specification states that milk coagulation must be achieved exclusively through the use of this type of rennet.

During the production process, natural whey starter, obtained from the previous day’s cheesemaking, is first added to the milk. Calf rennet is then added, causing the milk to coagulate and the curd to form.

This is one of the defining characteristics of the Parmigiano Reggiano production method and is not a choice left to the individual dairy.

At BONI dairies, Parmigiano Reggiano is produced in accordance with these rules: the milk is processed in traditional copper vats with the addition of natural whey starter and calf rennet.

This marks the beginning of a process that continues with curd breaking, cooking, extraction, moulding, salting and aging.

Also in the production of Grana Padano PDO, according to the Product Specification currently in force, milk coagulation is achieved using calf rennet, after the addition of natural whey starter.

Rennet therefore performs the same essential function: it allows the milk to coagulate and form the curd, which is then broken, cooked and processed according to the rules established for this PDO cheese.

To learn more about the product’s composition and characteristics, see the pages dedicated to Grana Padano ingredients and lysozyme.

These are three terms related to cheesemaking, but they refer to different things.

Rennet
It is the coagulant that acts on the milk and allows the curd to form.

Curd
It is the mass that forms when the milk coagulates. It is then broken and processed to produce the cheese.

Natural whey starter
It is a natural culture of lactic acid bacteria obtained from the whey of the previous day’s cheesemaking. In Parmigiano Reggiano production, it is added to the milk before the rennet and contributes to the development of the lactic fermentation required by the production process.

Rennet and natural whey starter therefore do not perform the same function, and the curd is not an added ingredient: it is the result of milk coagulation.

Yes. Rennet is one of the three ingredients of Parmigiano Reggiano PDO, together with milk and salt, with no additives or preservatives added.

Natural whey starter is used during cheesemaking, but it does not change this definition of the product’s ingredients.

In Parmigiano Reggiano production, moreover, not just any type of rennet can be used: as established by the Product Specification, it must be calf rennet.

At BONI dairies, rennet comes into play during the early stages of Parmigiano Reggiano production.

The milk from the evening milking, after the cream has naturally risen to the surface, is combined with whole milk from the morning milking and transferred to the traditional copper vats. Natural whey starter and calf rennet are then added.

At this stage, the milk coagulates and the curd forms. The cheesemaker then uses the spino, a traditional tool used to break the curd into small granules before cooking.

It may appear to be a simple stage, but it is essential to everything that follows. At BONI dairies, this process is carried out every day to produce around 350 wheels of Parmigiano Reggiano, in full compliance with the PDO Product Specification.