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Rennet

Rennet is a set of enzymes used to coagulate milk, especially in cheesemaking.

By Ethan Anderson| Reviewed by Olivia Bennett, Standards & Fact-Checking Lead| Updated July 26, 2026|2 min read| Fact-checked

Rennet is a set of enzymes used to coagulate milk, especially in cheesemaking. Its main active component is chymosin, which curdles casein proteins so milk separates into curds and whey. Traditional rennet comes from the stomachs of young ruminants, but microbial and plant-based alternatives are also used.

Definition and composition

Rennet refers to a milk-coagulating enzyme preparation. In traditional usage, it is derived from the stomachs of young ruminants and contains chymosin, with pepsin and lipase sometimes also present.

Role in cheesemaking

Rennet is used to coagulate milk so it separates into solid curds and liquid whey. This step is central to the production of many cheeses.

Sources and alternatives

Animal rennet is the traditional form, but microbial rennet and plant-derived coagulants are used as alternatives. These substitutes are common in modern cheesemaking, including for vegetarian products.

Terminology

The word rennet can also refer to the lining membrane of the stomach used for curdling milk, or more broadly to any preparation used for this purpose.

Key facts

  • Rennet is used to cause milk to clot during cheesemaking.
  • Its chief enzyme, chymosin, acts on casein in milk.
  • Traditional rennet is derived from the stomachs of unweaned ruminants such as calves, lambs, or kids.
  • Rennet can also refer to the stomach lining or the preparation made from it.
  • Microbial and plant-based substitutes are available and widely used.
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Canadian context

Rennet is relevant to Canadian cheesemaking, dairy science, and food labelling, including vegetarian and microbial alternatives used by Canadian producers and consumers.

Frequently asked questions

What is rennet used for?
Rennet is used to coagulate milk, a key step in making cheese and some other dairy products.
What is the main enzyme in rennet?
The main active enzyme is chymosin, which acts on casein in milk.
Where does traditional rennet come from?
Traditional rennet is obtained from the stomachs of young ruminant animals, especially calves.
Are there non-animal forms of rennet?
Yes. Microbial and plant-based coagulants are used as alternatives to animal rennet.
Does rennet only mean an enzyme?
No. It can also mean the stomach lining used to make the enzyme preparation, depending on usage.

References

  1. Wikipedia — https://en.wikipedia.org/wiki/Rennet
    Supports: General definition, composition, traditional source, and use in cheesemaking
  2. Merriam-Webster — https://www.merriam-webster.com/dictionary/rennet
    Supports: Dictionary definitions and alternate meanings
  3. Cambridge Dictionary — https://dictionary.cambridge.org/us/dictionary/english/rennet
    Supports: Plain-language definition as a milk-thickening substance
  4. Wisconsin Cheese — https://www.wisconsincheese.com/about-cheese/what-is-rennet
    Supports: Rennet’s role in coagulating milk and separating curds and whey
  5. Cheesegrotto — https://cheesegrotto.com/blogs/journal/what-is-rennet-cheesemaking
    Supports: Forms of rennet and use in cheesemaking
  6. ScienceDirect — https://www.sciencedirect.com/topics/food-science/rennet
    Supports: Functional definition, animal and plant sources, and role of chymosin