We all know the popular nursery rhyme about Little Miss Moffett eating her curds and whey, but you might not be quite as familiar with rennet. Simply put, rennet is an enzyme that coagulates milk and separates it into the aforementioned curds and whey. But where does it come from, and what does it do for the cheese?
What Is Rennet in Cheese?
Rennet is a natural enzyme used in cheesemaking to transform liquid milk into solid curds. It works by acting on casein, the primary protein in milk, causing it to coagulate and separate into curds (solids) and whey (liquid). These curds are then cut, cooked, pressed, or aged to create different styles of cheese.
Without rennet, many of the cheeses we recognize today, especially firm, aged varieties, wouldn’t exist. While some cheeses rely on acid or bacterial cultures alone to coagulate milk, rennet allows cheesemakers greater control over texture, structure, and aging potential. This is why it plays such a crucial role in producing everything from supple, sliceable cheeses to hard, long-aged wheels.
Rennet can come from animal, microbial, or plant-based sources, but regardless of origin, its purpose remains the same: to initiate the transformation of milk into cheese.
Why Is Rennet Used in Cheese Making?
So, what is rennet in cheese? Rennet is a vital enzyme mixture used in the cheese-making process. It’s used to coagulate milk. As casein protein breaks down in liquid milk, clumps form. The rennet enzymes help separate solid curds from the liquid whey.
The end result creates the texture and structure of cheese. By controlling the firmness of these curds, cheese producers can create everything from a soft cheese like Brie to a hard one like Parmesan.
Coagulation is the first step of cheesemaking, separating the solids (curds) from the liquid (whey) so that the solids can be manipulated in different ways, eventually resulting in cheese. Some cheeses are lactic- or acid-coagulated, meaning the process is driven by lactic acid bacteria present in the milk. However, many cheeses—especially hard or aged varieties—are made using rennet.
Rennet-coagulated cheeses use animal, microbial, or vegetable-based rennet to form curds with a firmer structure. Traditional rennet is derived from the fourth stomach (abomasum) of young ruminant animals, while vegetable-based rennet comes from plant sources such as thistle. Microbial rennet is produced using organisms like mold, yeast, or fungi.
Each type of rennet offers distinct benefits. Traditional rennet is often preferred for longer-aged cheeses, while microbial and vegetable rennets are suitable for vegetarians and are more widely used today. Some cheesemakers also use a combination of rennet types to achieve specific textures and flavors.
Types of Rennet
Traditionally, animal rennet from the stomach lining of a lamb, kid, or calf is used in the cheesemaking process. It contains a complex combination of enzymes to help curdle the milk.
However, the use of something like calf rennet is on the decline for both ethical and religious reasons, as well as due to availability. Vegetable rennet (also known as plant rennet) is drawn from sources like figs or thistles, and has similar coagulating properties needed for the liquid rennet used in making cheese.
Which Cheeses Use Rennet?
Traditionally, many hard cheeses have used animal rennet. These include Cheddar, Parmesan, Provolone, and Gorgonzola. Many soft cheeses, like cream cheese or ricotta, rely on plant rennet to help the milk coagulate.
More and more, cheese makers are turning to vegetable or plant-based options to replace animal rennet. Look for labels including “microbial rennet” or “plant-based enzymes.”
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Is Rennet Vegetarian or Vegan?
Of course, traditional animal rennet is used in the cheesemaking process for certain kinds of cheese. Therefore, it’s definitely not vegetarian or vegan.
However, more manufacturers are turning to plant-based rennets for the fermentation process needed to make cheese. Check labels for “microbial rennet,” “plant rennet,” or “vegetable rennet.”
Frequently Asked Questions
What is rennet made from?
Traditionally, rennet has been made from an enzyme mixture found in the stomach linings of calves, sheep, and kids. However, there is plant-based or vegetable rennet with a similar acidifying ability to coagulate milk for cheese production.
What cheeses contain animal rennet?
Many traditional hard cheeses use animal rennet. These include many types of Cheddar, Stilton, Parmigiano-Reggiano, Swiss, and Gorgonzola. Labels will include either “animal rennet” or simply “enzymes.”
Is rennet safe to consume?
Rennet is an enzyme complex used for centuries to produce cheese. Whether it’s animal rennet or plant-based, it’s safe to consume.
Is microbial rennet vegetarian?
Yes. It’s derived from fungi, yeast, or molds versus the stomachs of animals. It has the same milk-clotting effect needed for cheese production, making it a great option for vegetarian or Kosher diets.
