What Is Rennet?

Buying, Cooking, and Recipes

Rennet in a large copper pot

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A very important part of classic cheese making involves rennet, a substance used to break the solid particles in milk away from the water content in order to form a solid mass. Traditionally, rennet is made from the stomach lining of young ruminants, but there are other ingredients that can mimic this chemical reaction. Almost all formed cheeses contain rennin, the enzyme used to make rennet.

Fast Facts

  • Most common use: cheesemaking
  • Origin: stomach lining of young ruminants
  • Substitutes: cardoon thistle, artichokes, and nettles

What Is Rennet?

Rennet comes from an organic substance that contains the enzyme rennin. It's mainly found in the lining of the fourth stomach of young goats, calves and lambs. It only occurs in these animals when their main diet is still milk. Once they start to eat only grass that enzyme disappears; it curdles milk as part of digestion—that's why it is found in the stomachs of dairy-consuming young animals.

But it's not just animals that have this component. Plants such as cardoon thistle, artichokes, and nettles also contain a form of the enzyme. These are often used to make vegetarian cheeses and many are traditionally been the cheesemaker's choice in areas of Spain and Portugal. Of the two camps, animal-derived rennet is better for aged cheeses since it lasts longer. Plant rennet can add an element of bitterness to the product if aged for too long, capping most vegetarian cheeses at six months.

In most cheesemaking, rennet is added to the milk either in liquid or paste form. Modern practices also incorporate rennet in tablet and powdered applications, a method that allows the ingredient to be stored longer in warmer climates.

Varieties

There are two main varieties of natural rennet: those derived from plant or animal. The three animals with the rennin enzyme are lambs, goats, and calves.

The plants that contain this special enzyme are artichokes, nettles, and cardoon thistle. Some cheesemakers also use Mucur miehei, a type of mold that offers a similar reaction (but leaves no mold in the actual end product).

There's also a synthesized rennet produced through fermentation, which creates chymosin, or rennin. Fermentation-produced chymosin can be made through manipulating the genes of a young ruminant and/or synthesizing its genes. This method is used commonly in contemporary cheesemaking because it is cost-effective and reliable.

Rennet Uses

The real use of rennet is to separate the solid milk particles from the water in the milk. This allows the curds of the cheese to form. The enzymes are activated only when temperatures reach 85 to 105 F. The rennet will continue to help coagulate the milk until the liquid hits 140 F. This is important in cheesemaking because different types of cheeses have different levels of firmness, thanks to the role of rennet. For a soft brie, it's best to let the rennet make loose curds; whereas a hard Romano will benefit from firmer curds. (Cheeses such as ricotta, however, are the exception and don't typically contain rennet.)

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Rennet

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What Does It Taste Like?

Rennet isn't something eaten on its own. Instead, it's mixed into the warm milk to make cheese. There's no residing taste from the rennet, and most people can't tell the difference between cheeses made with animal rennet versus vegetarian rennet.

Where to Buy Rennet

Rennet is available for purchase in powder or tablet form at most grocery stores and mass merchandisers.

Traditional rennet is made from the fourth stomach lining of a young ruminant, but it's not the only way to get the rennin enzyme. You can substitute animal rennet for vegetable or microbial rennet, which comes from a fungus.

Recipes

The only thing rennet is really used for is making cheese. Here are some examples of easy-to-make cheeses to create at home using this unique ingredient.

Where To Buy Rennet

The easiest rennet to find is in the shelf-stable tablet form, mainly by the company Junket. It's available in powder, liquid, and paste form.

Animal-based liquid rennet proves harder to source, but most cheese shops, specialty grocers and cooking stores will carry some form. Vegetable rennet also can be found at these sorts of markets, though it won't last as long as the animal version. All types, be that powder, liquid or paste, are available online.

Storage

Liquid rennet can stay good in the refrigerator for almost a year if it's in a sealed container. Vegetable-based rennet is good for about half of that time. Tablets of rennet can be frozen and keep for up to three years as long as they're kept in an airtight container.