What Is Rennet?

Buying, Cooking, and Recipes


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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 have some sort of rennin, the enzyme used to make rennet, in them.

Fast Facts

  • Used to make cheese 
  • Made From: 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 eating solely grass that enzyme disappears.

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 or have been the cheese making 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 a 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 and last better in warmer climates.


There are two main varieties of natural rennet, those derived from plant or animal. The three animals that have the rennin enzyme are lambs, goats and calves. They must be young animals in order to ensure they only drink milk for substance. It doesn't matter which animal stomach is used for the rennet, there's no distinct flavor or nuance to any of them.

The plants that contain this special enzyme are artichokes, nettles and cardoon thistle. Some cheese makers also use Mucur Miehei, a type of mold that offers a similar reaction (but leaves no mold in the actual end product). Because the enzyme in the plants also has no taste, any of these plants will give off the same notes when making vegetarian cheese.

There's also a synthesized rennet produced through fermentation, which creates chymosin, one of the primary coagulating enzymes in animal rennet. Fermentation-produced chymosin can be made through manipulating the genes of a young ruminant and/or synthesizing its genes. Today this method is used a lot in the United States in commercially produced cheeses.

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. However rennet can't just get dumped into milk to start this process. The enzymes only get activated when temperatures reach 85- to 105-degrees. The rennet will continue to help coagulate the milk until the liquid hits 140-degrees. With cheese making this is important since the firmness of the curds determines the type of cheese being made. For a soft brie it's best to let the rennet make loose curds; where as a hard Romano will benefit from firmer curds.

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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 verses plant rennet.


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. Substitute animal rennet for vegetable or microbial rennet, which comes from a fungus.


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. 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.


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