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What Is Rennet, and How Is It Used in Cheesemaking?

What’s more satisfying than enjoying a creamy, tender ball of fresh mozzarella? Making your own mozzarella or burrata at home! Before you begin your next home cheesemaking project, you’ll need to gather some special ingredients, including rennet—the substance that turns milk into cheese.

This transformation from liquid to solid seems like magic, but it’s easy to do with the right supplies and equipment. Here’s what you need to know about rennet for home cheesemaking.

What Is Rennet?

overhead view of round white ball of homemade burrata on slate

Rennet is an enzyme used to coagulate milk proteins during the cheesemaking process. It’s the ingredient that lets us efficiently and quickly coagulate liquid milk into solid cheese. It's sold in solid tablet and liquid forms. 

The earliest rennet was made by extracting the enzyme chymosin from the fourth stomach of a young ruminant several thousand years ago. In fact, it’s theorized that observing solid curds in the stomach of young lamb or kid gave Neolithic humans the idea to use this ingredient in cheesemaking to draw more moisture out of the cheese. Animal rennet made from chymosin is widely used in commercial cheesemaking.

In the 1970s, researchers developed rennet that could be made in a lab and works similarly to chymosin. This is called microbial rennet or sometimes vegetarian rennet, and these days, it’s used to produce most of the cheese made at commercial creameries in the U.S. and U.K.

Plants can also be a source of coagulant for cheesemaking. Fig sap, thistles, nettles, caper leaves, and other plants have been used throughout history to coagulate milk into curd in cheesemaking recipes. Spain and Portugal are known for their traditional cheeses coagulated with thistles. Today, vegetable-derived coagulants are hard to come by in the U.S. for commercial or home cheesemaking, but efforts are being made to make them commercially available.

Rennet’s Role in Cheesemaking

cheesemaker stirring rennet from home cheesemaking kit into large pot of milk

Once the milk is acidified, whether through fermentation or direct acidification with citric acid, it’s time to add rennet. The enzymes in rennet coagulate casein, the main protein in milk.

Casein is contained in little bundles called micelles. Each micelle is covered in hairlike structures called kappa-caseins that keeps it from sticking to other micelles. Rennet removes or deactivates the kappa-casein, which allows the casein micelles to stick together and form solid curd.

stainless steel cooking pot with coagulated cheese curd and metal spoon

About 10 minutes after stirring rennet into the milk, it will become a gel-like solid. At this point, you can use a sharp knife to cut the curd and continue with your cheesemaking recipe.

Note that not all cheeses require rennet. Cheeses like whole-milk ricotta, mascarpone, and paneer are coagulated by a combination of acid and heat, and traditional recipes for cheeses like chevre and fromage blanc are coagulated only with acid. However, fresh pasta filata cheeses like mozzarella and burrata require rennet, as do most of the aged cheeses we enjoy.

Home Cheesemaking Kit With Rennet

red and white home cheesemaking kit box

A good home cheesemaking kit will include everything you need to make fresh cheeses at home quickly and easily. In addition to rennet, a cheesemaking kit should include citric acid for acidifying milk, a milk thermometer for checking temperature, and cheesecloth for straining curds. Some will also include small molds for making cheeses like ricotta.

The Farmsteady Italian Fresh Cheesemaking Kit offers all the ingredients and equipment you need to begin making cheese at home—just add milk. Plus, we at Cheese Grotto have developed a series of simple, easy-to-follow recipes for making fresh cheeses like mozzarella, ricotta, mascarpone, burrata, and more. Check them out and get started on your cheesemaking journey!

Have you tried using rennet in your homemade cheese experiments? Tag us @cheesegrotto on Instagram, Facebook, and Twitter and let us know!

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