What is Cheese Rennet?

If you love cheese, you may have noticed that the ingredient list usually includes milk, salt, cultures - and rennet. But what exactly is cheese rennet? This key ingredient plays a vital yet unseen role in transforming milk into delicious cheeses.

What is Cheese Rennet

Rennet contains enzymes that cause milk to coagulate, separating the solid curds from the liquid whey. This allows cheesemakers to shape and age the curds into countless cheese varieties. Without rennet, we wouldn't have cheddar, parmesan, brie and all the other cheeses we know and love!

What is Cheese Rennet?

Humans have been making cheese for over 8,000 years, yet the discovery of rennet was likely by accident. There's a charming legend that cheese was created when a traveling merchant filled his sheep stomach sack with milk, only to find curds by the end of his journey. Something in that sheep's stomach - rennet enzymes - caused the milk to curdle!

This chance encounter revealed how young animals' stomachs contain special enzymes to help them digest milk. Cheesemakers eventually learned to harvest animal stomachs for their rennet, using it to coagulate milk in a controlled way. Animal rennet from the stomach linings of calves, lambs, and kids (young goats) became a key ingredient in cheese production across Europe.

Over thousands of years, cheesemakers also found sources of rennet in certain plants. Vegetal rennets from thistle, artichokes or nettles proved excellent substitutes when animal stomachs were hard to come by. They remain a time-honored tradition in Spain, Portugal and other regions.

These days, most cheese in America is made with fermentation-produced rennets. By genetically modifying bacteria and fungi, scientists manufacture vegetarian chymosin - the key milk-coagulating enzyme in animal rennet. The development of these fermentation rennets has made cheese production more efficient and affordable worldwide.

A Closer Look at Rennet Enzymes

So what exactly does rennet do to milk? Let's zoom in on how its enzymes trigger curd formation.

First, understand that milk is an emulsion - tiny blobs of fat and proteins suspended in water. The key protein is casein, which exists in micelles (clusters). When rennet is added and slowly stirred into warm milk, its protease enzymes begin attacking the casein micelles.

The main enzyme chymosin clips proteins called kappa caseins located on the outside of the casein micelles. This destabilizes their structure and causes clustering. With their protective surface layer removed, the micelles lose charge repulsion and form larger masses. These casein solid curds then separate as the liquid whey is drained away for cheesemaking.

Besides chymosin, rennet also contains other milk-clotting enzymes like pepsin. Together, they drive the reactions that change liquid milk into semisolid curd. Without them, we simply couldn't make any ripened, molded cheeses like brie, blue cheese or cheddar!

Different Kinds of Rennet

Rennet TypeSourceConsiderations
AnimalStomach lining of young calves, lambs or kidsGold standard, imparts specific flavors during aging. Not vegetarian.
VegetableThistle, cardoon, nettles, artichokesTraditional plant source in some areas. Adds bitterness if aged too long.
MicrobialMolds like Mucor mieheiGives slight mushroom aroma. GMO-free.
FermentationGenetically engineered bacteria, yeast and fungiWidely used commercially. Cost effective, vegetarian.

As the table summarizes, animal rennets are still considered the gold standard for aged cheeses where flavor complexity develops over time. But vegetal, microbial and fermentation-based options now allow for both artisanal and mass production of cheese using non-animal enzymes.

Within each rennet type, there is also variety in forms and applications. Let's compare:

Animal rennets are commonly sold in paste, liquid, tablet or powder form. Tablets and powder provide longer shelf life in warmer conditions. Some farmstead cheesemakers also use strips of dried animal stomachs in the traditional style - but this makes standardizing strength more difficult.

Vegetable rennets usually come as aqueous extracts. Certain flowers, leaves, stems or fruits yield key clotting enzymes when processed correctly.

Microbial rennet consists of cultures from selected, safe molds, yeast or fungi strains. Producers grow the cultures in fermentation tanks, then filter, concentrate and purify the resulting enzyme extracts.

Finally, fermentation-produced (FPC) rennets use controlled bioreactors to generate chymosin and other clotting factors. This high-tech approach allows custom enzymes and yields using genetic engineering.

The variety of rennet types and forms provides today's cheesemakers tremendous choice and flexibility! Whether using ancient animal stomachs or lab-grown chymosin, all these rennets cause the same crucial step: curdling milk into curds and whey.

How Cheesemakers Use Rennet

Understanding the different rennet varieties helps clarify the cheesemaking process. Here is the basic approach:

  1. Warm milk to correct temperature (depends on cheese style)
  2. Add starter bacterial culture
  3. Mix in measured amount of rennet and let sit until clean break
  4. Cut curd; stir and heat curds, releasing whey
  5. Drain whey; salt; shape; press; age curds

The key role for rennet comes after the milk is warmed and bacterial cultures are blooming. Adding just the right amount of rennet, the cheesemaker watches closely for the "clean break" signal.

This means the coagulated curds make a clean edge when cut, without crumbling back into the whey. Cutting the curds right at this point ensures maximal transfer of fat and protein from the milk into the finished cheese.

Later steps involve heating, stirring and draining away the whey. But that vital early reaction mediated by rennet enzymes sets the stage for how much yield and moisture will remain in the final cheese. Master cheesemakers get to know their different rennets intimately - judging how animal stomach lining, microbial extract or fermented chymosin will each behave.

An ideal rennet leads to a firm curd that holds its shape later - neither too crumbly dry nor too moist. With experience, cheese crafters choose the right strength and amount of their preferred rennet to make cheese magic!

How Rennet Impacts Your Cheese

As a cheese consumer, the different rennet types mainly affect two factors: nutrition/diet, and flavor.

On the diet front, animal rennet generally doesn't work for vegetarians, kosher rules, or halal laws. Luckily, the vegetable, microbial and fermentation options provide great alternatives for specialty cheeses following religious guidance or vegetable diets. Labels will specify if a cheese is vegetarian-friendly.

In terms of taste, longer-aged cheeses may benefit most from animal rennets. Their unique mix of enzymes seems to offer complexity over time by breaking down fats and proteins. Short-to-medium aged cheeses can employ any rennet type successfully. So both artisanal and industrial producers make flavorful cheeses with vegetal, microbial or fermentation rennet options today.

Finally - what about allergies? No rennet remains in the finished cheese, meaning they are not true allergens or ingredients. Even folks with dairy allergies can often tolerate hard, aged cheeses made with animal rennet better compared to soft cheeses. As always, check with your doctor if concerned about reactions.

Where To Buy Rennet

New and hobbyist cheesemakers may wonder where to source their first rennet supply. Check local homebrew stores, cheesemaking shops or upscale grocers with artisanal cheese sections. Online specialty retailers offer all kinds of vegetable, microbial and even animal rennets for delivery across the country.

Rennet tablets or small containers of liquid rennet can be found in some mainstream stores as well. But these products, sold for "making junket," may not perform as consistently for serious cheese projects. Home cheese crafters quickly learn to rely on specialty producers targeting their niche!

In general, the variety and availability of cheesemaking rennets continues to grow with the hobby. Whether using tablet, paste, powder or liquids, modern home cheese creators have affordable access to high quality products for their craft.

Proper Storage and Handling

Like any natural ingredient for food processing, proper storage preserves peak enzyme activity in cheese rennets. Follow each product's guidelines, but some useful rules of thumb:

  • Liquid rennet lasts about 1 year refrigerated; 6 months frozen
  • Vegetable rennet keeps 6 months refrigerated
  • Tablet or powder rennet stays active for 2-3 years frozen

Once diluted or opened, liquid rennet loses potency faster - use within a few months. For all types, tightly seal containers and avoid temperature fluctuations to get the longest life.

When handling rennet, avoid touching tablets and don't ingest powders, liquids or pastes. Wear gloves when measuring out caustic microbial or animal rennet. Dispose properly or dilute down the drain. Lastly, carefully label containers to avoid confusion with other ingredients!


Can you make cheese without rennet?

Some fresh cheeses rely on acid coagulation rather than rennet - including cottage cheese, paneer, ricotta and cream cheese. But most ripened cheeses require rennet enzymes to properly curdle milk then age without spoilage.

Is vegetarian cheese made with rennet?

Yes! Vegetable and microbial rennets allow delicious cheesemaking without animal content. Also, fermentation-produced chymosin yields pure vegan rennet from bacteria. Labeling clearly states when cheeses follow vegetarian, kosher or halal guidelines.

Does rennet have lactose or dairy allergens?

No - rennet contains enzymes, not milk components. Even dairy-allergic individuals can often eat harder cheeses made with animal rennet, since aging reduces residual lactose and milk proteins. Always exercise caution and check labels when avoiding dairy.

What is better: tablet or liquid rennet?

Both work well, with tablets easier to store and liquids more precise to measure. Commercial cheesemakers prefer standardized liquid rennets. For small home use, tablets provide affordable convenience. Some advanced hobbyists use both!


Understanding the different animal, vegetable and microbial rennet varieties helps inform not only artisanal cheese crafting, but also your cheese buying choices. Whether ensuring a kosher ingredient, vegetarian diet or favorable flavor profile - consider the type of rennet used in your next cheese.

From its fabled origins curdling milk in sheep stomachs, to high tech fermentation tanks - humble rennet remains a crucial cheesemaking element.

AGAH Productions
AGAH Productions