Rennet for Cheese Making: History and Types

by | Oct 3, 2025 | Uncategorized | 0 comments

What Is Rennet in Cheesemaking?

Rennet (sometimes called cheese rennet) is a set of enzymes that coagulate milk into solid curds. These enzymes occur naturally in the stomachs of young ruminant animals, where they curdle milk to aid digestion. Cheesemakers add rennet to fresh milk to separate the solid curds from liquid whey. A critical step in making animal rennet cheese. This coagulation process transforms liquid milk into a gel-like curd, which is then transformed into cheese.

Traditional animal rennet is obtained from the fourth stomach of a suckling calf, lamb, or kid goat. It contains the enzyme chymosin (which causes curdling) and some pepsin, in a natural ratio optimal for cheese aging. Only a small amount of liquid rennet is needed to set a large volume of milk. For example, a few milliliters of liquid animal rennet can curdle many liters of milk into cheese. Because rennet curdles milk quickly and efficiently, it has been the best rennet for cheese making for thousands of years in traditional cheesemaking.

Ancient Discovery of Rennet in Cheese Making

Humanity has been making cheese for millennia, and the role of rennet was discovered very early. According to legend, the first cheese was accidentally made by an Arabian nomad. He stored milk in a pouch made from an animal’s stomach. After a long journey in the hot sun, the milk had separated into curds and whey due to the stomach’s natural rennet enzymes. The nomad found the curds to be both tasty and nourishing. This animal rennet in the stomach bag “magically” transformed milk, revealing a new way to preserve and enjoy it. While this story is likely a fable, it illustrates how ancient people may have first discovered that rennet, used for cheese making, could coagulate milk into solid cheese.

Earliest Written Records of Rennet Use

Cheesemaking certainly predates written history, but the earliest known writings about rennet date back over 2,500 years. The Greek poet Homer in the Iliad (8th century BC) used a simile describing how quickly a healer worked, “as when fig juice is added to milk” to make it curdle. This suggests that ancient Greeks knew an extract of fig tree sap could coagulate milk. An early form of natural rennet from plants. In the 5th and 4th centuries BC, both Hippocrates and Aristotle wrote about using fig-tree latex (sap) to curdle milk. Aristotle even described a detailed cheese recipe using fig juice as a substitute for rennet. He wrote of stretching wool on a wooden frame and dripping fig sap through it to curdle the milk. These early texts are the earliest written remembrance of humans deliberately using a coagulant – whether animal or plant – to make cheese.

By the time of the Roman Empire, the practice of using rennet was well established. The Roman author Columella (1st century AD) provided one of the first clear descriptions of various rennet types. In his agricultural writings, he advised that milk “should usually be curdled with rennet obtained from a lamb or a kid.” He also noted alternatives: wild thistle flowers, safflower seeds, or the “liquid which flows from a fig-tree” (fig sap) could be used to coagulate milk. Columella remarked that the best cheese used only a minimal amount of these coagulating “drugs.” He is implying that minimal animal rennet made the highest quality cheese. This Roman documentation indicates that both animal rennet and vegetable rennet were known 2,000 years ago. And that people recognized the value of rennet in producing high-quality cheese.

Types of Rennet for Cheese Making

Over the centuries, cheesemakers have developed several types of rennet. All serve the same purpose – curdling milk – but they come from different sources. Every kind of rennet for cheese making has unique advantages and is used in various ways in the modern world.

Animal Rennet (Traditional Rennet)

Animal rennet is the classic form and is sometimes referred to as natural rennet. It is derived from the stomach lining of unweaned animals, most often calves, lambs, or kid goats. Young nursing animals produce especially potent rennet (rich in chymosin) to digest their mothers’ milk. To extract rennet, the dried stomach is sliced and soaked in a brine or wine solution, yielding a liquid rennet extract that can coagulate milk. This method of obtaining liquid animal rennet has been used since ancient times and is still employed by traditional cheesemakers. Modern producers also manufacture animal rennet by grinding frozen stomachs and refining the enzymes into a liquid or powder form.

The Role of Animal Rennet in Aging Cheese

Animal rennet is prized for making high-quality cheese. It typically yields excellent curd formation and flavor development, especially for aged cheeses. Chymosin from calves or lambs creates curds with a clean flavor, and the small amount of pepsin present helps develop robust flavors during long aging. Many artisanal and heritage cheeses exclusively use animal rennet. For example, Italy’s Pecorino Romano must be made with rennet from grass-fed lambs, as per tradition and PDO regulations, and is not suitable for vegetarians. In general, animal rennet cheese tends to have the classic textures and flavors that cheesemakers have valued for centuries. However, because animal rennet comes from slaughtered calves or lambs, it is limited in supply. It is also not acceptable to vegetarians or specific religious consumers. This led cheese producers, even as far back as Roman times, to seek rennet alternatives.

(Note: Milk-specific rennets exist: each species’ young produce enzymes best suited to curdle that species’ milk. For instance, lamb rennet is traditionally used for sheep’s milk, and kid-goat rennet for goat’s milk. This specialization is one reason we use lamb rennet for our sheep’s milk cheeses – it is naturally optimized for curdling ewe’s milk.)

Vegetable Rennet (Plant-Based Coagulants)

Not all rennet comes from animals. Many plants produce substances that can curdle milk, and humans have long used them to make natural rennet alternatives. Vegetable rennet refers to coagulant enzymes from plant sources. Examples include the sap of fig trees, the flowers of the cardoon thistle (Cynara cardunculus), dried caper leaves, stinging nettles, mallow, and certain wild weeds that various cultures have traditionally used. In ancient Greece, as noted, fig juice was used to clot milk. In the Mediterranean region, thistle rennet (derived from wild artichoke thistles) has been used for thousands of years. It remains a component in some artisanal cheeses today. For example, several traditional Portuguese and Spanish sheep cheeses (like Torta del Casar) rely on thistle flower extract as the coagulant.

People turned to plant rennet often for cultural or practical reasons. In parts of India, where cows are sacred and killing calves is taboo, early cheesemakers used plant extracts from fruits and leaves to curdle milk. In medieval Europe, vegetable rennet gained popularity in some communities (such as among Iberian Jews). It allowed cheese making without violating dietary laws against mixing milk and meat. These plant-based rennets enable the creation of animal-rennet-free cheeses for vegetarians or those avoiding animal products.

Challenges of Plant-Based Rennet

In the modern world, pure vegetable rennets are less common than other alternatives. But they persist in certain specialty cheeses and “vegetarian cheese” products. They are also of interest to innovative cheesemakers experimenting with flavors. However, plant rennets can be tricky to work with. Many plant-derived enzymes are not as specific as animal chymosin, and they may break down proteins too much. This can lead to bitter or inconsistent flavors, especially in aged animal rennet cheese made from cow’s milk.

Cheesemakers have found that thistle rennet works best with fatty sheep or goat milk, yielding creamy, spreadable textures, but in cow’s milk it may impart a sharp or bitter note. Due to these challenges, vegetable rennet today remains a niche tool, used deliberately for certain traditional styles or to create vegetarian artisanal cheeses. Commercial “vegetable rennet” sold in stores is often not purely plant-based but may be a blend or a microbial rennet (described next) labeled as vegetable, so consumers should read labels carefully.

Microbial Rennet (Fungal Enzymes)

By the mid-20th century, scientists and cheese producers developed microbial rennet as another alternative to animal rennet. Microbial rennet is produced by certain molds or fungi that naturally create milk-coagulating enzymes. For instance, molds like Rhizomucor miehei or Rhizomucor pusillus can generate protease enzymes that mimic chymosin. These fungi are grown in fermentation tanks, then the enzyme is extracted and purified for use in cheesemaking. The result is a rennet for cheese making that involves no animal ingredients, so it is vegetarian-friendly and also acceptable for kosher and halal cheese when properly prepared.

Microbial rennets became popular because they are less expensive and avoid the supply limitations of calf stomachs. Today, many mass-produced “vegetarian” or inexpensive cheeses use microbial coagulants. For example, a number of industrial cheddar and mozzarella producers use fungal-derived rennet to curdle milk. This liquid rennet often comes in concentrate form and can simply be poured into vats like traditional rennet.

Early on, microbial rennet had a reputation for causing bitterness or off-flavors in aged cheeses. The fungal enzymes were less specific than calf chymosin, sometimes breaking down cheese proteins excessively over time, which led to the formation of bitter peptides in long-aged wheels. However, over the years, the quality of microbial rennet has improved. Producers have learned to remove or suppress the unwanted side enzymes from the mold extracts. Many short-aged or fresh cheeses can be made quite successfully with modern microbial coagulants, and even some longer-aged cheeses now turn out well using these rennets. Still, animal rennet is often considered superior for very long-aged cheeses due to its purity and the unique mix of enzymes that contribute to flavor. Plus, it is natural in every way.

Fermentation-Derived (Artificial) Rennet – FPC

The newest and most widespread type is fermentation-produced chymosin (FPC), also known simply as artificial rennet. This is a product of genetic engineering. Scientists in the 1980s isolated the gene for calf chymosin and inserted it into microbes, such as certain bacteria, yeasts, or molds. These genetically modified microorganisms (GMO) are then fermented in large vats to produce chymosin enzyme. After fermentation, the enzyme is harvested and purified, while the GMO microbes themselves are removed and not present in the final product. The resulting chymosin is chemically identical to the chymosin from calf stomach, but it is produced without needing any animal tissue.

FPC rennet was first introduced commercially in 1990, when the biotechnology company Pfizer brought the product to market. It was the first enzyme from GMO technology approved for use in food by the U.S. Food and Drug Administration. This artificial rennet changed cheese making. Chymosin produced in a factory could be made in virtually unlimited supply, solving the problem of seasonal or limited calf rennet. It was also far cheaper. Very soon after its introduction, many large cheese manufacturers switched to FPC. By 1999, about 60% of hard cheeses in the U.S. were made with FPC instead of animal rennet. As of 2017, over 90% of the global cheese industry was using fermentation-produced chymosin as the coagulant. (In 2008 it was estimated around 80–90% of cheeses in the U.S. and UK were made with FPC, and that percentage grew further through the 2010s.)

Is Fermentation Rennet Considered GMO?

Today, FPC is the invisible workhorse of cheese coagulation. Practically all large-scale, mass-produced cheeses – from factory cheddar and Jack to big-brand pizza mozzarella – use fermentation-derived rennet. The consistency and purity of FPC give cheesemakers very predictable curd formation. Because it is produced without animal parts, FPC rennet is suitable for vegetarians (though not for strict vegans, since it originates from an animal enzyme gene). It also avoids the ethical or religious issues of animal slaughter. The organic rennet standard is another matter: most organic cheeses do not allow FPC because it is derived from GMOs.

Significant difference with fermentation rennet is that it contains only the single enzyme chymosin, whereas traditional calf or lamb rennet also contains pepsin and other enzymes. Cheesemakers like those at Shelburne Farms have noted that pure chymosin (FPC) might not produce the same aging characteristics, since pepsin in animal rennet can influence texture and flavor development in long-aged cheeses. Nonetheless, the overall performance of FPC has proven to be very high. It is a very clean enzyme that typically does not impart bitterness in cheese, and the amount required can be standardized easily. For these reasons, fermentation-produced (GMO) rennet has overtaken older rennet types in modern cheesemaking.

That said, there is ongoing debate about whether FPC should be considered a GMO product in the final cheese. The enzyme itself is purified and does not contain living GMO cells or DNA, so regulators generally do not classify cheeses made with it as “GMO foods.” Some countries, especially in the EU, were initially cautious or even banned FPC in the 1990s, but today it is widely accepted except by certain organic or traditional cheese.

Choosing the Best Rennet for Cheese Making

Cheesemakers now have a range of rennet options. The best rennet for cheese making depends on one’s priorities. If maximum consistency, efficiency, and low cost are the goal (as in large industrial production), then fermentation-produced chymosin is usually the top choice due to its reliability and purity. If a cheese must be vegetarian or kosher, the maker may choose a plant-based rennet or a microbial rennet, as these involve no animal slaughter. 

However, many artisan producers (like ourselves) and traditionalists believe the highest-quality cheeses come from traditional animal rennet. They argue that calf or lamb rennet produces a superior flavor and texture, especially in long-aged cheeses, and is connected to centuries of heritage. In fact, some protected designation cheeses (like Parmigiano-Reggiano, Pecorino Romano, or traditional Cheddar) require animal rennet by their very standards or laws. On the other hand, some delicate fresh cheeses or specialty recipes might work better with a specific plant rennet to create a desired flavor profile.

In practice, each type of rennet is used in the modern world. Many mass-market cheeses use FPC rennet – it is estimated that only 20–30% of global cheese production still uses animal rennet today. Most vegetarian cheeses rely on microbial enzymes for their production. Plant rennets are comparatively rare, but remain essential for traditional farmhouse cheeses in parts of Europe and for innovative cheesemakers seeking unique flavors. Ultimately, the “best” rennet is a matter of context. Our view is that for creating authentic, wholesome cheese with rich flavor, natural rennet from animals is ideal – and we choose it for our cheeses. Next, we explain how we apply this choice in our own cheesemaking.

Our Cheesemaking with Natural Lamb Rennet

Lamb Rennet in Our Traditional Cheesemaking

At our dairy in Ontario, we continue the old-world tradition of using pure lamb rennet to craft our cheeses. We specialize in making classic sheep’s milk cheeses (our main varieties are Pecorino and Caciotta), and we use natural rennet specifically suited for sheep’s milk. In fact, we source a liquid animal rennet made from lambs, ordered from an organic farm in France. This organic rennet comes from pasture-raised lambs and contains no GMOs or artificial additives. Using lamb rennet honors the traditional methods for sheep cheese – for example, authentic Pecorino Romano has historically been made with lamb’s rennet for the best flavor. By using the same type of rennet, we ensure that our Pecorino-style cheese and our Caciotta maintain that rich, time-tested taste and texture.

It’s important to note that we do not sell rennet as a product; we are cheese makers, not rennet producers. The organic lamb rennet we import from France is solely for crafting our own cheeses. We choose this high-quality natural rennet as part of our commitment to clean, traditional ingredients. In our view, this is the cleanest and most natural way to make cheese. Just as artisans have done for centuries. While many large producers use artificial rennet or other shortcuts, we adhere to the simplicity of traditional animal rennet cheese-making. This means our cheeses are truly authentic, made with only milk, cultures, salt, and lamb rennet. No synthetic enzymes, no preservatives, and nothing unnecessary.

Natural Ingredients for Healthy Cheese

Our cheesemaking philosophy is simple: use natural, nutrient-rich ingredients and let nature do the work. We start with whole sheep’s milk from our own grass-fed flock. The milk is high in butterfat and proteins, and it’s free of any antibiotics or hormones. We then add kefir as our starter for fermentation. Kefir grains contain a diverse community of probiotic cultures that ferment the milk, much like traditional cheese cultures but with even more variety.

This fermentation process (using kefir instead of standardized industrial cultures) slowly acidifies the milk and develops complex flavor. It also consumes most of the lactose in the milk. By the time the cheese is aged, virtually all lactose is gone – so our cheeses are naturally lactose-free and easy to digest, even for people sensitive to dairy. We then coagulate the fermented milk using our lamb rennet, forming curds that are cut, pressed, and aged according to our recipes for Pecorino or Caciotta.

Because we rely on grass-fed milk, live cultures, and natural rennet, our cheeses are as pure and real as it gets. They contain nothing that could harm your body – no chemical additives, no genetically modified ingredients, and no fillers. This results in a cheese that is clean and healthful, yet rich in flavor. We firmly believe that food made in the traditional way, with organic and natural components, is better for you.

Choose Cheese Made with Natural Rennet

We encourage our customers and all cheese lovers to take a close look at the products they buy and to be mindful of the ingredients. The type of rennet and fermentation used in a cheese can tell you a lot about its purity and heritage. Are you buying a real cheese made with organic rennet and natural fermentation, or an ultra-processed cheese product made with cheap substitutes? We want you to be informed and choose what’s best for your health and palate.

If you have any questions about rennet or our cheeses, please let us know! We are always happy to answer inquiries and share our knowledge. In fact, many of our articles (like this one) are inspired by customer questions. We are passionate about cheesemaking and healthy eating. And we’re glad to create new articles to explain any aspect of our process. Our goal is to help you enjoy cheese that is authentic, nutritious, and delicious. Natural rennet is just one part of that story. And as you now know, it’s a tradition and science that stretches back to the very beginning of cheese. We’re proud to continue that tradition here in Ontario. Crafting cheese the way it’s meant to be made: with wholesome ingredients and care. Enjoy our rennet-crafted cheeses, and stay healthy!

Sources:

  • Kindstedt, P. (2012). Cheese and Culture: A History of Cheese and its Place in Western Civilization. (For historical references on ancient cheesemaking and rennet use.) pongcheese.co.uken.wikipedia.org
  • Homer. Iliad. (8th century BC) – simile comparing fig juice curdling milk pmc.ncbi.nlm.nih.gov.
  • Columella. De Re Rustica (c. 65 AD) – instructions on curdling with lamb or kid rennet, and plant alternatives tavolamediterranea.com.
  • Shelburne Farms – “Demystifying Rennet” (2020). Insight on rennet types and modern usage shelburnefarms.org.
  • Wikipedia – “Rennet” and “Cheese” articles. Statistics on modern rennet usage and vegetarian rennet options en.wikipedia.org.
Your Cart
Your cart is emptyReturn to Shop