Rennet Explained: The Magic Behind Cheesemaking

Cheesemaking combines art, science, and a touch of magic. While milk is the star of the show, a few supporting ingredients play crucial roles in creating your favorite cheeses. Among these, rennet might be the most mysterious. This guide explains what rennet is, how it works, and the different types available.

What is Rennet and Why Do We Need It?
Simply put, rennet is an enzyme that performs an essential task in cheesemaking: it turns liquid milk into solid curd. When added to cultured milk, rennet triggers a series of changes that transform the milk from liquid to gel.

This transformation is critical because it allows cheesemakers to cut the curd, separating the liquid (whey) from the solids that will become cheese. These solids contain the good stuff: proteins, sugars, calcium, fat, minerals, and vitamins.

How Does Rennet Work? A Simple Explanation
Think of milk as a liquid filled with tiny protein bundles called "micelles." These micelles float freely throughout the milk, keeping it in liquid form.

Each micelle is like a fuzzy ball with tiny hair-like projections (called kappa casein) sticking out from its surface. These "hairs" love water molecules and keep the protein bundles suspended in the liquid milk.

Rennet works in two steps:

1. First, it snips off those tiny "hairs" from the protein bundles

2. When the hairs are cut, the protein bundles no longer repel each other

Without these protective "hairs," the protein bundles naturally stick together, forming a gel-like mass—what we call "curd." 

What Does Rennet Come From?
One of the most common questions we hear as cheesemongers is about the source of rennet. The enzyme can be derived from animals (sometimes referred to as “traditional” rennet) or from three alternative sources can be considered suitable for vegetarians.

Animal Rennet
Traditional animal rennet consists mostly of an enzyme called chymosin, which is naturally found in the fourth stomach (abomasum) of unweaned calves, kids, or lambs. In young animals, chymosin helps turn milk into a soft curd in the stomach, slowing digestion so nutrients can be absorbed. Cheesemakers harness this same enzymatic process to coagulate milk into cheese.

A little chymosin goes a long way—just one part enzyme to 15,000 parts milk is enough to achieve coagulation. It’s also important to note that rennet is a byproduct of the meat industry; these young animals are not slaughtered specifically for rennet production.

Traditionally, cheesemakers obtained rennet from local butchers in the form of vells—abomasums that were cleaned, inflated, and dried. Strips of vell were rehydrated in a saltwater solution to extract the enzyme when needed.

In 1874, the Hansen Company in Scandinavia isolated the enzyme from vell, allowing for the commercial production of rennet. This innovation gave cheesemakers greater control over the quality and strength of the rennet, leading to more consistent cheese production. Today, many traditional European cheesemakers continue to prefer animal rennet, as they believe vegetable and microbial rennets can impart a bitter flavor, especially in aged cheeses.

True Vegetable Rennet
True vegetable rennet comes from plants such as cardoon thistle, butterwort, artichoke, nettles, safflower, melon, yellow bedstraw, and fig leaves. The coagulant—an enzyme similar to chymosin—is extracted by steeping these plants in water to create a tea, which is then added to milk.

The challenge with true vegetable rennet is its inconsistency. Its coagulating strength varies widely, requiring significant skill to produce a consistent cheese. Additionally, cheeses made with plant-based rennet often develop a slight bitterness as they age.

Because of these factors, true vegetable rennet is rarely used in large-scale cheese production. However, some traditional cheeses made with vegetable coagulants include Queso de la Sierra and Torta del Casar from Spain, Serra da Estrela from Portugal, Retorta from Finca Pascualete and certain Pyrenean sheep’s milk cheeses from France.

Microbial Rennet
By the 1970s, the demand for cheese had outpaced the supply of animal rennet. In response, scientists discovered that certain molds—Rhizomucor miehei and Rhizomucor pusillus—naturally produce enzymes similar to chymosin, capable of coagulating milk. This microbial rennet became widely embraced by the vegetarian community and proved especially effective for soft and young cheeses.

At the same time, many small American farmstead cheesemakers were emerging, and the rising cost of animal rennet made microbial rennet an attractive, cost-effective alternative.

GM Chymosin
Since the 1980s, advances in DNA engineering have made it possible to produce rennet substitutes using genetically modified microorganisms. Scientists extract the chymosin gene from a calf’s stomach and implant it into yeast or bacterial cultures, which then grow, producing chymosin identical to the natural enzyme. The enzyme is then extracted and purified in the lab.

Although the final product contains no animal matter, it originates from animal DNA, which makes it a gray area for some vegetarians. Others object to the use of GMOs in general. (Some argue that because the original enzyme’s structure remains unchanged, this process isn’t technically genetic engineering.)

A Side Note on Rennet in India
In India, where cows are considered sacred, rennet is sometimes extracted from live animals in a unique way. While the calf is still nursing, a small hole is bored into its hide and abomasum to drain a small amount of gastric fluid containing chymosin. The hole is then closed, and the calf remains unharmed, preserving its sacred status—albeit with a temporary discomfort.