Rennet is the generic name for the family of enzymes used to coagulate milk in the cheesemaking process. Rennet is derived from one of three sources: Animal, microbial, or vegetable. All introduce similar action in the milk; the enzyme causes a chemical reaction that separates the milk proteins, separating the milk into curd and whey. Once the initial action of the enzyme is completed, the traces of the enzyme remaining in the milk play a role in the aging process, and the creation of various aromatic and flavor profiles.
Animal rennet is derived from the fourth stomach (abomasa) of an unweaned ruminant. The stomachs are dried and either powdered, or cut into pieces for later use. The enzymes found in animal rennet are chymosin (80%) and pepsin. The young animal uses the enzymes to coagulate the milk in the stomach for better digestion.
There is no clear indication of when cheesemaking started; the earliest evidence dates back about 5,000 years, but it is very likely that the use of milk as a nutritional component extends much farther back in time, to the period when man moved from a hunter-gatherer (nature-provided) form of food generation to an agricultural / pastoral form. Evidence of herding dates back some 30,000 – 40,000 years, so it is likely that the use of milk in the diet extends back almost as far.
Milk, when left to sour, naturally separates into solid and liquid, as the acid level of the milk increases. Given that the origins of man place us in the Middle East, and that the climate there was quite arid, it is likely that the souring of milk, and the use of the liquid and solid was common. It is not much of a stretch to imagine man figuring out that draining the liquid extended the life of the solid material, and then to using some form of pressure to force more liquid out.
When rennet came to be used as a coagulant is not known. The popular “creation myth” for rennet is that of a nomad galloping across the desert, his milk ration hanging from the saddle in a stomach being used as a container. Although that has a nice, romantic “ah-ha!” feel, it is far more likely that the discovery of rennet’s coagulating properties was a mundane accident. Given that early man would have made use of all of the parts of the animal, cooking stomach (tripe) in milk is the more likely source of the discovery. It is likely that the realization that combining milk and stomach took some time, as it had to be the perfect storm of events: the correct stomach, in milk, at the (reasonably) correct temperature. Speculation would say that the use of rennet as a coagulant probably occurred over several thousand years. I checked on-line for recipes using tripe and milk, and found quite a few. Here’s one from Gordon Ramsey…
In addition to coagulating the milk, rennet also plays an important part in the development of the flavor of the cheese. The biggest difference in the impact of rennet on flavor is found in cheeses made from thistle, or Cardoon, rennet, especially those from Portugal. Thistle-rennet cheeses often have more pronounced vegetal aromas and flavors, and are not as prone to the “piquant” or peppery feel of animal or microbial rennet cheeses.
When looking at the ingredients list of cheese, it is sometimes difficult to discern which rennet was used in the making of the cheese. For vegetarians, this is an important consideration. The obvious labeling shows the cheese as being vegetarian, but if that is not included, you must look at the ingredients list. Clues are “microbial rennet”, “vegetable rennet”, and “thistle rennet.” If the labeling is “animal rennet or “traditional rennet” then animal rennet was used. The problem comes in when the labeling simply says “enzymes” with no reference to the type used. Some manufacturers will include “no animal rennet used” on the label, but for others, you would have to contact the cheesemaker directly.
Cheeses made in Europe use mainly animal rennet, while cheeses made in the US use mainly microbial. Most Portuguese cheeses are made with vegetable rennet.
Sources for information in this article: American Farmstead Cheese, by Paul Kindstedt, Brined Cheeses, by A.Y. Tamime, and The Wisconsin Center for Dairy Research publication, Dairy Pipeline, October 2000, Volume 12, Number 3.