It is hard to sort out all the different types and categories of rennet. For that reason, you often hear them mixed up, even by some experts! Here is a little guide that will hopefully help you sort them out – and keep them straight!
Rennet vs. Coagulant
While the term rennet originated from the use of enzymes (formerly called rennin) from the true stomach (the abomasum) of a young ruminant, it is perfectly acceptable these days to use it to refer to any product, compound, or ingredient that is added to milk to produce a slow coagulation (as opposed to adding acid for a fast coagulation). The term coagulant is synonymous.
Vegetarian vs. Vegetable
Vegetable coagulants are truly made from vegetables. Examples include thistle rennet (usually from the cardoon thistle), fig sap, lady’s bedstraw, and several others. All vegetable rennets are vegetarian, but not all vegetarian rennets are made from vegetables. Vegetarian rennets are made two ways, one by the growing and collection of a natural enzyme produced by the microbe rhizor mucor miehei and the other by the fermenting action of microbes that have had the animal gene for producing chymosin spliced into their DNA (genetically modified).
Traditional or Animal Rennet
Animal rennet contains chymosin, the primary enzyme desirable for cheesemaking. Chymosin and pepsin (another enzyme) are both produced in the true stomach (the 4th compartment) of cows, goats, and sheep (other ruminants as well). The younger the animal, the more chymosin is present, compared to pepsin. The best rennet for cheesemaking has over 80% chymosin. The percentage should be guaranteed by the company making the rennet. Comes in single strength liquid or tablet form.
Fermented chymosin involves the use of genetically engineered (or modified) organisms. In this case, microbes are modified to produce chymosin (no pepsin). The microbes produce chymosin through a fermentation process. This type of rennet is microbial, but not all microbial rennets are the product of engineered microbes. Usually sold as “double-strength”.
Most often refers to a coagulant produced naturally (but grown and harvested in a laboratory) by the microbe rhizor mucor miehei. The enzyme produced by this microbe is not chymosin, but acts in a similar fashion. Has an undeserved reputation for causing bitterness. Can be purchased in an organic version. Usually sold as “double-strength”. Available in shelf-stable tablets.
A coagulant made from the true stomach of an adult cow. Mostly comprised of pepsin which will coagulate milk, but is not desired for cheesemaking as it will produce bitterness progressively as the cheese ages.