By Claire Gillespie, Updated Mar 24, 2022
Rennin, also known as chymosin, is a naturally occurring proteolytic enzyme that is exclusively produced in the fourth stomach of young ruminants—cows, sheep, and goats. In the cheese‑making process, rennin catalyzes the transformation of soluble caseinogen into insoluble casein, a step called coagulation. This reaction is crucial for converting liquid milk into a firm curd that can be further processed into cheese.
Milk proteins are dominated by casein, which exists in four main forms: alpha‑s1, alpha‑s2, beta, and kappa. While alpha and beta caseins readily precipitate when calcium is introduced, kappa casein stabilizes the micellar structure and prevents spontaneous coagulation. Rennin deactivates kappa casein by converting it into para‑kappa‑casein and a small peptide called macropeptide. Because para‑kappa‑casein cannot stabilize the micelles, calcium‑insoluble caseins precipitate, forming the curd.
In young mammals, this curdling mechanism keeps milk in the stomach long enough for the proteins to be initially digested. Humans, lacking rennin, rely on pepsin—a potent gastric enzyme—to break down milk proteins during digestion.
Rennet is the commercial form of rennin used in most cheese production. Traditionally, it is sourced from the stomach lining of slaughtered newborn calves. Alternative animal sources include ewes (female sheep) and kids (baby goats). For vegetarian cheeses, rennet is derived from bacterial or fungal cultures, or from genetically engineered microorganisms that produce chymosin.
Today, the majority of cheese is made with enzymes produced by genetically modified microbes rather than by animal digestion. Regardless of origin, the term “rennet” now refers to any enzymatic preparation that can clot milk.
Beyond cheese, rennet is employed as a coagulant in certain yogurts and in the pudding‑like dessert known as junket. Some cheeses, such as the Indian paneer, do not require rennet; instead, they rely on acid (lemon juice or vinegar) to curdle heated milk.
