What does chymosin mean?
Chymosin is a digestive enzyme found chiefly in the gastric juice of young ruminants such as calves, lambs, and goats, where its natural role is to curdle milk so that it stays longer in the stomach and can be digested more slowly. It achieves this by cleaving a specific protein in milk, kappa-casein, which destabilises the casein micelles and makes them aggregate into a solid curd. That same clotting action is the foundation of cheesemaking: chymosin is the key active component of rennet, the preparation long used to turn milk into cheese. Traditionally extracted from the stomach linings of slaughtered calves, chymosin today is mostly produced by fermentation, using microbes engineered to carry the bovine gene for the enzyme. Because this recombinant form is chemically identical to the animal-derived protein, it suits vegetarian diets and dominates industrial cheese production worldwide. The word derives from Greek chymos, meaning juice, reflecting the enzyme's discovery in gastric fluid.
nounAn enzyme that coagulates milk, occurring naturally in the gastric juice of young ruminants and used commercially as the active ingredient of rennet in cheesemaking.
- The milk-coagulating aspartic protease enzyme found in the gastric juice of young ruminants, responsible for curdling milk during digestion.
- The commercial preparation of this enzyme — traditionally from calf stomachs, now largely produced by microbial fermentation or recombinant technology — used as the active agent of rennet in cheesemaking.
"Most cheese produced today is set with chymosin made by genetically engineered microbes rather than with calf stomachs."
"Chymosin cleaves kappa-casein, causing the milk proteins to clump into a soft curd."
"Vegetarian cheeses typically rely on chymosin produced by fermentation rather than on traditional animal rennet."
Rarely pluralised except when referring to different forms or variants of the enzyme, such as bovine versus porcine chymosins.
"Comparative studies have examined the milk-clotting efficiency of chymosins from several ruminant species."
Nearly every slice of modern cheese owes its existence to chymosin — and since the 1990s most of it is brewed by fungi carrying a cow gene, not taken from calf stomachs.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of chymosin
The word chymosin was coined in the late nineteenth century from Greek chymos, meaning juice or humour, combined with the chemical suffix -in used for enzymes and other substances. The same Greek root gives English chyme, the semi-liquid product of digestion, and appears in related biochemical terms such as chyle and chymotrypsin. An earlier common name for the enzyme was rennin, formed from rennet, though this term declined because of confusion with the blood-pressure enzyme renin.
How chymosin is actually used
A technical term of biochemistry, dairy science, and food manufacturing; rarely encountered outside those contexts. The older name 'rennin' has fallen out of favour precisely because it invites confusion with 'renin'.
Easily confused with chymosin
Chyme is the semi-fluid mass of partly digested food that passes from the stomach to the small intestine, whereas chymosin is a milk-clotting enzyme secreted in the stomach.
Rennin is an older synonym for chymosin itself, but it is often confused with renin, a completely unrelated enzyme involved in blood-pressure regulation.