rennet

Rennet derives from Old English gerinnan, meaning to cause to run together, to coagulate — sharing a root with the modern verb 'run.' The process it names is equally ancient: enzymes present in the stomach lining of young ruminants — calves, lambs, kids — cause the protein casein in milk to coagulate into curds. Without rennet, or something that performs its function, cheese cannot exist.

The discovery of rennet is likely prehistoric, probably accidental. Nomadic herders who stored milk in pouches made from the stomachs of slaughtered young animals would have found the milk transformed — thickened, pleasantly soured, durable in ways that raw milk was not. The stomach lining had done what it was designed to do: prepare milk for digestion. Human food preservation was a lucky consequence. No one planned the invention of cheese.

For millennia, rennet meant animal rennet — dried and salted pieces of calf stomach, dissolved in brine to create a working enzyme solution. Cheesemakers in every tradition guarded their rennet preparations carefully; the quality of the rennet determined the quality of the cheese. In the 20th century, shortages of calf stomachs and vegetarian market demand drove development of alternatives: vegetable rennet from plants like fig sap and cardoon thistle, and eventually microbial and fermentation-produced rennets.

Today, over 90 percent of commercial cheese is made with fermentation-produced chymosin — the specific rennet enzyme isolated and produced in genetically modified yeast and fungi. The same coagulation that a Neolithic herder discovered in a skin pouch is now achieved in stainless steel bioreactors. The enzyme is identical. The ancient accident has been industrialised, and most cheesemakers have never held a calf stomach.