🧫 Rennet

Rennet is a milk-clotting enzyme preparation used to make cheese. Its main active enzyme is chymosin (rennin), an aspartic protease produced by glandular cells of the abomasum of newborn mammals. Traditionally, the chymosin used in cheesemaking was extracted from the abomasum of calves.

How does it work? The casein micelles of milk are covered by a layer of the hydrophilic end of the protein κ-casein, which keeps them stable. Chymosin breaks one specific peptide bond (Phe105–Met106) in κ-casein, which produces insoluble para-κ-casein and begins the first phase of renneting. In the second phase the micelles aggregate and form the curd. The whey is then separated from the curd.

Today, chymosin is also made by fermentation: the gene of the animal chymosin is placed into a microorganism that is generally recognised as safe, and the product is identical to the enzyme from the animal. Calf chymosin was the first recombinant enzyme approved by the US Food and Drug Administration, and one review states that more than 90% of the rennet used is produced by fermentation. Other options are microbial coagulants, from the moulds Rhizomucor miehei and Rhizomucor pusillus, and plant coagulants, for example from cardoon flowers; the use of plant coagulants is ancient, and in the 1st century BC Columella mentioned wild thistle flowers as a coagulant. Microbial coagulants conform with kosher, halal and vegetarian principles, but they are less specific and make bitterness in the cheese more likely.

Why it matters to you. Whether a cheese suits a vegetarian depends on the coagulant: consumers who avoid animal products for ethical or religious reasons prefer not to eat cheese made with animal rennet, while microbial coagulants conform with vegetarian principles, and chymosin made by fermentation is kosher and halal.