Rennin (Chymosin) in Cheese Making: How Milk Solidifies

The enzyme that solidifies milk is called rennin, also known as chymosin. It is produced in the stomachs of young mammals, including calves, and is essential for the digestion of milk. When rennet is added to milk, it causes the milk proteins, particularly casein, to coagulate and form a semi-solid gel, also known as curds. This process is the first step in making cheese.

Rennin works by cleaving the peptide bond between phenylalanine and methionine in the kappa-casein molecule, which changes the structure of the casein proteins and allows them to aggregate and form a gel.

Commercially, rennin is often derived from microbial sources, such as fungi or bacteria, to produce vegetarian or kosher cheese.