How does rennet help turn milk into cheese curds?

Rennet is an enzymatic preparation, typically obtained from the fourth stomach of calves, that plays a critical role in the transformation of milk into cheese. It contains a mixture of proteolytic (protein-degrading) enzymes, most notably rennin and pepsin.12

During the cheese-making process, rennet acts as a coagulant by targeting the milk protein casein. In the presence of rennet—and often aided by the acidic environment created by lactic acid from starter cultures—these proteins are transformed from a soluble or dispersed state into an insoluble form.3412

This chemical change causes the casein to clump together and precipitate out of the liquid, a process known as curdling or coagulation. The resulting solid or gel-like structure, called the curd, traps milk fats, calcium, and other solids, while the remaining liquid, known as whey, is separated and drained away.324