Bacteria feed on the lactose in milk and produce lactic acid as a waste product. With time, increasing amounts of lactic acid lower the pH of the milk. Vinegar and lemon juice are two common acids added to milk instead of using bacteria. Coagulation and dehydration . Coagulation is converting milk into curds and whey.
Dehydration is the removal of water.
As the pH of the milk changes, the structural nature of the casein proteins changes, leading to curd formation. Essentially, the casein proteins in the milk form a curd that entraps fat and water. The physical properties of enzyme-coagulated milk are better than that coagulated with acid. Curds produced by enzyme coagulation have better syneresis and achieve a lower moisture content without excessive hardening. The traditional source of enzyme is rennet.