Why does casein precipitated as sour milk?

Casein precipitates as sour milk due to the action of lactic acid produced by bacteria during fermentation. When milk sours, the pH decreases, causing casein proteins to unfold and aggregate, leading to the formation of curds. This process is essential in cheese-making, where the coagulation of casein is desired. The resulting curds trap fat and other components, creating the thicker consistency of sour milk.