What term describes the process when milk spoils and forms curds?

The term that describes the process when milk spoils and forms curds is curdling. This occurs when the proteins in the milk, primarily casein, aggregate and form solids in the presence of certain conditions, such as the introduction of acid or bacterial activity. During spoilage, the natural bacteria in the milk produce lactic acid, which lowers the pH and causes the casein proteins to coagulate and separate from the liquid whey, resulting in curds.

Fermentation, while related to the action of bacteria in milk, typically refers to the process of converting sugars into acids, gases, or alcohol. It does not specifically pertain to the formation of curds from spoiled milk but rather to the transformation of lactose into lactic acid in controlled conditions, as in the production of yogurt or cheese. Coagulation could also describe the process of curd formation but is a broader term that encompasses various natural and artificial processes where liquids turn into solids or semi-solids. Evaporation refers to the transition of liquid to gas and does not relate to the spoilage of milk or curd formation.