Which acid is generally responsible for the natural souring of milk?

The acid primarily responsible for the natural souring of milk is lactic acid. This process occurs when bacteria, particularly lactic acid bacteria, ferment lactose, the sugar found in milk. During fermentation, these bacteria convert lactose into lactic acid, resulting in an increase in acidity, which causes the milk to sour. This process not only alters the taste but also affects the texture by causing proteins in the milk to coagulate.

Lactic acid is a key component in various dairy products, such as yogurt and cheese, where controlled souring is essential for developing flavor and texture. The presence of lactic acid is indicative of fermentation and is a crucial aspect of dairy processing in public health and food safety, as it helps to preserve the milk by inhibiting the growth of spoilage microorganisms.

Other acids listed play different roles: acetic acid is associated with vinegar and some fermented foods, ascorbic acid is commonly known as vitamin C and is not related to the souring of dairy, and citric acid is found in citrus fruits and is used to add sour flavor but does not typically result from the souring of milk.