When milk turns sour, the lactose (a sugar) in the milk is converted into lactic acid by bacteria. This process is known as fermentation. The lactic acid gives milk its sour taste and characteristic smell.

The chemical change that takes place during fermentation can be represented by the following equation:

C12H22O11 (lactose) + H2O (water) -> 4 C3H6O3 (lactic acid)

This reaction is catalyzed by the enzyme lactase, which is produced by the bacteria.

The fermentation of milk is a natural process that occurs when milk is exposed to bacteria. However, the process can be accelerated by adding a starter culture, which contains a high concentration of lactic acid bacteria. Starter cultures are often used in the production of yogurt, cheese, and other fermented milk products.