Milk sours due to the activity of lactic acid bacteria which enters the milk through a variety of channels, it ferments the lactose (milk sugar) present in milk and produces lactic acid as a byproduct. This increase in acidity causes the proteins in the milk, mainly casein, to coagulate, leading to the formation of curd and a change in the texture and taste of the milk which is when the milk is said to sour.

The process of milk souring can occur naturally or due to chemical reactions, as lactic acid bacteria are commonly present in the environment and can be introduced to the milk during the milking process or through exposure to air and milk can also sour due to chemical reactions that occur during storage, such as oxidation or exposure to heat, which can also cause changes in taste and texture. This chemical reactions, however, are different from the fermentation process driven by lactic acid bacteria.
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