Why and how does milk sour?
Milk sours due to the activity of bacteria, specifically lactic acid bacteria, which ferment the lactose (milk sugar) present in milk and produce lactic acid as a byproduct. This increase in acidity causes the proteins in the milk, mainly casein, to denature and coagulate, leading to the formation of curds and a change in the texture and taste of the milk.

The process of milk souring can occur naturally, 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. However, this process can also be deliberately initiated to produce fermented dairy products such as yogurt, kefir, and certain types of cheese, by adding specific strains of lactic acid bacteria or other microorganisms to milk under controlled conditions.

In addition to bacterial activity, 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. These reactions, however, are different from the fermentation process driven by lactic acid bacteria.

To prevent or slow down the souring process, milk is typically pasteurized to kill most of the bacteria present and is stored at low temperatures to inhibit the growth of any remaining microorganisms. Additionally, proper handling and storage of milk can help minimize the risk of contamination and extend its shelf life.
Can you explain that a bit shorter and simpler?
Milk sours when bacteria in the milk consume the lactose (a sugar in milk) and produce lactic acid as a byproduct. This acid lowers the pH of the milk, making it more acidic, which causes the milk proteins to denature and form clumps, leading to the sour taste and thickened texture.