A question asked by someone just like you.

How does milk sour?

Milk contains a sugar called lactose. There are bacteria that make their living by causing lactose to react with oxygen to make lactic acid. Like other acids, lactic acid is sour.

Today most milk is pasteurized. It’s heated to about 150 degrees Fahrenheit to kill most bacteria. If milk is handled in a sanitary manner and kept cool (except for pasteurizing) all the way from the cow to your table, it may take a long time for bacteria to grow and make it sour.