How Does Milk Sour?
Milk contains a sugar called lactose. It also contains harmless bacteria called lactobacillus, which uses lactose for energy and creates lactic acid as a by-product. It is the lactic acid which makes milk taste 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.
Pasteurizing milk kills off harmful bacteria and greatly reduces the number of lactobacillus ensuring that milk will last up to three weeks in a fridge. Lactobacillus is a very useful bacterium as the acidic conditions it creates makes a milk protein called casein turn to curd. Without curd we would not have dairy food products like cheese.