What is an explanation for why milk sours

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. Pasteurising 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.

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