Question

If the milk gets curdled for 7 8 hours in summer it curdles . Why does it happens​

Answer 1

As they digest lactose, a number of byproducts are created, including lactic acid. When the amount of lactic acid in the milk begins to increase, the pH drops and the casein molecules begin to clump. The high levels of lactic acid are also what give spoiled milk its characteristically sour smell.

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Answer 2

Milk contains lactose which is easily fermented into lactic acid with the helpful bacteria called lactobacillus and become sour. This is the reason why the milk gets easily curdled in summers as compared in winters.
This process can be seen at your home also when your mother prepares curd.