Question

why does the milk turn sour In summer ?

Answer 1

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 milktaste sour. ... Lactobacillus is a very useful bacterium as the acidic conditions it creates makes a milk protein called casein turn to curd.

Answer 2

In summers the temperature isoptimum for the multiplication of the Lactobacillus ( bacteria that convertsmilk into curd) than in winters. Under these favorable conditions, the Lactobacillus multiplies very fast and produces more lactic acid which coagulates milk proteins and thus converts milk into curd