When pasteurized milk is heated to a temp. of 30-40 degrees, and a small amount of curd is added to it. The lactobacillus in that curd starts to grow which convert lactose into lactic acid. 
A spoon ful of curd is added to warm milk,which turns into curd overnight. Thid happnes because a protien called casein present in milk coagulates to form curd.for casein to coagulate, the milk has to be made acidic. A bacterium called lactobacillus, present in spoonful of curd added to milk, converts the lactous sugar present in milk to lactic acid. This creates the acidic environment needed for casein coagulation.
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