The reaction of milk to lemon juice depends on the temperature of the milk. If the milk is hot, adding lemon juice causes the milk to curdle almost immediately. If the milk is cold when the lemon juice is added, the chemical reaction does not happen for some time.
Cold milk with lemon juice added to it is often used as a substitute for buttermilk while cooking. The ratio for the substitution is one tablespoon of lemon juice per cup of milk. The mixture should be left to sit for a few minutes before using it. Milk curdles when the pH balance of the liquid changes. The protein molecules in the milk attract one another and clump up to form "curdles."