A milkman adds a very small quantity of baking soda to fresh milk, as it helps the milkman to keeps the milk unspoiled for little more time than usual. (if there is no baking soda in the milk, it will not last for a longer period of time and will convert into curd really soon). After adding soda the pH value of the milk (the pH value of milk is generally 6 before adding soda), increases and it turns milk to slightly alkaline. When the milk is boiling, and when we consume the boiled milk it tastes little bitter, the bitterness is due to the presence of soda in the milk. Now that there is soda in the milk and it has turned alkaline, the alkaline base willl prevent milk from turning directly to curd. Now the milk will take longer time to covert itself from milk to curd.