A milkman adds a very small amount of baking soda to fresh milk.
(a) Why does he shift the pH of the fresh milk from 6 to slightly alkaline?
(b) Why does this milk take a long time to set as curd?
(a) The pH of milk is 6. As the pH of milk reduces , it starts turning into curd.
Hence, to prevent that, the milkman adds a small amount of baking soda, which is a base.
By doing this he increases the pH of the milk and prevents it from turning into curd sooner.
(b) As the pH of this milk containing baking soda (a base) is slightly higher than that of the milk not containing baking soda, the lactic acid produced when the milk gets converted into curd has to first neutralise the baking soda. As a result, it takes longer time for the pH of the milk to go down. As the pH goes down, it slowly gets converted into curd.