The pH value of milk is 6 since it is acidic in nature. When the milk is converted into curd due to the action of bacteria lactic acid is formed which is more acidic in nature. Therefore, the pH value of the milk is reduced as it turns to curd.
(a) To prevent the milk from being sour due to the development of lactic acid, milkman will adjust the pH of the fresh milk from 6 to slightly alkaline.
(b) Such milk takes long time to set into curd because the lactic acid produced here first neutralises the pH then the pH is reduced to turn milk to curd.