Question

Fresh milk has a pH of 6. Explain why the pH changes as it turns into curd?

Answer 1

Hint: - Lactic acid is an organic acid. The molecular formula of Lactic acid is $C{H_3}CH(OH)COOH$. In solid state it is white and is miscible with water. In a liquid state it is colourless.

Complete answer:-
Being acidic in nature, the pH value of milk is 6. When milk is converted into curd due to the action of bacteria, lactic acid is formed which is more acidic in nature. Therefore, as milk turns into curd the pH value gets reduced. Simple carbohydrates such as glucose, sucrose, or galactose get converted into lactic acid by the process known as lactic acid fermentation which is done by lactic acid bacteria. The bacteria can also grow in the mouth. This formation of bacteria in the mouth leads to tooth decay which is known as caries.
Hence, we can say that due to formation of lactic acid due to fermentation when fresh milk turns to curd, the pH value changes from 6 and becomes more acidic.

Note: - The presence of lactic acid or lactate in milk is due to the fermentation of lactose caused mainly by lactic bacteria. Generally speaking, just fresh milk does not contain lactic acid, but this increases after a while and it makes it more acidic.