Question

When milk is converted into curd, the sour taste is due to:
A. acetic acid
B. citric acid
C. lactic acid
D. tartaric acid

Answer 1

Hint: The sour taste of curd is due to its acidic nature. The acids have a characteristic taste which is sour. The acid formed lowers the pH level. The pH of milk is 6 while the pH of curd is between 4.5-5.5. The Lactobacilli bacteria present in the curd multiply and convert it into curd.

Complete step by step answer:
Let us discuss the conversion of milk into curd and chemical processes involved
Milk contains globular proteins called casein. The curd is formed by the chemical reaction between the lactic acid bacteria and casein.
By the process of fermentation, milk is converted into curd. Fermentation process requires temperature range of ${{35}^{\text{o}}}\text{C}-{{40}^{\text{o}}}\text{C}$. During fermentation, the Lactobacilli bacterium uses enzymes to produce energy from lactose. The by-product of this energy is lactic acid.
Lactic acid in turn acts on the globular proteins present in the milk and thus, denatures them. This destroys the tertiary and quaternary structures of proteins. The globular proteins are converted into fibrous proteins giving a thick texture to the milk due to the coagulation of the proteins.
The chemical reaction involved in the conversion is
${{\text{C}}_{12}}{{\text{H}}_{22}}{{\text{O}}_{11}}+{{\text{H}}_{2}}\text{O}\to \text{4C}{{\text{H}}_{3}}\text{CH(OH)COOH}$. In this reaction, lactose reacts with water to give lactic acid.
When milk is converted into curd, the sour taste is due to lactic acid, which is option ‘c’.


Additional Information: The structure of lactic acid is

Lactic acid has a molecular formula $\text{C}{{\text{H}}_{3}}\text{CH(OH)COOH}$. Lactic acid has a hydroxyl group adjacent to the carboxyl group, thus it is an alpha-hydroxy acid.

Note: The formation of curd from the milk. A new substance is formed. This is an irreversible process which means that we cannot obtain milk back from curd. Formation of new substance and irreversible process indicates that the formation of curd is a chemical change.