Question

The acid present in the curd/sour milk is
A. Acetic acid
B. Formic acid
C. Lactic acid
D. Hydrochloric acid

Answer 1

For solving this question, first we need to understand how curd or sour milk is formed. Sour milk is produced by the acidification of milk. Acidification is the process in which a tart taste is obtained by the milk. When an acid like vinegar or lemon juice is added to the milk or through bacterial fermentation the process of acidification is achieved and sour milk is obtained.

Complete step by step answer:
The acid like vinegar and lemon juice added to milk is called acidification. Thus, acidification is the process in which the milk obtains a tart or sour taste. This acidic taste is obtained from vinegar or lemon juice added to the milk or through bacterial fermentation the process of acidification is achieved and sour milk is obtained.
Curd or yoghurt is obtained by controlling the fermentation of the milk. In chemical laboratories, when milk is tested then it is found that lactic acid or lactate is found in milk. Lactic acid is formed due to the fermentation of lactose that is caused by the lactic bacteria. Also, the lactic acid that curdles the milk is responsible for separation that further forms curds which are further used to produce whey and cheese.
The chemical formula of the acid present in sour milk or curd is $\;C{H_3}CH\left( {OH} \right)COOH$ i.e., lactic acid
$\therefore$The correct option is the option C.

Note:
The IUPAC name of lactic acid is $2 - Hydroxypropanoic{\text{ }}acid$. The normal range of acidity in cow’s milk lies between 0.10 to 0.26. And in titrated milk the acidity ranges more than 0.18%. Remember not to mix Alpha hydroxy Acids and Retinol with lactic acid.