Question

Fresh milk has a pH of 6. On changing into curd, its pH becomes:
A. more than 6
B. less than 6
C. does not change
D. 7

Answer 1

Hint: To answer this question, we need to have an idea about how curd is formed. Once we know the process we can find whether the curd is more acidic or basic or neutral as compared to that of milk.

Step by step answer:
We should know that curd is a fermented milk product. It is formed by fermentation of milk by Lactic Acid bacteria to Lactic Acid. Milk consists of globular proteins called casein. The curd forms because of the chemical reaction between the lactic acid bacteria and casein.
During fermentation, the bacteria use enzymes to produce energy (ATP) from lactose. The by-product of ATP production is lactic acid. Lactic acid acts on the globular proteins present in the milk and denatures them.
This denaturation destroys the tertiary and quaternary structures of proteins and the globular proteins are converted into fibrous proteins. Thus, giving a thick mixture to the milk due to the coagulation of the proteins.
Therefore, even though the pH of milk is 6 the pH of curd is less than 6. The pH of curd is 4.5 – 5.5. The pH of milk on changing into curd reduces because of the formation of lactic acid. So, we can say that curd is more acidic in nature as compared to milk.
So, the correct answer is Option B.

Note: There are various bacteria which are present in curd. Some of them are Lactobacillus acidophilus, Lactococcus lactis, Lactococcus lactis cremoris etc.