Question

Which acid is produced when milk turns sour?

• A. Acetic acid
• B. Tartaric acid
• C. Lactic acid
• D. Butyric acid

Hint:

Bacteria ferment the sugar in milk, called lactose, and the acid formed from that sugar gives sour milk its name.

Answer 1

The Correct Option is C

Step 1: Recall what milk contains.
Fresh milk carries a sugar called lactose, along with proteins such as casein, fats, and water. Lactose is the main food source for certain bacteria that live in milk, whether they get in naturally from the air or are added on purpose, as in making curd.

Step 2: Understand the souring process.
Bacteria such as Lactobacillus feed on the lactose sugar and break it down through a process called fermentation. This fermentation converts lactose into lactic acid. As lactic acid builds up, it lowers the pH of the milk, which is what makes the milk taste sour and also causes the milk proteins to clump and curdle. This is the same chemistry behind curd and yogurt formation.

Step 3: Rule out the other acids listed.
(A) Acetic acid is the acid in vinegar, made when ethanol is oxidized by acetic acid bacteria, not from milk sugar.
(B) Tartaric acid is found naturally in grapes and is linked to wine making, not to milk.
(D) Butyric acid is the acid that gives rancid butter its sharp, unpleasant smell, formed by different bacteria acting on milk fat, not the acid responsible for the everyday souring of milk.

Step 4: Match the correct acid.
Since the souring of milk comes specifically from bacterial fermentation of lactose into a single well known acid, the answer is lactic acid.

Final Answer:
Bacterial fermentation of milk sugar (lactose) produces lactic acid, which sours the milk.