Question

Curdling of milk is caused due to the formation of which of the following acids by the bacteria?

• A. Formic acid
• B. Acetic acid
• C. Lactic acid
• D. Maleic acid

Hint:

Milk turns into curd because bacteria convert lactose into lactic acid, which coagulates the milk protein casein.

Answer 1

The Correct Option is C

Step 1: Recall what curdling of milk involves.
Milk contains the protein casein, which remains in suspension due to its negative charge (in the colloidal state). Curdling occurs when an acid neutralizes the charge on casein, causing it to precipitate (coagulate).
Step 2: Identify the acid responsible for natural milk curdling.
In natural milk souring, bacteria (Lactobacillus species) ferment the lactose (milk sugar) in milk through lactic acid fermentation: \[ C_6H_{12}O_6 \xrightarrow{\text{bacteria}} 2CH_3CH(OH)COOH \] (Glucose $\rightarrow$ Lactic acid).
Step 3: Role of lactic acid in curdling.
Lactic acid (CH₃CH(OH)COOH) lowers the pH of milk. When pH drops below the isoelectric point of casein (approximately pH 4.6), casein precipitates, causing milk to curdle (form curd/yogurt).
Step 4: Distinguish from other acids.
While other acids (HCl, H₂SO₄, acetic acid) can also curdle milk artificially, the naturally occurring acid responsible for NATURAL curdling of milk is specifically lactic acid produced by bacterial fermentation of lactose.
Step 5: Confirm the answer.
Lactic acid is the answer. This is why sour milk, yogurt, and cheese all involve lactic acid bacteria (LAB) in their production.
Step 6: Final answer.
Curdling of milk is caused by lactic acid (CH₃CH(OH)COOH), produced by bacterial fermentation of lactose in milk.
\[ \boxed{\text{Lactic acid (option 3)}} \]