Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Care should be taken to test only fresh samples of milk for freezing point depression, as aged samples can actually depress the freezing point such that the presence of extraneous water could be masked.
Reason (R) : The production of four molecules of lactic acid from one molecule of lactose during storage of milk increases freezing point depression.
In light of the above statements, choose the correct answer from the options given below.

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is true but (R) is false.
• D. (A) is false but (R) is true.

Hint:

For accurate cryoscopic testing, always check the titratable acidity of the milk sample first.
If the titratable acidity exceeds $0.18\%$ (expressed as lactic acid), the milk is starting to sour, and the measured freezing point will be artificially low, making it invalid for detecting added water.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The freezing point of milk is a colligative property that depends on the concentration of dissolved solutes (primarily lactose and mineral salts).
Adding water (adulteration) dilutes these solutes, raising the freezing point of the milk toward $0^\circ$ C (reducing the freezing point depression).
Analyzing the chemical stability of milk solutes during storage is essential to performing accurate cryoscopic testing.

Step 2: Detailed Explanation:
Let us analyze Assertion (A):
Fresh, unadulterated milk has a freezing point of approximately $-0.525^\circ$ C to $-0.560^\circ$ C.
If milk is stored for too long or is not kept cold, lactic acid bacteria grow and ferment lactose into lactic acid.
This fermentation process breaks down larger molecules into more numerous, smaller molecules, increasing the total solute concentration (osmolarity) of the milk.
This increase in solute concentration further lowers the freezing point of the milk (increasing the freezing point depression).
If water has been added to the milk (which raises the freezing point), the subsequent bacterial acid production will lower the freezing point again.
This can make the adulterated milk appear to have a normal freezing point, masking the presence of the added water.
Therefore, only fresh or properly preserved milk samples should be tested.
Thus, Assertion (A) is correct.
Now let us analyze Reason (R):
During fermentation, homofermentative lactic acid bacteria convert lactose into lactic acid.
Lactose is a disaccharide ($C_{12}H_{22}O_{11}$).
The bacteria hydrolyze lactose and metabolize the resulting sugars to yield four molecules of lactic acid ($CH_3CH(OH)COOH$):
\[ \text{1 Molecule of Lactose} \rightarrow \text{4 Molecules of Lactic Acid} \] This conversion multiplies the number of active solute particles in the solution by four.
According to the laws of colligative properties, this increase in solute concentration increases the freezing point depression.
Therefore, Reason (R) is correct and provides the direct scientific explanation for Assertion (A).

Step 3: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A).