Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A) : Specific gravity of whole milk increases when held at lower/cold temperature. This phenomenon is known as Recknagel phenomenon.
Reason (R) : Reckangel phenomenon occurs mainly due to loss of entrapped gas when milk is held at lower temperature.
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 not correct but (R) is correct.

Hint:

To eliminate the error caused by the Recknagel phenomenon when measuring specific gravity using a lactometer:
Warm the milk sample to $40^\circ$ C to melt the crystallized fat, then cool it back down to the lactometer calibration temperature ($15.5^\circ$ C, $20^\circ$ C, or $27^\circ$ C) and read the value immediately.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Recknagel's phenomenon refers to the gradual upward shift in the specific gravity of freshly drawn or warm-processed milk when it is cooled and held at refrigeration temperatures over several hours.
This physical change is related to the state of fat and protein complexes in milk at lower temperatures.

Step 2: Detailed Explanation:
Let us analyze Assertion (A):
The assertion states that the specific gravity of whole milk increases when stored at cold temperatures, which is known as the Recknagel phenomenon.
This statement is correct.
When milk is cooled and stored at $4^\circ$ C to $10^\circ$ C, its specific gravity increases by approximately 0.001 to 0.0015 units over 12 to 24 hours.
Now let us analyze Reason (R):
The reason states that this phenomenon is mainly due to the loss of entrapped gas.
This statement is incorrect.
While some dissolved gases may escape as milk cools, the Recknagel phenomenon is primarily caused by:
1. Fat crystallization: Milk fat within the globules slowly solidifies at low temperatures.
Solid crystalline fat has a smaller specific volume (higher density) than liquid fat.
Since crystallization is a slow process, the density of the milk increases gradually over several hours.
2. Hydration changes in proteins: Cooling causes structural changes in the casein micelles and increases their hydration, which also contributes to the rise in specific gravity.
Therefore, the primary cause is fat phase transition, not the loss of entrapped gas.

Step 3: Final Answer:
Assertion (A) is true, but Reason (R) is false.