Question

Given below are two statements with respect to heat stability of milk:
Statement (I) : The heat stability of concentrated milk increases considerably upon preheating of milk.
Statement (II) : Interaction of denatured whey proteins with \(\kappa\)-casein and reduction of calcium ion activity during preheating are responsible for increased heat stability of concentrated milk.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both Statement (I) and Statement (II) are correct.
• B. Both Statement (I) and Statement (II) are incorrect.
• C. Statement (I) is correct but Statement (II) is incorrect.
• D. Statement (I) is incorrect but Statement (II) is correct.

Hint:

Preheating (forewarming) milk before concentration is a crucial industrial step.
It prevents coagulation during sterilization by:
1. Coating casein micelles with denatured \(\beta\)-lactoglobulin.
2. Precipitating soluble calcium, which reduces ionic \(\text{Ca}^{2+}\) activity.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Concentrating milk via evaporation increases the concentration of proteins and minerals, which reduces its heat stability and makes it susceptible to coagulation during subsequent sterilization.
Preheating (or forewarming) the milk before concentration is a critical processing step used to prevent this coagulation.

Step 2: Detailed Explanation:
Let us analyze both statements:
- Statement (I) is correct: Raw milk that is concentrated without preheating will coagulate during sterilization at \(115\text{--}120^\circ\text{C}\).
Subjecting the milk to a preheating step (typically at \(85\text{--}95^\circ\text{C}\) for \(10\text{--}15\) minutes) before evaporation increases the heat stability of the resulting concentrated milk, allowing it to withstand sterilization.
- Statement (II) is correct and explains the biochemical mechanism:
During preheating, the heat-sensitive whey proteins, particularly \(\beta\)-lactoglobulin, denature and expose their sulfhydryl groups.
These denatured whey proteins form a stable, sterile complex with \(\kappa\)-casein on the surface of the casein micelles via thiol-disulfide exchange.
This complex acts as a protective barrier that prevents the micelles from aggregating during subsequent high-temperature sterilization.
Additionally, preheating causes soluble calcium and phosphate ions in the milk serum to precipitate as insoluble colloidal calcium phosphate.
This reduction in free calcium ion (\(\text{Ca}^{2+}\)) activity in the serum prevents the formation of calcium bridges between caseins, further stabilizing the concentrated milk.

Step 3: Final Answer:
Both Statement (I) and Statement (II) are correct.