Question

During high preheating treatment of milk during powder manufacture, _________ undergoes denaturation and interacts with _________ to form a complex.

• A. \(\beta\)-lactoglobulin; \(\beta\)-casein
• B. \(\alpha\)-lactalbumin; \(\alpha_{s1}\)-casein
• C. \(\alpha\)-lactalbumin; \(\alpha\)-casein
• D. \(\beta\)-lactoglobulin; \(\kappa\)-casein

Hint:

Remember the classic heat-induced protein interaction in milk:
- Heat denatures \(\beta\)-lactoglobulin (whey protein).
- Exposed sulfhydryl (-SH) groups react with \(\kappa\)-casein (on the micelle surface).
- This forms a stable \(\beta\)-lactoglobulin/\(\kappa\)-casein complex, improving heat stability.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Bovine milk contains two major classes of proteins: caseins (which exist in organized micelles) and whey proteins (which are soluble globular proteins).
When milk is subjected to high heat treatments (above 70°C), the globular whey proteins unfold and denature, exposing active functional groups that can interact with other milk proteins.

Step 2: Detailed Explanation:
Let us analyze the specific biochemical interactions that occur during the preheating stage of milk powder manufacture:
- \(\beta\)-lactoglobulin is the most abundant whey protein in bovine milk, containing two disulfide bonds and one free sulfhydryl (-SH) group buried within its native structure.
- When milk is preheated to temperatures above 70°C, the tertiary structure of \(\beta\)-lactoglobulin denatures and unfolds, exposing this highly reactive free sulfhydryl group.
- At the same time, \(\kappa\)-casein, which is located on the outer surface of the casein micelles and stabilizes them, contains disulfide bonds.
- The exposed sulfhydryl group on the denatured \(\beta\)-lactoglobulin undergoes a thiol-disulfide exchange reaction with the disulfide bonds of \(\kappa\)-casein on the micelle surface.
- This reaction binds the denatured \(\beta\)-lactoglobulin to the \(\kappa\)-casein, forming a stable \(\beta\)-lactoglobulin/\(\kappa\)-casein complex.
This complex formation has significant technological effects on the milk powder:
1. It increases the heat stability of the reconstituted milk.
2. It improves the water-binding capacity of the proteins.
3. It prevents the premature coagulation of casein during subsequent concentration steps.

Step 3: Final Answer:
During preheating, \(\beta\)-lactoglobulin undergoes denaturation and interacts with \(\kappa\)-casein to form a complex.