Question

Which milk protein can withstand heating at 140$^\circ$C for 20 minutes without coagulation?

• A. $\alpha$-lactalbumin
• B. $\beta$-lactoglobulin
• C. Immunoglobulin
• D. Casein

Hint:

Casein is an "intrinsically disordered protein" lacking tertiary structure, which gives it remarkable thermal stability compared to globular whey proteins.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Milk proteins are divided into two major classes based on their solubility at pH 6: caseins (which precipitate) and whey proteins (which remain soluble).
These protein classes behave differently under thermal stress due to differences in their molecular structures.

Step 2: Detailed Explanation:
Casein exists in milk as highly hydrated, colloidal aggregates known as casein micelles.
These micelles are exceptionally heat-stable due to the unique structural properties of caseins.
Caseins ($\alpha_{s1}$-, $\alpha_{s2}$-, $\beta$-, and $\kappa$-casein) are open, flexible, and intrinsically disordered proteins with very little tertiary structure.
Because they lack a rigid globular conformation and contain few disulfide bonds, they do not undergo traditional thermal denaturation and subsequent aggregation.
At normal milk pH, casein micelles can withstand temperatures up to 140$^\circ$C for 20 minutes without coagulating.
Conversely, whey proteins like $\beta$-lactoglobulin, $\alpha$-lactalbumin, and immunoglobulins are globular proteins with highly ordered tertiary structures.
These globular proteins easily denature, unfold, and coagulate at temperatures above 70$^\circ$C to 80$^\circ$C as their hydrophobic cores and free sulfhydryl groups are exposed.

Step 3: Final Answer
The correct option is (D).