Question

Arrange the following whey proteins based on the order of heat stability (ascending order) measured by loss of solubility
(A) \(\alpha\)-lactalbumin
(B) Immunoglobulin
(C) Bovine Serum Albumin
(D) \(\beta\)-lactoglobulin

• A. (A), (B), (C), (D).
• B. (B), (A), (C), (D).
• C. (C), (D), (A), (B).
• D. (B), (C), (D), (A).

Hint:

To order major whey proteins by heat stability (least to most stable):
\[ \text{Immunoglobulins (least)} < \text{BSA} < \beta\text{-lactoglobulin} < \alpha\text{-lactalbumin (most)} \] The reversibility of \(\alpha\)-lactalbumin denaturation makes it the most heat-stable fraction.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Whey proteins are globular proteins that lack the highly disordered, heat-stable structure of caseins.
Upon exposure to elevated temperatures, these proteins denature, lose their tertiary structure, and precipitate out of solution, leading to a loss of solubility.
The different whey protein fractions exhibit varying degrees of thermal stability based on their molecular structures and disulfide bond arrangements.

Step 2: Detailed Explanation:
Let us examine the thermal denaturation temperatures of the major whey protein fractions:
1. Immunoglobulins (B): These are the largest and most heat-sensitive whey proteins.
They begin to denature and lose solubility at temperatures as low as \(60\text{--}65^\circ\text{C}\). They are the least heat-stable fraction.
2. Bovine Serum Albumin (BSA) (C): BSA is a large globular protein containing multiple disulfide bridges.
It undergoes irreversible thermal denaturation and loss of solubility in the temperature range of \(64\text{--}68^\circ\text{C}\).
3. \(\beta\)-lactoglobulin (D): This is the most abundant whey protein in bovine milk.
It denatures at temperatures above \(70^\circ\text{C}\), exposing a highly reactive free sulfhydryl group that can form disulfide linkages with \(\kappa\)-casein.
4. \(\alpha\)-lactalbumin (A): This is the most heat-stable major whey protein.
It denatures at approximately \(74^\circ\text{C}\), but this denaturation is highly reversible.
When cooled, up to \(90\%\) of the denatured \(\alpha\)-lactalbumin can refold into its native conformation, allowing it to remain soluble. This is due to its compact structure stabilized by four disulfide bonds and a bound calcium ion.

Step 3: Final Answer:
The ascending order of heat stability (least stable to most stable) is: Immunoglobulins (B) \(\rightarrow\) Bovine Serum Albumin (C) \(\rightarrow\) \(\beta\)-lactoglobulin (D) \(\rightarrow\) \(\alpha\)-lactalbumin (A).