Question

The cooked flavor of milk on heating develops due to the release of SH groups by heat denaturation of

• A. Whey proteins
• B. Alpha-casein
• C. Beta-casein
• D. Kappa-casein

Hint:

Heated Milk Flavor: COOKED FLAVOR is caused by the release of reactive -SH groups from the heat denaturation of WHEY PROTEINS (specifically $\beta$-Lactoglobulin at $> 74^\circ\text{C}$).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Thermal unfolding and sulfhydryl group activation in whey proteins: heating milk above $74-80^\circ ext{C}$ denatures globular whey proteins (predominantly $eta$-lactoglobulin), exposing masked free cysteine sulfhydryl (-SH) groups that undergo thermal cleavage into volatile hydrogen sulfide ($ ext{H}_2 ext{S}$) to generate cooked flavor.
Key Formula or Approach:
\[ \text{Heating } (T > 74^\circ\text{C}): \mathbf{\beta\text{-Lactoglobulin (Whey Protein)}} \xrightarrow{\text{Denaturation}} \mathbf{\text{Exposes Free -SH Groups}} \longrightarrow \mathbf{\text{H}_2\text{S} \uparrow \implies Cooked \text{ } Flavor} \]

Step 2: Detailed Explanation:
In milk chemistry and thermal processing kinetics:
- Casein proteins ($\alpha_{s1}, \alpha_{s2}, \beta$) possess open, random-coil, non-globular structures with virtually no secondary/tertiary folding, making them remarkably heat stable ($> 140^\circ ext{C}$ for 20 min).
- In contrast, Whey Proteins (predominantly $\beta$-Lactoglobulin, which contains 5 cysteine residues per monomer including one free reactive -SH group and 2 disulfide bonds, and $\alpha$-lactalbumin) possess compact globular tertiary conformations:
1. Heating milk above 74°C to 80°C (pasteurization, boiling, UHT) causes thermal unfolding and Heat Denaturation of Whey Proteins (A).
2. Unfolding unmasks the buried reactive sulfhydryl (-SH) groups of $\beta$-lactoglobulin.
3. Thermal cleavage of these thiol groups releases volatile Hydrogen Sulfide ($\text{H}_2\text{S}$), Methanethiol, and Dimethyl Sulfide, which are solely responsible for the characteristic 'Cooked Flavour' of heated milk.

Step 3: Final Answer:
Therefore, cooked flavor develops due to heat denaturation of Whey proteins, corresponding to option (A).