Dairy Chemistry & Milk Constituents

When milk is subjected to severe heat treatment ($\gt 75^\circ\text{C}$) during the preheating (forewarming) stage of milk powder production, the globular whey protein $\beta$-lactoglobulin undergoes conformational unfolding and denaturation, exposing its buried free sulfhydryl ($-SH$) group at Cys121. This active thiol interacts through thiol-disulfide interchange with the disulfide bonds of $\kappa$-casein located on the outer hairy coat of casein micelles, forming a covalent $\beta$-lactoglobulin-$\kappa$-casein complex. This complex critically influences heat stability, rennet clotting, and powder properties.

Option analysis:

• $\beta$-lactoglobulin; $\beta$-casein: Incorrect. $\beta$-casein is located internally and lacks available surface disulfide links in normal micelles.
• $\alpha$-lactalbumin; $\alpha_{s1}$-casein: Incorrect.
• $\alpha$-lactalbumin; $\alpha$-casein: Incorrect.
• $\beta$-lactoglobulin; $\kappa$-casein: Correct. Forms the well-known heat-induced whey protein-casein complex.

In milk powder manufacture, the pre-heating (forewarming) step is deliberately designed to control the heat classification of the powder (low-heat, medium-heat, or high-heat powder as measured by the Whey Protein Nitrogen Index, WPNI). Severe pre-heating regimes ($85\text{ to }95^\circ\text{C}$ for up to 30 minutes, or ultra-high heating at $115\text{ to }130^\circ\text{C}$) cause up to $80\%$ to $90\%$ denaturation of native whey proteins. In subsequent stages (vacuum evaporation at $45\text{ to }70^\circ\text{C}$ and spray drying where evaporative cooling keeps droplet temperature low), relatively minimal additional denaturation takes place.

Option analysis:

• Pre-heating: Correct. The major thermal treatment responsible for nearly all whey protein denaturation in powder manufacture.
• Concentration: Incorrect. Carried out under vacuum at low temperatures ($45\text{ to }65^\circ\text{C}$).
• Spray-drying: Incorrect. Despite high inlet air temperature ($180\text{ to }200^\circ\text{C}$), rapid latent heat absorption during evaporation keeps droplet temperatures well below denaturation thresholds.
• Atomization: Incorrect. A mechanical shearing process.

At $20^\circ\text{C}$, the initial solubility of $\alpha$-lactose is approximately 7.0 g / 100 g water, while $\beta$-lactose initial solubility is ~50 g / 100 g water. Upon mutarotation to equilibrium ($\beta : \alpha$ ratio $\approx 1.6:1$), the final total solubility of lactose at $20^\circ\text{C}$ reaches approximately 18.2 g per 100 g water ($7.0 + 1.6 \times 7.0 \approx 18.2\text{ g}$).