• Post-Translational Phosphorylation Site on Caseins:
• Phosphoserine Residues:
• Caseins ($\alpha_{s1}, \alpha_{s2}, \beta$-casein) are phosphoproteins where orthophosphate groups are monoesterified to the hydroxyl group of serine residues (as O-phosphoserine).
• Phosphorylation occurs within the recognition sequence motif $\text{Ser-X-Glu/SerP}$.
• These phosphoserine clusters bind calcium ions, forming nanoclusters of colloidal calcium phosphate that stabilize casein micelles.
• Conclusion: Serine (Option 3).

Correct option: (3) Serine

• Role of Potassium Sulphate in Kjeldahl Nitrogen Digestion:
• Boiling Point Elevation:
• Concentrated sulfuric acid boils at $338^\circ\text{C}$, which is insufficient for rapid organic matter destruction.
• Adding potassium sulphate ($\text{K}_2\text{SO}_4$) raises the boiling point of the digestion mixture to $370 - 380^\circ\text{C}$, accelerating oxidative digestion without causing nitrogen volatilization losses.
• Conclusion: Increasing the boiling point of sulphuric acid (Option 1).

Correct option: (1) Increasing the boiling point of sulphuric acid

• Functional Comparison: Casein vs Whey Proteins:
• Emulsifying Capacity:
• Caseins ($\alpha_s, \beta, \kappa$) possess flexible, unfolded random-coil structures with amphiphilic hydrophobic and hydrophilic domains.
• Rapidly adsorb to the oil-water interface to form cohesive steric barriers, imparting superior emulsifying ability compared to rigid globular whey proteins.
• Conclusion: Emulsifying ability (Option 1).

Correct option: (1) Emulsifying ability

• Salting-Out of Casein with Sodium Chloride:
• Precipitation Behavior:
• Saturation of raw milk with sodium chloride ($\text{NaCl}$) at $37^\circ\text{C}$ quantitatively precipitates casein micelles by dehydrating hydrophobic patches and screening stabilizing surface charges, leaving whey proteins in the supernatant.
• Conclusion: Casein (Option 1).

Correct option: (1) Casein