Question

At Natural pH of milk the casein occur in the form of

• A. Gel
• B. Coagulum
• C. Micelle
• D. Emulsion

Hint:

At pH 6, casein is stable and suspended as colloidal particles called micelles, stabilized by calcium phosphate and surface $\kappa$-casein. Acidification to pH 6 destabilizes this suspension, forming a coagulum.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Casein is the primary protein group in milk, accounting for approximately 80% of the total milk protein.
Casein is not a single protein but consists of several fractions ($\alpha_{s1}$-, $\alpha_{s2}$-, $\beta$-, and $\kappa$-casein).
These hydrophobic proteins associate with calcium, phosphate, and other minerals to form stable colloidal particles.
The physical form of these colloidal aggregates depends on the pH, ionic strength, and temperature of the milk.

Step 2: Detailed Explanation:
At the natural pH of fresh milk (approximately 6 to 7), the casein fractions are organized into spherical colloidal particles called casein micelles.
These micelles are stabilized by colloidal calcium phosphate (CCP), which acts as an inorganic cement holding the hydrophobic protein sub-units together.
The outer surface of the micelle is coated with hydrophilic $\kappa$-casein, which projects a negatively charged glycomacropeptide "hairy" layer into the aqueous serum.
This negative charge and steric hindrance prevent the micelles from aggregating, keeping them in a stable colloidal suspension.
If the pH is lowered to the isoelectric point of casein (pH 6), the negative charge is neutralized, causing the micelles to aggregate and form a gel or coagulum.
Thus, at the natural pH of milk, casein occurs in the form of stable micelles.

Step 3: Final Answer
At the natural pH of milk, casein occurs in the form of micelles.