Question

Given below are two statements:
Statement (I) : Primary phase of rennet coagulation involves hydrolysis of \(\kappa\)-casein.
Statement (II) : The calcium ions are essential for coagulation of casein in the secondary phase.
In light of the above statements, choose the most appropriate answer from the options given below.

• A. Both Statement (I) and Statement (II) are correct.
• B. Both Statement (I) and Statement (II) are incorrect.
• C. Statement (I) is correct but Statement (II) is incorrect.
• D. Statement (I) is incorrect but Statement (II) is correct.

Hint:

Rennet coagulation occurs in two phases:
1. Primary Phase: Enzymatic cleavage of \(\kappa\)-casein (independent of calcium).
2. Secondary Phase: Aggregation of micelles (highly dependent on temperature and free \(\text{Ca}^{2+}\) ions).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Rennet-induced coagulation of milk occurs in two distinct, sequential phases: a chemical enzymatic phase (primary phase) followed by a physical aggregation phase (secondary phase).

Step 2: Detailed Explanation:
Let us evaluate both statements:
- Statement (I) is correct: The primary phase is strictly enzymatic.
The active enzyme in rennet, chymosin (an aspartic protease), specifically hydrolyzes the peptide bond between phenylalanine (Phe\(_{105}\)) and methionine (Met\(_{106}\)) in the \(\kappa\)-casein molecule.
This cleavage splits \(\kappa\)-casein into hydrophobic para-\(\kappa\)-casein, which remains on the micelle, and hydrophilic glycomacropeptide (GMP), which diffuses into the serum.
This process removes the protective, negatively charged steric barrier of the casein micelle.
- Statement (II) is correct: The secondary phase is physical and non-enzymatic.
Once approximately \(85\text{--}90\%\) of the \(\kappa\)-casein is hydrolyzed, the destabilized para-casein micelles begin to aggregate.
This aggregation requires free divalent calcium (\(\text{Ca}^{2+}\)) ions.
These calcium ions act as salt bridges that link the exposed phosphate groups on adjacent para-casein micelles, forming a three-dimensional gel network.
If calcium ions are absent or chelated, the primary enzymatic phase still occurs, but the micelles cannot aggregate, preventing clot formation.

Step 3: Final Answer:
Both Statement (I) and Statement (II) are correct.