Question

Proteolytic cleavage of Kappa casein in the cheese-making process is referred to as

• A. Secondary phase of calcium coagulation
• B. Primary phase of rennet coagulation
• C. Tertiary phase of rennet coagulation
• D. Secondary phase of rennet coagulation

Hint:

Rennet Coagulation Phases:
PRIMARY PHASE = Enzymatic Cleavage of $\kappa$-Casein at Phe105-Met106 (GMP released).
SECONDARY PHASE = Non-enzymatic $\text{Ca}^{2+}$-dependent micellar aggregation curd gelation ($T > 20^\circ\text{C}$).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Two-stage mechanism of enzymatic milk clotting (Berridge model): the Primary Enzymatic Phase involves chymosin cleavage of the Phe105-Met106 bond of $\kappa$-casein, followed by the Secondary Calcium-dependent Coagulation Phase.
Key Formula or Approach:
\[ \underbrace{\kappa\text{-Casein} \xrightarrow{\text{Chymosin}} \text{Para-}\kappa\text{-Casein} + \text{GMP}}_{\mathbf{Primary \text{ (Enzymatic) Phase of Rennet Coagulation}}} \quad \longrightarrow \quad \underbrace{\text{Para-Micelles} + \text{Ca}^{2+} \xrightarrow{T > 20^\circ\text{C}} \text{Gel / Curd}}_{\text{Secondary (Coagulation) Phase}} \]

Step 2: Detailed Explanation:
In the biochemistry of rennet coagulation and cheese manufacturing (Berridge 1942, Dalgleish):
- Enzymatic coagulation of milk by rennet (chymosin) proceeds through two distinct sequential phases:
1. Primary Phase of Rennet Coagulation (Enzymatic Phase) (B): Chymosin specifically hydrolyzes the sensitive Phe105-Met106 peptide bond of $\kappa$-Casein on the outer hairy layer of the casein micelle. This cleaves off the hydrophilic, negatively charged C-terminal Glycomacropeptide (GMP Caseinomacropeptide, residues 106--169), leaving hydrophobic Para-$\kappa$-casein (residues 1--105) on the micelle surface. This enzymatic cleavage occurs even at low temperatures ($0^\circ - 4^\circ ext{C}$) with a low $Q_{10} \approx 1.5 - 2.0$.
2. Secondary Phase (Coagulation Phase): Once $> 85\%$ of $\kappa$-casein is cleaved, steric repulsion is abolished; in the presence of $\text{Ca}^{2+}$ at temperatures $> 20^\circ ext{C}$, the hydrophobic micelles aggregate into a 3D coagulum gel.

Step 3: Final Answer:
Therefore, proteolytic cleavage of Kappa casein is the Primary phase of rennet coagulation, corresponding to option (B).