Question

Addition of 1.0 percent sodium chloride to milk has the following effect on rennet clotting time (RCT).

• A. Decrease RCT
• B. Increase RCT
• C. No effect on RCT
• D. No clotting

Hint:

Adding monovalent salts like NaCl to milk increases ionic strength, which shields protein charges and competes with calcium ions. This delays the aggregation phase, increasing the rennet clotting time (RCT).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Rennet clotting of milk consists of an enzymatic cleavage phase followed by a non-enzymatic aggregation phase.
The second phase relies on electrostatic interactions and calcium bridge formation between destabilized paracasein micelles.
Adding neutral salts like sodium chloride (NaCl) alters the ionic environment of the milk, which affects these electrostatic interactions.

Step 2: Detailed Explanation:
When sodium chloride is added to milk at a concentration of 1.0%, it dissociates into sodium ($Na^+$) and chloride ($Cl^-$) ions.
This dissociation increases the ionic strength of the milk serum.
The increased concentration of monovalent sodium ions has two primary inhibitory effects on the aggregation phase:
1. It shields the charged groups on the surface of the paracasein micelles, reducing the electrostatic attraction needed for aggregation.
2. Sodium ions compete with divalent calcium ions ($Ca^{2+}$) for binding sites on the casein molecules, preventing the formation of calcium bridges between micelles.
As a result of this interference with the secondary aggregation phase, the rennet clotting time (RCT) increases significantly.
At very high salt concentrations (e.g., above 3%), clotting can be completely prevented, but at 1.0% NaCl, clotting is delayed, increasing the RCT.

Step 3: Final Answer
The addition of 1.0% sodium chloride to milk increases the rennet clotting time (RCT).