Question

Which of the following statements is incorrect with respect to rennet coagulation of milk

• A. pH has no effect on the secondary phase of coagulation
• B. Coagulation is very heat sensitive and does not occur below 18°C
• C. Calcium content mainly influences the primary phase of coagulation
• D. Rennet coagulation time generally increases with the severity of heat treatment of milk

Hint:

To remember the mineral requirements of coagulation:
- Primary phase \(\rightarrow\) Enzymatic cleavage (Calcium-independent).
- Secondary phase \(\rightarrow\) Physical aggregation (Strictly Calcium-dependent; no clot forms without soluble \(\text{Ca}^{2+}\) ions).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Rennet coagulation of milk occurs in two distinct stages:
- The primary (enzymatic) phase, where chymosin cleaves \(\kappa\)-casein to release glycomacropeptide, exposing the hydrophobic core of the micelle.
- The secondary (non-enzymatic) phase, where the modified micelles (para-casein micelles) aggregate in the presence of calcium ions to form a curd.

Step 2: Detailed Explanation:
Let us analyze each statement to find the incorrect one:
-Option (A): "pH has no effect on the secondary phase of coagulation."
While pH primarily accelerates the enzymatic primary phase, it also influences the charge on the micelle surface, meaning it can have a minor effect on aggregation. However, compared to other options, this is not the main incorrect statement.
-Option (B): "Coagulation is very heat sensitive and does not occur below 18°C."
This is correct.
The enzymatic primary phase can still occur at low temperatures (even at 0°C to 4°C).
However, the secondary aggregation phase is highly temperature-dependent and does not occur below 18°C because hydrophobic interactions are too weak at low temperatures to support gel network formation.
-Option (D): "Rennet coagulation time generally increases with the severity of heat treatment of milk."
This is correct.
Heating milk causes the denaturation of whey proteins (primarily \(\beta\)-lactoglobulin) and their interaction with \(\kappa\)-casein, which sterically hinders the access of rennet to its target cleavage site, prolonging the clotting time.
-Option (C): "Calcium content mainly influences the primary phase of coagulation."
This statement is incorrect.
The primary phase is purely enzymatic cleavage, which does not require calcium ions.
Soluble calcium (\(\text{Ca}^{2+}\)) is essential for the secondary phase of coagulation, where it neutralizes the negative charges on the para-casein micelles and forms ionic salt bridges that link the micelles together into a cohesive gel.
Therefore, calcium content mainly influences the secondary phase of coagulation, not the primary phase.

Step 3: Final Answer:
The incorrect statement is that calcium content mainly influences the primary phase of coagulation, which corresponds to Option (C).