Question

Given below are two statements, one is labelled as Assertion (A) and and other one labelled as Reason (R).
Assertion (A): Due to the interaction of thermally denatured \(\beta\)-lactoglobulin with \(\kappa\)-casein via disulphide bonds the Rennet coagulation is prolonged
Reason (R): The effect of pH is mainly on the first (enzymatic) stage of rennet coagulation. As the pH of the milk decreases, the enzyme moves closer to its pH optimum speeding up the reaction.
In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both (A) and (R) are true and (R) is the correct explanation of (A).
• B. Both (A) and (R) are true but (R) is NOT the correct explanation of (A).
• C. (A) is correct but (R) is not correct.
• D. (A) is not correct but (R) is correct.

Hint:

Thermally induced whey protein-casein interaction acts as a physical barrier to renneting, whereas pH reduction chemically accelerates the enzymatic cleavage rate of the rennet.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Rennet coagulation of milk involves two main stages: an enzymatic stage (cleavage of \(\kappa\)-casein by chymosin) and a non-enzymatic aggregation stage (coagulation of destabilized micelles in the presence of calcium ions).

Step 2: Detailed Explanation:
Let us analyze both statements:

• Assertion (A): When milk is heated to high temperatures, the major whey protein \(\beta\)-lactoglobulin denatures and exposes its free sulfhydryl group. It then interacts with the sulfhydryl groups of \(\kappa\)-casein via disulfide exchange, forming a \(\beta\)-lactoglobulin–\(\kappa\)-casein complex. This complex sterically hinders the access of chymosin to its target cleavage site (Phe105–Met106) on \(\kappa\)-casein, thereby prolonging the rennet coagulation time. This statement is True.

• Reason (R): The enzyme chymosin has a highly acidic pH optimum (around 5.5). As the pH of milk decreases from its normal value (6.7) toward this optimum, the enzymatic cleavage of \(\kappa\)-casein proceeds much faster. This statement is True.
While both statements are scientifically true, the effect of pH described in (R) does not explain the thermal interaction of \(\beta\)-lactoglobulin with \(\kappa\)-casein described in (A).
Thus, (R) is not the correct explanation of (A).

Step 3: Final Answer
Both (A) and (R) are true, but (R) is not the correct explanation of (A).