Question

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

• A. Coagulation does not occur below 15$^\circ$C.
• B. An increase in enzyme concentration results in reduction of rennet coagulation time
• C. Calcium content mainly influences the secondary phase of coagulation.
• D. Rennet coagulation time generally decreases with the severity of heat treatment.

Hint:

Heating milk denatures whey proteins, which form a protective coat around casein micelles, and decreases ionic calcium, both of which prolong (increase) the rennet coagulation time.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Rennet coagulation of milk is a two-phase process consisting of a primary enzymatic phase and a secondary non-enzymatic aggregation phase.
The rate of the primary phase is governed by enzyme-substrate kinetics, temperature, and pH.
The secondary aggregation phase is highly dependent on temperature and the presence of divalent calcium ions ($Ca^{2+}$) to form salt bridges between paracasein micelles.
Severe heat treatment of milk alters the state of proteins and minerals, significantly affecting both phases.

Step 2: Detailed Explanation:
The primary enzymatic phase involves the specific cleavage of $\kappa$-casein at the Phe105-Met106 bond to release glycomacropeptide.
This enzymatic reaction can occur at low temperatures, but the secondary aggregation phase has a very high temperature coefficient and virtually ceases below 15$^\circ$C.
Thus, statement (A) is correct.
Increasing the concentration of rennet increases the rate of enzymatic hydrolysis, thereby reducing the rennet coagulation time (RCT).
Thus, statement (B) is correct.
Calcium ions are essential for neutralizing negative charges on paracaseinate micelles and forming calcium bridges that allow micellar aggregation during the secondary phase.
Thus, statement (C) is correct.
Severe heat treatment of milk (such as pasteurization or sterilization) denatures soluble whey proteins (mainly $\beta$-lactoglobulin).
These denatured whey proteins form complexes with $\kappa$-casein via disulfide exchanges, sterically hindering chymosin access to its cleavage site.
Furthermore, heat treatment precipitates soluble calcium phosphate into insoluble forms, reducing the concentration of ionic calcium needed for the secondary aggregation phase.
This combination of factors increases the rennet coagulation time, meaning RCT increases with the severity of heat treatment, rather than decreasing.
Therefore, statement (D) is incorrect and is the correct choice.

Step 3: Final Answer
The incorrect statement is that rennet coagulation time decreases with the severity of heat treatment.