Question

Which of the following indigenous enzymes in milk has capability of reactivation from an inactivated state thereby giving a false positive test during a quality check?

• A. Lactoperoxidase
• B. Plasmin
• C. Xanthine oxidase
• D. Alkaline phosphatase

Hint:

ALP reactivation is particularly common in high-fat products (like cream) and products heated at ultra-high temperatures.
A laboratory differentiation test (using magnesium salts) can distinguish reactivated phosphatase from residual raw milk phosphatase.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Alkaline phosphatase (ALP) is an indigenous enzyme present in raw milk.
The inactivation of ALP is used as the standard index to verify the efficiency of HTST pasteurization ($72^\circ\text{C}$ for 15s) because its thermal resistance is slightly greater than that of the most heat-resistant vegetative pathogen found in milk (Coxiella burnetii).

Step 2: Detailed Explanation:
When milk is heated during pasteurization, the alkaline phosphatase enzyme is denatured and inactivated, leading to a negative phosphatase test.
However, under certain conditions, particularly when milk or cream is subjected to high-temperature short-time (HTST) treatment and subsequently stored at warm ambient temperatures, a portion of the denatured ALP enzyme can spontaneously fold back into its active conformation.
This phenomenon is known as enzymatic reactivation.
When reactivated, the enzyme can yield a positive phosphatase test result, which can be mistakenly interpreted as a pasteurization failure or post-pasteurization contamination with raw milk.
Therefore, alkaline phosphatase has this reactivation capability.

Step 3: Final Answer:
Alkaline phosphatase has the capability of reactivation, potentially giving a false-positive pasteurization test.