Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): The fruity flavor in milk is due to the production of esters.
Reason (R): The esters are produced by lipases and esterases elaborated by Pseudomonas spp.
In light of the above statements, choose the correct 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 true but (R) is false
• D. (A) is false but (R) is true

Hint:

The synthesis of the volatile ester ethyl butyrate is catalyzed by Pseudomonas fragi esterases.
This chemical reaction is the direct cause of the sweet, fruity spoilage aroma in cold-stored milk.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Psychrotrophic bacteria can grow in raw milk at refrigeration temperatures (4°C to 7°C).
These bacteria produce extracellular lipolytic and proteolytic enzymes that break down milk lipids and proteins, generating volatile compounds that cause distinct off-flavors.

Step 2: Detailed Explanation:
Let us evaluate both statements:
- Assertion (A):
The fruity off-flavor in refrigerated milk (often resembling apple, pear, or pineapple) is caused by the synthesis of volatile organic esters.
These esters have low sensory thresholds and can be detected by human senses at very low concentrations.
Thus, Assertion (A) is true.
- Reason (R):
Psychrotrophic Gram-negative bacteria, particularly Pseudomonas fragi, are the primary organisms responsible for this defect.
These bacteria produce heat-stable extracellular lipases and esterases.
The lipases hydrolyze milk triglycerides to release short-chain fatty acids (such as butyric and hexanoic acids).
The esterases then catalyze the esterification of these fatty acids with ethanol (produced by microbial fermentation), forming volatile esters like ethyl butyrate and ethyl hexanoate.
Thus, Reason (R) is true and directly provides the biochemical mechanism that explains the assertion.

Step 3: Final Answer:
Both (A) and (R) are true, and (R) is the correct explanation of (A), which corresponds to Option (A).