Fruity flavour defects in milk and dairy products (such as Cheddar cheese or pasteurized milk) are caused by volatile ethyl esters formed by enzymatic esterification of free fatty acids with ethanol, catalyzed by microbial esterases (predominantly from Pseudomonas fragi). Ethylbutanoate (ethyl butyrate) and ethyl hexanoate are the primary volatile esters imparting sweet, apple- or pineapple-like fruity aromas.

Option analysis:

• methylbutanol: Responsible for the malty flavor defect produced by Lactococcus lactis biovar maltigenes.
• methoxy formazine: Chemical intermediate, not a dairy fruity odorant.
• Ethylbutanoate: Correct. Key ethyl ester responsible for fruity off-flavour in dairy products.
• Trimethylamine: Responsible for fishy off-flavour in stored butter and milk.

Assertion (A) is true: The distinctive sweet fruity defect in cold-stored raw and pasteurized milk is caused by volatile esters (principally ethyl butyrate and ethyl hexanoate).
Reason (R) is true: Psychrotrophic Pseudomonas species (most notably Pseudomonas fragi) produce heat-stable extracellular lipases and esterases that enzymatically synthesize these ethyl esters from free fatty acids and ethanol.
Because Reason (R) describes the exact enzymatic and microbial mechanism causing the ester production in Assertion (A), both (A) and (R) are true and (R) is the correct explanation of (A).

Option analysis:

• Correct. Both (A) and (R) are true and (R) is the correct explanation of (A).
• Incorrect because (R) is indeed the direct explanation of (A).
• Incorrect because (R) is true.
• Incorrect because (A) is true.