Question

Given below are two statements, one is labelled as Assertion (A) and other one labelled as Reason (R).
Assertion (A): Frozen tuna exhibit green/brown on cooking.
Reason (R): Browning is due to the formation of metmyoglobin in the muscle through autoxidation of ferrous myoglobin.
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:

To prevent browning/metmyoglobin formation, tuna should be frozen rapidly to extremely low temperatures ($-60^\circ$C). Standard freezers ($-18^\circ$C) are often not cold enough to stop myoglobin oxidation in tuna.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
The color of tuna meat is primarily due to the pigment myoglobin. In fresh tuna, myoglobin is in the form of oxymyoglobin (bright red). During storage, especially if frozen improperly or for long periods, chemical changes occur that alter the color of the flesh, often becoming apparent only after cooking.
Detailed Explanation:
Analysis of Assertion (A): A common quality defect in tuna is "greening" or "browning." This refers to the development of an off-color (dull green or brown) in the meat when it is cooked. This is a significant commercial issue in the canning industry. Therefore, (A) is true.

Analysis of Reason (R): Myoglobin contains an iron atom in the ferrous ($Fe^{2+}$) state. Through a process called autoxidation, this iron is oxidized to the ferric ($Fe^{3+}$) state, forming metmyoglobin. Metmyoglobin is naturally brown. During the cooking process, this metmyoglobin can further react with other muscle components (like hydrogen sulfide or trimethylamine) to form sulfmyoglobin or choleglobin, which appear green. The underlying cause for these discoloration pathways is the initial oxidation to metmyoglobin. Therefore, (R) is true.

Evaluating the Relationship: The Reason (R) provides the biochemical explanation (oxidation of myoglobin to metmyoglobin) for the physiological change observed in the meat (browning/greening) mentioned in the Assertion (A).

Step 2: Final Answer:
Both (A) and (R) are true and the reason correctly explains the biochemical cause of the color defect.