Question

Conversion of chlorophyll to pheophytin occurs due to _______.

• A. oxidation
• B. acidic conditions
• C. alkaline conditions
• D. enzyme action

Hint:

Acid replaces magnesium:
* Chlorophyll + Acid $\rightarrow$ Pheophytin (Olive-brown)
* Chlorophyll + Heat (without acid control) $\rightarrow$ color loss

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the primary chemical factor responsible for converting green chlorophyll pigments into olive-brown pheophytin.

Step 2: Key Concepts and Approach:
Chlorophyll has a porphyrin-like ring structure containing a central magnesium ($Mg^{2+}$) ion.
The displacement of this magnesium ion by hydrogen ions ($H^+$) changes the chemical structure and changes the pigment's color from bright green to olive-brown.

Step 3: Detailed Explanation:

• Reaction Mechanism: In the presence of acids (high concentration of $H^+$ ions), two hydrogen ions replace the magnesium ion in the center of the porphyrin ring.
\[ \text{Chlorophyll (Green)} + 2H^+ \rightarrow \text{Pheophytin (Olive-brown)} + Mg^{2+} \]

• Processing Effects: During thermal processing (like canning green peas or spinach), cell walls break down, releasing organic acids from plant vacuoles.
These acids lower the pH, driving the conversion of chlorophyll to pheophytin and turning the vegetables a dull olive-brown color.

• Effect of Alkali: In contrast, alkaline conditions help retain the magnesium ion (keeping the bright green color by forming chlorophyllin), although it can make the vegetable texture unacceptably soft.

• Effect of Enzymes: Chlorophyllase removes the phytol tail to form chlorophyllide, but does not remove the magnesium ion to form pheophytin.

Step 4: Final Answer:
The conversion of chlorophyll to pheophytin is caused by acidic conditions.