The compound that makes palm wine taste sour after exposure to the air for few days is

Analyze the question: Palm wine tastes sour after a few days of exposure to air. We need the compound responsible for that sour taste.

Concept:

• Fresh palm sap contains sugar. Yeast (naturally present) ferments the sugar into ethanol (alcohol). This is why palm wine becomes alcoholic.
• If the palm wine is left exposed to air, bacteria (Acetobacter) oxidise the ethanol to ethanoic acid (acetic acid), the same acid found in vinegar. Acids taste sour.

Step-by-step solution:

1. Fermentation of sugar: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (catalysed by zymase in yeast)

2. Oxidation of ethanol on exposure to air: C₂H₅OH + O₂ → CH₃COOH + H₂O (by the action of Acetobacter bacteria)

3. Ethanoic acid (CH₃COOH) is what gives the sour taste.

Checking the options:

• A. Ethanol: this is the alcohol produced first, not the sour compound.
• B. Ethanoic acid: this is the sour product formed by oxidation of ethanol in air. ✔
• C. Methanol: it is toxic and not formed in the fermentation of palm sap.
• D. Methanoic acid: it is found in ant stings, not in souring palm wine.

Tip: Remember the sequence: Sugar → Ethanol → Ethanoic acid. Fermentation makes the alcohol, and air exposure (oxidation) makes it sour, just like wine turning to vinegar.