Concept:Souring of palm wine is caused by oxidation of its alcohol to an organic acid.Explanation:Fresh palm wine contains ethanol, C2H5OH.When exposed to air, oxygen oxidises ethanol to ethanoic acid.The reaction is:C2H5OH+O2→CH3COOH+H2OEthanoic acid has a sour taste and is responsible for the sour smell and taste of stale palm wine.Answer:Ethanoic acid.