What functional group is responsible for the sour taste on citrus fruits?

The sour taste in citrus fruits is primarily due to the presence of organic acids, particularly citric acid. This functional group, characterized by its carboxyl groups (-COOH), contributes to the tart flavor that is a hallmark of citrus fruits like lemons, limes, and Oranges. Additionally, other acids such as ascorbic acid (vitamin C) may also contribute to the overall taste profile.