Final answer:
The sour taste in foods like lemons, grapes, and oranges is due to the presence of citric acid. Acetic acid, found in vinegar, also has a sour taste.
Step-by-step explanation:
The sour taste in foods like lemons, grapes, and oranges is due to the presence of citric acid. Citric acid is a weak acid found naturally in citrus fruits and is responsible for their sour flavor. On the other hand, acetic acid, which is found in vinegar, also has a sour taste.