If you’ve ever taken a bite out of a lemon, you already know they pack some serious sour power. This is because they contain citric acid, which is a weak organic acid found in many fruits and vegetables.
True to their name, citrus fruits have particularly high concentrations of citric acid. Lemons and limes have even higher concentrations of citric acid than their sweeter citrus cousins, such as oranges and grapefruits.
The tip of your tongue senses sweet and salty foods, while the back of your tongue senses bitter flavors. When you bite into a lemon, the citric acid activates taste buds along the sides and center of your tongue. These taste buds let your brain know when something is sour.