Why is lemon juice sour ?

Sour taste is one of the five basic tastes, along with sweet, salty, bitter, and umami. It is often described as sharp, tangy, or acidic. Sour taste is typically caused by the presence of acids, such as citric acid in lemon juice or acetic acid in vinegar. When you taste something sour, it's because the taste receptors on your tongue are reacting to the presence of hydrogen ions in the acid. Some sour foods and drinks include lemons, limes, grapefruits, sour candies, pickles, and yogurt. Sour taste is often used to balance out sweetness in foods and beverages, or to add a refreshing, tangy flavor to dishes.