Acidic foods are believed to taste sour because of their hydrogen ion concentration. Scientists believe that the human tongue determines taste through several different channels. The taste buds that register sour and salty flavors do so through ion channels.
According to the Brønsted-Lowry definition, acids include any substance that gives up its hydrogen ions when mixed with another substance. Thus, acids increase the hydrogen ion concentration, which provides more ions for the tongue. Because the tongue gains ions, it registers the flavor as sour. Bases, on the other hand, gain ions when mixed. For instance, if one mixed an acid and a base together, the mixture would create water and salt. Bases taste more bitter because fewer hydrogen ions reach the tongue. Acidity can be measured by the pH level of a solution or compound. The lower the solution or compound on the pH scale, the more sour it tastes due to the concentration of hydrogen ions.
Contrary to popular belief, all portions of the tongue can register all types of flavors, and the tongue is not divided into specific flavor-registering sections. Taste buds are essentially clusters of 100 to 150 cells that all experience taste in a slightly different way.