Sourness describes the taste that perceives acidity. See acidity. The mechanism for detecting sour taste is similar to that which detects salt taste. Hydrogen ion channels detect the concentration of hydronium ions that are formed from acids and water. Sour taste is mainly recognized on the left and right sides of the tongue.

By a combination of direct intake of hydrogen ions (which itself depolarizes the cell) and the inhibition of the hyperpolarizing channel, sourness causes the taste cell to fire in this specific manner. It has also been suggested that weak acids—such as carbon dioxide, which is converted into the bicarbonate ion by the enzyme carbonic anhydrase—mediate weak acid transport. The food group that most commonly contains naturally sour foods is fruit, with examples such as lemon or grape.

Sourness normally is not one of the main taste impressions in beer and is usually considered an off-flavor. Sourness in beers usually is indicative of an infection, and acidity tends to clash with hop bitterness. However, Belgian lambic beers (gueuze, kriek), which undergo spontaneous fermentation, traditionally have a significant sour tang. See lambic. The same is true of Flanders brown and red ales. Wild yeasts and bacteria varieties such as Brettanomyces bruxellensis, Brettanomyces lambicus, and Pedioccocus are responsible for the occurring sourness. Other organisms, including Lactobacillus bacteria, produce acids which may cause a sour taste in beer. Sour ales such as Berliner weisse and Leipziger gose are inoculated with lactobacillus bacteria. See lactobacillus.

The Belgian and old German sour beer styles have inspired considerable interest in the brewing of sour ales among craft brewers, particularly in the United States.

See also sour beer.