Acid. The general definition of an acid is any compound that yields H+ (hydrogen ions or protons) in solution or a chemical that reacts with alkalis (bases) to form salts. Solutions with pH values below 7.0 are acidic (see ph). In beer the term often pertains to sour, tart, acidic, and pungent aroma and/or taste. In most beer styles, notable acidic character is considered undesirable.

Most acids in beer are organic acids and include, most commonly, acetic (vinegar flavor), pyruvic, lactic (sour milk-like flavor), malic, and citric acid (strong tartness). They derive from raw materials, wort boiling and, mainly, yeast metabolism. Lactic and acetic acids can also derive from unwanted or uncontrolled microbial contamination. See acetic acid and lactic acid. The fall in pH during fermentation (typically by about one unit, for instance, from 5.20 to 4.20) depends in part on organic acid excretion.

These and other acids in beer influence flavor both directly, when present above their threshold levels, and by their influence on beer pH. Lactic acid, which is relatively weak, is sometimes used in the brewery for water composition and pH adjustment purposes (as the anions can aid the brewing process or promote flavor enhancement from raw materials). Stronger mineral acids, such as sulfuric acid and phosphoric acid, are used for passivating stainless steel and for cleaning brewing vessels and tanks. Phosphoric acid may also be used in acid-washing to reduce or eliminate bacterial infections in brewing yeast. Peracetic acid is an effective no-rinse sanitizer for brewery applications. Ascorbic acid (vitamin C) is sometimes used in beer as an antioxidant.