Lactic Acid (2-hydroxypropanoic acid), also known as milk acid, is a chemical compound that plays a role in several biochemical processes. It was first isolated in 1780 by Swedish chemist Carl Wilhelm Scheele and is a carboxylic acid with a chemical formula of C3H6O3. It has a hydroxyl group adjacent to the carboxyl group, making it an alpha hydroxy acid. See alpha acids. In solution, it can lose a proton from the acidic group, producing the lactate ion. Industrially, lactic acid fermentation is performed by lactobacillus bacteria, among others. Lactic acid is primarily found in sour milk products, such as koumiss, leban, yogurt, kefir, and some cottage cheeses. The casein in fermented milk is coagulated by lactic acid. In most types of beer, noticeable lactic character (sourness) is considered an off-flavor. In Germany, some breweries will maintain lactic fermentations and use the resulting liquid to make pH adjustments during the brewing process without breaking Reinheitsgebot. See reinheitsgebot. In other countries, lactic acid may be added directly to brewing water, the mash, or the kettle to make pH adjustments.

Pediococcus is, along with other lactic acid bacteria such as leuconostoc and lactobacillus, responsible for the fermentation of cabbage, turning it into sauerkraut. See lactobacillus and pediococcus. In this process the sugars in fresh cabbage are transformed to lactic acids, which give it a sour flavor. Pediococcus and lactobacillus bacteria are usually considered contaminants of beer, although their presence is sometimes desired in beer styles such as lambic (gueuze, kriek), Berliner weisse, or Leipziger gose. Certain pediococcus isolates also produce diacetyl, which gives in addition to the sour taste a buttery or butterscotch aroma to beer.