Potassium Metabisulfite (K2S2O5), commonly abbreviated KMS, is a powdery, white, strongly sulfurous-smelling (burned match) chemical used in many food and beverage industries, as well as in breweries as an antioxidant. When dissolved in water or beer, KMS releases free sulfite ions, and these are responsible for the antioxidative properties of the compound. KMS is by far the most popular antioxidant used in the brewing industry. Its popularity within some large breweries is because it is highly effective at preventing oxidation and because the added sulfites improve the naturally occurring sulfites produced by the yeast during fermentation. KMS is also active in limiting growth of wild yeasts and bacteria in beer.

Free sulfite reacts very readily with and thus eliminates free oxygen in foods (including beer), but also with many intermediary chemicals compounds (most important, aldehydes and ketones) that can combine with free oxygen into the stale and papery-tasting compounds so undesirable in beer.

KMS is allowed by Food Health and Safety authorities in most countries (it has the E-number 224 on the EU positive list of food additives) around the world, but it is not permitted by Germany’s Reinheitsgebot (the German Purity Law, which is not, however, despite its name, an official law). KMS is rarely used by craft brewers anywhere, but it is common in mass-market beers in many countries. In most countries the use of KMS is regulated by specific limits either to the total concentration of sulfites in beer or by maximum allowed dosing rates. The reason for these limitations is that sulfites are known to provoke and worsen certain allergies in humans. For example, in the United States, levels above 10 ppm will require the words “contains sulfites” to be printed on the label.

KMS is very widely, almost universally, used in the wine industry for the same purposes as in brewing, but in wine KMS is used at concentrations 10–20 times higher than in beer.