Iron (chemical symbol Fe) occurs naturally in brewing and cleaning water as salts or ions, including Fe²⁺ and Fe³⁺ cations. Iron, however, is usually kept at concentrations of no more than 1 mg/l because at higher concentrations it would have a detrimental effect on the finished beer’s taste and color. To keep the iron content in check, brewers often aerate and filter their water before using it. In addition to brewing water, diatomaceous earth preparations used in beer filtration, as well as hot water from fobber jets between the filler and the capper at the bottling line, are also potential sources of iron in beer.

In most finished beers, iron is no more than just a trace element of perhaps 0.1 mg/l. Otherwise, tannins—derived from grain husks and hops—could form chemical linkages with iron ions, which would add slightly metallic or ink-like off-flavors and a brown tinge to the beer. Even these low levels of iron can be damaging to the stability of beer because they potentiate the production of reactive oxygen species that can cause the staling of beer and the oxidation of polyphenols that leads to haze development.

Iron, however, has one positive effect. It promotes beer foam by enhancing the bridge-building capacity, elasticity, and stability of polypeptide chains on the surface of the carbonation bubbles that form the head. In countries where food safety regulations permit, therefore, ferrous salts are sometimes added to beer as foam stabilizers at a dosage of up to 0.6 g/hl, but always in conjunction with reducing compounds that keep the foam from turning a rather unattractive rust brown.

Excessive amounts of ferrous salts in beer are extremely undesirable because ferrous precipitates may serve as nucleation points for large carbon dioxide bubbles inside the bottle, causing gushing problems. See also gushing.