Dissolved oxygen in packaged beer is mainly of concern to the brewing industry because the oxidation of various beer constituents can generate off flavours, but it can also have adverse effects on foam stability. On the basis of the known properties of hydrogen, oxygen and water, the author explains how, in an acidic aqueous medium such as beer, dissolved oxygen becomes ionized and the oxygen ions then react with water to form hydrogen peroxide, a reaction particularly favoured by low temperatures. If the beer is subsequently heated, the peroxide breaks down, releasing the oxygen ions which can then oxidize other beer constituents. However, if the beer is not exposed to heat after packaging (i.e. it is not pasteurized and is kept cold during storage and distribution) the release of pressure when the bottle or can is opened also causes the breakdown of some of the hydrogen peroxide to oxygen and water, but instead of oxidation taking place, the oxygen thus released (whether as ions or molecules or a mixture of the two) and the remaining hydrogen peroxide rise out of the liquid beer into the foam head, where they react with water molecules in the bubble film, which results in the very rapid collapse of the foam.
Keywords : beer chemistry foam stability hydrogen peroxide oxygen