Understanding reactions of oxygen in beer.

Selles, J.F.

Abstract
Following on from a previous paper (MBAA TQ vol. 34, Number 1, Pages 290-292) in which the author postulated that oxygen which dissolves in an acidic aqueous medium, such as beer, becomes ionized and in that state reacts with water to form hydrogen peroxide, from which oxygen ions can subsequently be released (e.g. by temperature increases, pressure drops or agitation) to oxidize other beer constituents, the possible reactions between different forms of oxygen (ions, molecules, hydrogen peroxide) and beer constituents are discussed. Some alcohols, including ethanol, are oxidized to aldehydes which in turn can be further oxidized to form acids such as acetic acid, while secondary alcohols, such as propanol, are oxidized to ketones. The acids formed by aldehyde oxidation can react with alcohols to form esters, or can catalyse the self condensation of alcohols to ethers. Low molecular weight phenolic compounds can be polymerized by reactions with peroxides, which can also break the double bond in alkenes and thus cause chains of alkanes to be formed. It has also been reported in the literature that hydrogen peroxide promotes the coagulation of proteins, suggesting the possible use of this reaction to assist trub formation and precipitation in the brewhouse. These findings, together with the fact that tannins similarly coagulate and precipitate proteins, form the basis of the author's theory that oxygen ions released by the breakdown of hydrogen peroxide might react with the hydrogen ion of one amino acid and the linked oxygen and hydrogen of the carboxyl of another, condensing the two amino acids together, after which each oxygen ion joins a water molecule to become hydrogen peroxide again. The possibility of testing this theory by experimentation is briefly discussed together with the theory's implications with regard to beer quality, including colloidal stability and foaming properties.
Keywords : beer chemistry deterioration oxidation oxygen quality