Oxygen and Oxygen Radicals in Malting and Brewing: A Review¹

Abstract

Because oxygen is a relatively stable molecule, it needs to be converted to one of several radical or activated forms before it can react with other species. Such activation may be caused by transition metal ions or by other radical species. Alternatively, activation may be brought about by enzymes such as lipoxygenase. In the context of oxidation in malting and brewing, it is argued that lipoxygenase is most active during malting and that the principal route by which oxygen is consumed in wort production is by peroxidase-catalyzed oxidation of polyphenols in mashing and by nonenzymatic oxidation of polyphenols in boiling. Oxygen, oxygen radicals, oxidation, and relevant protective systems are reviewed principally in the context of flavor instability. Oxygen and oxygen radicals are, however, relevant to other areas pertinent to the brewer, including enzyme inactivation: Hydroxyl radicals have been shown to inactivate malt β-glucanase. Phytic acid may protect against oxidation by scavenging metal ions.