Behavior and role of iron ions in beer deterioration

Abstract

The behavior of iron ions during beer oxidation was studied, using an electron spin resonance (ESR) technique at 77 K, to clarify the role of metal ions in beer deterioration. An ESR signal due to non-heme Fe3+ (g = 4.3) was detected in beer, and its peak height increased during beer oxidation. The increase during beer storage was accelerated by the addition of H2O2, Fe3+, or Fe2+. The signal for free radicals (g = 2.004) was detected in beer by the addition of KOH just before the analysis. It was increased by the storage of beer with or without the addition of iron ions and/or H2O2. The addition of Fe2+, Fe3+, H2O2, Fe2+-H2O2, or Fe3+-H2O2 accelerated the oxidative degradation of the main bitter components, isohumulones, during the storage of beer, while the addition of DETAPAC or EDTA inhibited the oxidative degradation. It was thought that Fe2+ ions dominant in fresh beer are oxidized to Fe3+ ions during the beer oxidation and that the strong complexes of Fe3+ with beer components (non-heme Fe3+) increase, leading to haze formation. It seems that the changes in iron ions following the metal-catalyzed free-radical reactions lead to the deterioration of beer quality.