Characterisation of a stable radical from dark roasted malt in wort and beer

摘要

An exceptionally stable radical was detected by electron spin resonance (ESR) spectroscopy at room temperature directly in dark beers and in sweet wort produced with dark malt. The ESR spectrum was a broad singlet signal without any hyperfine structure and a g-value of 2.0040. The radical was detected at the initial step of the mashing process and the radical intensity increased with higher proportions of dark malt suggesting a water-soluble radical complex that is formed during the roasting of malt. The radical was further characterised by several precipitation protocols suggesting a melanoidin-derived radical. Size-exclusion chromatography and light scattering measurements showed that the radical was associated with a high molecular weight complex of approximately 106-108 g/mol. The radical proved to be stable towards Fenton reactants, freezing or heating temperatures. Radical intensity was shown to be stable in the presence of oxygen and decreased reversibly under a nitrogen atmosphere.