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 10 6–10 8 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.