Electron spin resonance spin trapping identification of radicals formed during aerobic forced aging of beer

Using 2-methyl-2-nitrosopropane, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), phenyl-N-tert-butylnitrone, and 4-pyridyl-1-oxide-N-tert-butylnitrone as spin traps, spin adducts of free radicals formed in beer under aerobic conditions have been characterized by ESR spectroscopy. The 1-hydroxyethyl radical is found to be the quantitatively most important radical, while the hydroxyl radical is trapped only by DMPO and only at high concentrations (0.5 M). Fenton-like oxidation processes in beer thus involve reaction of the 1-hydroxyethyl radical in an important reaction with oxygen, converting it to hydroperoxyl radicals.