Mechanism for formation of the lightstruck flavor in beer revealed by time-resolved electron paramagnetic resonance

dTime-resolved electron paramagnetic resonance (TREPR) data collected durin g the photodegradation of iso-a-acids (isohumulones), the principal bitteri ng agents from hops in beer, are presented and discussed, and, from the dat a, the photophysics leading to free-radical production as the primary step in the photo decomposition of iso-alpha -acids towards the development of " skunky" beer are explained. During laser flash photolysis of iso-a-acids at 308 run in toluene/methylcyclohexane (1:1), TREPR spectra exhibit net emis sive signals that are strongly spin polarized by the triplet mechanism of c hemically induced electron spin polarization. From two potential photochern ically active sites, the TREPR data show that although the first site, an e nolized triketone, is the primary light-absorbing chromophore, an uphill in tramolecular triplet energy transfer process leads to Norrish type I a-clea vage at a second site, an alpha -hydroxycarbonyl. The energy transfer mecha nism is supported by additional TREPR experiments with chemically modified hop compounds. Structural parameters (hyperfine coupling constants, g facto rs, line widths) for the observed free radicals, obtained from computer sim ulations, are presented and discussed.