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

Time-resolved electron paramagnetic resonance (TREPR) data collected during the photodegradation of iso-a-acids (isohumulones), the principal bittering agents from hops in beer, are presented and discussed, and, from the data, the photophysics leading to free-radical production as the primary step in the photodecomposition of iso-alpha-acids towards the development of "skunky" beer are explained. During laser flash photolysis of iso-alpha-acids at 308 nm in toluene/methylcyclohexane (1:1), TREPR spectra exhibit net emissive signals that are strongly spin polarized by the triplet mechanism of chemically induced electron spin polarization. From two potential photochemically active sites, the TREPR data show that although the first site, an enolized beta-triketone, is the primary light-absorbing chromophore, an uphill intramolecular triplet energy transfer process leads to Norrish type I alpha-cleavage at a second site, an alpha-hydroxycarbonyl. The energy transfer mechanism is supported by additional TREPR experiments with chemically modified hop compounds. Structural parameters (hyperfine coupling constants, g factors, line widths) for the observed free radicals, obtained from computer simulations, are presented and discussed.