title: Reactivity of beer bitter acids toward the 1-hydroxyethyl radical as probed by spin-trapping electron paramagnetic resonance (EPR) and electrospray ionization-tandem mass spectrometry (ESI-MS/MS).
abstract: The iso-α-acids or isohumulones are the major contributors to the bitter taste of beer, and it is well-recognized that they are degraded during beer aging. In particular, the trans-isohumulones seem to be less stable than the cis-isohumulones. The major radical identified in beer is the 1-hydroxyethyl radical; however, the reactivity between this radical and the isohumulones has not been reported until now. Therefore, we studied the reactivity of isohumulones toward the 1-hydroxyethyl radical through a competitive kinetic approach. It was observed that both cis- and trans-isohumulones and dihydroisohumulones are decomposed in the presence of 1-hydroxyethyl radicals, while the reactivities are comparable. On the other hand, the tetrahydroisohumulones did not react with 1-hydroxyethyl radicals. The apparent second-order rate constants for the reactions between the 1-hydroxyethyl radical and these compounds were determined by electron paramagnetic resonance (EPR) spectroscopy and electrospray ionization-tandem mass spectrometry [ESI(+)-MS/MS]. It follows that degradation of beer bitter acids is highly influenced by the presence of 1-hydroxyethyl radicals. The reaction products were detected by liquid chromatography-electrospray ionization-ion trap-tandem mass spectrometry (LC-ESI-IT-MS/MS), and the formation of oxidized derivatives of the isohumulones was confirmed. These data help to understand the mechanism of beer degradation upon aging.
authors.affiliation: Instituto de Química de São Carlos (IQSC), Universidade de São Paulo, São Carlos, São Paulo, Brazil.
authors.forename: Natália E C
authors.initials: NE
authors.lastname: de Almeida
authors.forename: Paula
authors.initials: P
authors.lastname: Homem-de-Mello
authors.forename: Denis
authors.initials: D
authors.lastname: De Keukeleire
authors.forename: Daniel R
authors.initials: DR
authors.lastname: Cardoso
mesh_terms: D000143:Acids / Q000032:analysis*; D001515:Beer / Q000032:analysis*; D002851:Chromatography, High Pressure Liquid; D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods*; D000431:Ethanol / Q000032:analysis*; D021241:Spectrometry, Mass, Electrospray Ionization / Q000379:methods*; D013113:Spin Labels; D053719:Tandem Mass Spectrometry / Q000379:methods*
chemical_list: D000143:Acids; D013113:Spin Labels; C065424:1-hydroxyethyl radical; D000431:Ethanol