On the autoxidation of bitter-tasting iso-alpha-acids in beer.
The iso-alpha-acids, the major contributor to bitter beer taste, is well-known to strongly degrade during beer aging. The storage of beer in brown glass bottles revealed a strong depletion of the trans-configured isomers in a highly specific manner, whereas the corresponding cis-iso-alpha-acids seemed to be hardly affected. In comparison, storage of beer in polyethylene terephthalate bottles, which are known to be permeable to oxygen, induced a drastic degradation of both isomers independent of their cis/trans configuration. To investigate the chemical transformation of iso-alpha-acids under oxidative storage conditions, suitable model experiments were performed, and the reaction products that formed were identified as previously not reported hydroperoxy- and hydroxyl-allo-iso-alpha-acids by means of one-/two-dimensional NMR and liquid chromatography-mass spectrometry experiments; for example, cis- and trans-configured hydroperoxy-alloisohumulone as well as the corresponding hydroxy-alloisohumulones were generated upon oxidation of cis- and trans-isohumulone independent of their cis/trans configuration. In addition, the oxidation products formed from the various iso-alpha-acid congeners were quantitatively determined in a series of beer samples stored under defined conditions. For the first time, these data help to understand the molecular mechanism involved in the autoxidative degradation of iso-alpha-acids in beer.
Intelmann
Daniel
D
Food Chemistry and Molecular Sensory Science, Technische Universitat Munchen, Freising, Germany.
Hofmann
Thomas
T
D000143:Acids / Q000737:chemistry*; D001515:Beer / Q000032:analysis*; D002853:Chromatography, Liquid; D009682:Magnetic Resonance Spectroscopy; D013058:Mass Spectrometry; D010084:Oxidation-Reduction
D000143:Acids