Researchers at the Technical University of Munich and the University of Münster have identified several new chemicals in the complex brew we call beer (Intelmann et al. 2009 DOI). Adhumulone is one of several compounds present in hops and converts in a pinacol rearrangement to iso-adhumulone - a key bitter compound - as a cis and trans-isomer during beer processing. The bitter taste of beer (a quality) disappears over time - limiting shelf life - as the amount of trans humulones decreases (significantly the cis isomer is stable) and the German researchers now propose a new chemical pathway.
They purified a trans iso-adhumulone, stored an aqueous solution of it at pH 4 @ 40 °C for 6 days, separated the degradation products by reversed phase LC-MS and analysed components by NMR spectroscopy to arrive at a tricylocohumol as one of the main degradation products, the result of a Prins reaction. The researchers then went on to detect the compound in a sample of commercial aged beer and even found some time to establish bitter-taste threshold concentrations (poor volunteers) - still inconclusive as some tricycles are more bitter (but more harshly so) than their precursors.
Intelmann, D., Kummerlöwe, G., Haseleu, G., Desmer, N., Schulze, K., Fröhlich, R., Frank, O., Luy, B., & Hofmann, T. (2009). Structures of Storage-Induced Transformation Products of the Beer?s Bitter Principles, Revealed by Sophisticated NMR Spectroscopic and LC-MS Techniques Chemistry - A European Journal DOI: 10.1002/chem.200902058