Identification and RP-HPLC-ESI-MS/MS Quantitation of Bitter-Tasting β-Acid Transformation Products in Beer

Thermal treatment of the hop beta-acid colupulone under wort boiling conditions, followed by LC-TOFMS and 1D/2D NMR spectroscopy, revealed cohulupone, hulupinic acid, nortricyclocolupone, two tricyclocolupone epimers, two dehydrotricyclocolupone epimers, two hydroxytricyclocolupone epimers, and two hydroperoxytricyclocolupone epimers as the major bitter-tasting beta-acid transformation products. Among these compounds, the chemical structures of the hydroxy- as well as the hydroperoxytricyclocolupone epimers have not previously been confirmed by 1D/2D NMR experiments, Depending on their chemical structure, these compounds showed rather low recognition thresholds ranging from 7.9 to 90.3 mu mol/L. The lowest thresholds of 7.9 and 14.7 mu mol/L were found for cohulupone, imparting a short-lasting, iso-alpha-acid-like bitter impression, and for hydroxytricyclocolupone, exhibiting a long-lasting, lingering, and harsh bitterness perceived on the posterior tongue and throat, Furthermore, HPLC-ESI-MS/MS analysis allowed for the first time a simultaneous detection and quantitation of these bitter-tasting beta-acid transformation products in a range of commercial beer samples without any sample cleanup. Depending on the type of beer, these studies revealed remarkable differences in the concentrations of the individual g-acid transformation products.