Structure determination and sensory evaluation of novel bitter compounds formed from [beta]-acids of hop (Humulus lupulus L.) upon wort boiling

In order to screen for the bitter compounds generated from hop-derived precursors during the wort boiling process, an ethanolic hop extract was fractionated; the fractions obtained were thermally treated under model wort boiling conditions and, then, sensorially evaluated for their bitterness. Besides the isomerisation of the [alpha]-acids into the intensely bitter iso- [alpha]-acids, the bitterness of the fraction containing the [beta]-acids was also found to be enhanced after wort boiling. To gain first insights into the [beta]-acid-derived bitter compounds, the [beta]-acid colupulone was isolated, thermally treated under wort boiling conditions and, then, investigated for bitter tasting degradation products by means of a taste dilution analysis (TDA). Besides the cohulupone, five previously unreported bitter-tasting colupulone degradation products, all of which exhibited a lingering, [beta]-acid-like bitter taste with low recognition thresholds between 37.9 and 90.3 [micro]mol/l, were isolated and their structures determined as two tricyclocolupone epimers, two dehydrotricyclocolupone epimers, and nortricyclocolupone, respectively, by means of LC-TOF-MS and 1D/2D-NMR spectroscopy.