Identification of (furan-2-yl)methylated benzene diols and triols as a novel class of bitter compounds in roasted coffee

Preliminary studies demonstrated that the identification of unknown bitter taste compounds in roasted coffee, by means of an analytical fractionation approach, is hampered by their limited oxidative, as well as chemical stability. A synthetic-constructive strategy was followed in the present investigation by performing targeted reactions of putative coffee-related precursors to give candidate bitter taste molecules. Binary mixtures of a di and trihydroxybenzene, namely pyrogallol, hydroxyhydroquinone, catechol, or 3- and 4-methylcatechole, and a furan derivative, namely furfuryl alcohol, furan-2-aldehyde, or 5-(hydroxymethyl)furan-2-aldehyde, all of which are known to be present in roasted coffee, were thermally treated. The reaction products were identified as (furan-2-yl)methylated benzene diols and triols, by means of LC-MS and NMR experiments, and their bitter taste thresholds determined by means of sensory analysis. Finally, LC-MS/MS studies verified the natural occurence of 4-(furan-2-ylmethyl)benzene-1,2-diol, 4-(furan-2-ylmethyl)benzene-1,2,3-triol, 4-(furan-2-ylmethyl)-5-methylbenzene-1,2-diol, and 3-(furan-2-ylmethyl)-6- methylbenzene-1,2-diol as a novel class of bitter taste compounds in roasted coffee. Depending on their chemical structure, the bitter taste recognition threshold of these compounds ranged between 100 and 537 μmol/l.