Structure Determination of 3-O-Caffeoyl-epi-γ-quinide, an Orphan Bitter Lactone in Roasted Coffee

Recent investigations on the bitterness of coffee as well as 5-O-caffeoyl quinic acid roasting mixtures indicated the existence of another, yet unknown, bitter lactone besides the previously identified bitter compounds 5-O-caffeoyl-muco-γ-quinide, 3-O-caffeoyl-γ-quinide, 4-O-caffeoyl-muco-γ-quinide, 5-O-caffeoyl-epi-δ-quinide, and 4-O-caffeoyl-γ-quinide. In the present study, this orphan bitter lactone was isolated from the reaction products generated by dry heating of 5-O-caffeoylquinic acid model, and its structure was determined as the previously unreported 3-O-caffeoyl-epi-γ-quinide by means of liquid chromatography−mass spectrometry (LC-MS) and one-/two-dimensional NMR experiments. The occurrence of this bitter lactone, exhibiting a low bitter recognition threshold of 58 μmol/L, in coffee beverages could be confirmed by LC-MS/MS (negative electrospray ionization) operating in the multiple reaction monitoring mode.