Bioresponse-guided decomposition of roast coffee beverage and identification of key bitter taste compounds

Sensory-guided fractionation of a roasted coffee brew by means of sequential solvent extraction, ultrafiltration, and RP-HPLC demonstrated a group of ethyl acetate soluble compounds formed from O-hydroxycinnamoyl quinic acid derivatives upon coffee roasting as the key compounds contributing to the bitter taste of roasted coffee beverages. LC-MS/MS studies, 1D- and 2D-NMR spectroscopy, syntheses, and model roast experiments with 5-O-caffeoyl- and 5-O-feruloylquinic acid led to the unequivocal identification of 3-O-caffeoyl-gamma-quinide (2a), 4-O-caffeoyl-gamma-quinide (3a), 5-O-caffeoyl-epi-delta-quinide (4a), 4-O-caffeoyl-muco-gamma-quinide (5a), 5-O-caffeoyl-muco-gamma-quinide (6a), 3-O-feruloyl-gamma-quinide (2b), and 4-O-feruloyl-gamma-quinide (3b) as intense coffee bitter tastants. Besides these individual bitter compounds, a highly complex and intensely bitter HPLC fraction was isolated from the ethyl acetate extractables of coffee brew. Application of COSY spectroscopy and alkaline hydrolytic degradation gave strong evidence that the bitter taste of that fraction is due to a multiplicity of rather complex quinic acid lactone isomers multiply esterified with p-coumaric acid, caffeic acid, ferulic acid, 3,4-dimethoxycinnamic acid, and quinic acid, respectively. As representatives of this fraction, 3,4-O-dicaffeoyl-gamma-quinide (10), 3,5-O-dicaffeoyl-epi-delta-quinide (11), and 4,5-O-dicaffeoyl-muco-gamma-quinide (12) have been isolated, purified, and identified as strongly bitter-tasting compounds in roasted coffee. For the first time, bitter taste recognition thresholds were determined for the individual compounds showing that, strongly depending on their chemical structure, the bitter threshold levels ranged between 9.8 and 180 mu mol/l (water).