Bitterness of Congeners and Stereoisomers of Hop-Derived Bitter Acids Found in Beer

The sensory bitterness intensity of the major hop bitter acids has been shown to vary. The cis- and trans-isomers of isocohumulone and isohumulone were isolated and purified by preparative high-performance liquid chromatography. These compounds were tested by seven tasters over a concentration range of 16.6–166 μM in a model phosphate-buffered system and compared with quinine sulfate (13.1–131 μM). More than 1,200 two-glass taste tests were carried out. Over these concentration ranges, the sensory attributes of these substances obeyed the Stevens' psychophysical power law. cis-Isohumulone was the most bitter hop acid studied while trans-isocohumulone was the least bitter. Further testing in an unhopped beer by six tasters demonstrated similar differences in the bitterness intensities of hop bitter acids. These differences were partially masked by the background flavor of the beer.