Pure hops-derived substances were added to unhopped beer. Illumination only produced a sun-struck flavour with cis-isohumulone, trans-isohumulone and racemic trans-isohumulone. It seems that 3-methyl-butenyl-mercaptan is not or not alone responsible for sun-struck flavour. Tests trials show that trans-isohumulone, cis-isohumulone, racemic trans-isohumulone, trans-1-isohumulone and cis-1-isohumulone do not show significant differences in bitter taste and bittering power. Fresh solutions of these bitter acids are not as bitter as solutions which have been kept for a few weeks.