The photochemistry of trans-isohumulone, a bitter flavouring component of beer

Abstract

Methanolic solutions of trans-isohumulone (2), a major bitter flavouring component in beer, were irradiated with UV light of 313 nm wavelength and yielded four primary products containing an enolized cyclic β-triketone moiety: cis-isohumulone (3), humulone (1), dehydro-isohumulone (7), and dehydro-humulinic acid (5). The last of these products results from loss of the 4-methyl-3-pentenoyl side chain of trans-isohumulone. Nine volatile products derived from this side chain were identified and quantitated. The identifications of all photoproducts were confirmed by independent preparation of authentic samples. No evidence of either intramolecular or intermolecular 2+2 cyclo-addition was observed. This work clarifies previous contradictory reports of the products of isohumulone photolysis and provides an example of unexpected photochemistry of an alkenyl-substituted enolized cyclic β-triketone.Key words: trans-isohumulone, photo-degradation, cyclic β-triketone.