Oxidative Degradation of Isohumulones in Relation to Flavour Stability of Beer

Isohumulones undergo oxidative degradation in bottled beer during shelf storage to give volatile compounds including 2-alkanones with chain lengths of C3 to C11, alkanals with chain lengths of C2 to C10 2-alkenals of C4 to C7 or more and 2,4-alkadienals of C6 to C7. Since some resultant aldehydes are involved in the formation of stale aldehydes, very high levels of isohumulones in beer may cause considerable staling of beer flavour. Isohumulones were decomposed to these carbonyl compounds more easily than their derivatives, such as di- or tetrahydroisohumulones and humulinic acids, because the double bond or carbonyl group of the isohexenoyl side chain of the isohumulone molecule is involved in this degradation. This oxidation of isohumulones was greatly suppressed by addition of 300 ppm of sodium ascorbate or 40 ppm of ferrous ion.