The Role of Free Radicals in Beer Oxidation

Influence of the Hopping Technology on the Storage-induced Appearance of Staling Aldehydes in Beer

“…11 and 12, although the levels of these aldehydes show only a minor increase in comparison with 2-methylpropanal. Yet, once more the increase of these branched aldehydes, also related to the decomposition of iso-α-acids in the literature 18,19,33 , cannot be associated to the bittering applied in the experimental beers in this study, as the results of both the unhopped and the bittered beers were similar. These results are not in agreement with the scientific findings reported by Hashimoto and Eshima 18 stating that beer brewed without hops hardly develops any typical stale flavour, even after prolonged storage.…”

“…Various authors have suggested chemical structures of putative degradation products of iso-α-acids 18,19,20,21,22,25,26,27,28,33 . Non-volatile trans-specific cyclic iso-α-acid transformation products, generated exclusively from trans-iso-α-ac- ids by proton-catalysed cyclisation during storage of beer, were very recently identified by simulation of the transspecific degradation in quantitative model experiments as well as in beer on a laboratory scale 22,27,28 .…”

“…Regarding the formation of volatile compounds from isohumulones, it has been reported that the oxidative degradation of isohumulones based on radical reactions produces aldehydes participating in stale flavour formation 18,19,20,33 . According to Kaneda et al 33 , free radicals, such as hydroxyl radicals and the superoxide radical anion, oxidise isohumulones directly into aldehydes and ketones. Besides, they proposed hydroxyl radicals to initiate a series of radical reactions which, in turn, also oxidise isohumulones into the undesirable staling aldehydes.…”

“…Upon storage in the absence of light, these compounds rapidly undergo degradation in the presence of oxygen species 18,24,32 . This oxidative degradation of iso-α-acids results in a significant decline in beer bitterness intensity and a quality change towards a lingering, more harsh bitterness, accompanied by an astringent note in the taste 10,22,27,28,36 , and has been linked to the development of volatile carbonyl compounds such as 2-methylpropanal, 2-methylbutanal and 3-methylbutanal, associated with staling of beer flavour 4,17,18,33,51 . On the other hand, the oxidative decomposition of iso-α-acids during beer ageing has also been linked to the formation of a series of nonvolatile trans-specific cyclic iso-α-acid transformation products, exhibiting the aforementioned harsh lingering bitter taste 22,27,28 .…”

“…Accordingly, the fundamental central question in this study was which compounds are to be regarded as the precursors for the staling volatile aldehyde compounds formed upon beer ageing. Specifically 2-methylpropanal, 2-methylbutanal and 3-methylbutanal are known as both Strecker degradation aldehydes 45 and as possibly originating from the conversion of hop-derived bitter acids 18,33 . In other words, are these ageing aldehydes derived from the raw material malt and the production process or should they mainly be considered as degradation products of trans-iso-α-acids?…”