Off-flavor in beer is often associated with the appearance of staling aldehydes. In this study, the factors amino acid concentration, carbohydrate concentration, Fe concentration, and oxygen concentration were investigated in terms of their effect on the formation of carbonyl compounds during storage using response surface methodology. From all factors tested, only amino acid concentration and oxygen concentration promoted Strecker aldehyde formation during storage while all other carbonyls measured were unaffected. A mixture of glucose/xylose, representing carbohydrate sources, as well as Fe concentration were insignificant factors; though, carbohydrate additions exhibited a significant role for the formation of 2-furfural. De novo formation of phenylacetaldehyde from phenylalanine during beer storage was observed using labeling experiments and a linear relationship between Strecker aldehydes formed and total packaged oxygen was identified. Capping beers with oxygen barrier crown corks and addition of 10 mg/L EDTA to beers effectively diminished Strecker aldehyde formation. Oxygen was additionally shown to significantly promote Strecker aldehyde formation during sweet wort production. A pathway for the reactive oxygen species-induced degradation of amino acids yielding Strecker aldehydes was proposed and was further scrutinized in buffered model solutions. The insignificant role of Fe in the response surface experiments is discussed.

J. Inst. Brew. 116(2), 157-166, 2010The oxidative stability of wort at different stages during wort production and the radical scavenging characteristics of α-, β-, iso-α-acids and a hop polyphenol extract were evaluated using ESR spectroscopy and DPPH• radical quenching measurements. Radical generation in unhopped wort was significantly, positively correlated with heating rate prior to boiling but not with dissolved oxygen content. Hop α-and β-acids showed similarly significant radical quenching abilities, while iso-α-acids and a hop polyphenol extract displayed a negligible effect. Isomerization of α-acids to iso-α-acids reduced wort's antioxidative capacity. Commercially available hop products with varying polyphenol content showed similar radical inhibiting properties, which were a function of hop acid content. Relative to an unhopped treatment, hop additions to wort resulted in significantly lower amounts of Strecker aldehydes in stored beer.

Different hopping regimes were evaluated to investigate the effect on the oxidative stability of wort and beer. Compared with a single hop dosage at the beginning of wort boil, it was possible to increase the concentration of α-acids in pitching wort and beer by applying incremental hop dosage, dry hopping or the use of a pre-isomerized hop product in combination with an α-acid extract, which concomitantly resulted in lower iron concentrations and an enhanced flavour stability as indicated by standard wort and beer analyses, atomic absorption spectroscopy, electron spin resonance spectroscopy and sensory analysis of fresh and force-aged beers. The functional principle of hop dosage variations is explained by saving of α-acids throughout the wort production process, which yields an increased formation and precipitation of pro-oxidative acting transition metal ions (e.g. Fe) in α-acid-complexes during the whirlpool rest and fermentation. Consequently, fewer reactive oxygen species are generated. Additional laboratory trials simulating wort cooling and beer storage in buffered model solutions proved that unisomerized α-acids are strong iron chelators and confirmed the functional principle of the applied hopping regimes. Negative effects of higher α-acid contents on fermentation performance and depletion of the zinc concentration, which is an essential nutrient for yeast, could be excluded.

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