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During fermentation, the yeast Saccharomyces cerevisiae produces a broad range of aroma-active substances, which are vital for the complex flavour of beer. In order to obtain insight into the influence of high-gravity brewing and fermentation temperature on flavour formation, we analysed flavour production and the expression level of ten genes (ADH1, BAP2, BAT1, BAT2, ILV5, ATF1, ATF2, IAH1, EHT1 and EEB1) during fermentation of a lager and an ale yeast. Higher initial wort gravity increased acetate ester production, while the influence of higher fermentation temperature on aroma compound production was rather limited. In addition, there is a good correlation between flavour production and the expression level of specific genes involved in the biosynthesis of aroma compounds. We conclude that yeasts with desired amounts of esters and higher alcohols, in accordance with specific consumer preferences, may be identified based on the expression level of flavour biosynthesis genes. Moreover, these results demonstrate that the initial wort density can determine the final concentration of important volatile aroma compounds, thereby allowing beneficial adaptation of the flavour of beer.

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The flavor profile of beer is subject to changes during storage. Since, possibly, yeast has an influence on flavor stability, the aim of this study was to examine if there is a direct impact of brewing yeast on aged aroma. This was achieved by refermentation of aged beers. It was shown that several aged aroma notes, such as cardboard, ribes, Maillard and Madeira, were removed almost entirely by brewing yeast, independently of the yeast or the beer type. This was explained by the reduction of aldehydes, mainly (E)-2-nonenal, Strecker aldehydes, 5-hydroxymethylfurfural and diacetyl, to their corresponding alcohols. Furthermore, it became evident that the reducing capacity of brewing yeast is high, but that yeast strain and compound specific residual concentrations remained in the refermented beer independently of the initial concentration. Finally, it appeared that aldehydes were not only reduced but also formed during refermentation.

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