How process and raw materials shape the flavour instability of German wheat beer: an exploratory study

Flavour instability is a critical quality attribute of beer, as consumers expect a consistent sensory profile throughout shelf life. Even small changes in aroma-active compounds can markedly reduce product acceptance, making the control of beer ageing a central technological challenge for brewers. While lager beer stability has been extensively studied, much less is known about German wheat beer, despite its high market relevance and unique composition. In this study, 25 commercial wheat beers were examined in forced and natural ageing up to nine months. Analytical focus was set on esters, volatile phenols, and carbonyls, complemented by sensory evaluation using trained panels over the course of ageing. In fresh beers various other parameters including precursors for ageing compounds were assessed.Principal component analysis confirmed aldehydes as the main drivers of ageing, with Strecker aldehydes (3-methylbutanal, 2-methylpropanal, methional) and furfural showing the highest increase. In contrast, yeast-derived esters (isoamyl acetate, ethyl acetate) proved largely stable, while volatile phenols (4-vinylguaiacol, 4-vinylphenol) showed continuous degradation. Sensory deterioration was reflected in increasing descriptors such as buttermilk, sherry, sweetish and sour, while cardboard remained negligible. Technological parameters influenced the course of ageing: yeast strain, pitching rate, fermentation method and flash pasteurization showed significant impact. Raw material factors, such as wheat malt proportion and specialty malt use, showed moderate, non-significant impact on flavour instability. Overall, German wheat beer displayed a high variability of ageing behaviour across breweries, underlining the complex interaction of matrix composition, yeast activity, and process choices.