Studies on the Simultaneous Formation of Aroma-Active and Toxicologically Relevant Vinyl Aromatics from Free Phenolic Acids during Wheat Beer Brewing

During the brewing process of wheat beer, the desired aroma-active vinyl aromatics 2-methoxy-4-vinylphenol and 4-vinylphenol as well as the undesired and toxicologically relevant styrene are formed from their respective precursors, free ferulic acid, p-coumaric acid, and cinnamic acid, deriving from the malts. Analysis of eight commercial wheat beers revealed high concentrations of 2-methoxy-4-vinylphenol and 4-vinylphenol always in parallel with high concentrations of styrene or low concentrations of the odorants in parallel with low styrene concentrations, suggesting a similar pathway. To better understand the formation of these vinyl aromatics, each process step of wheat beer brewing and the use of different strains of Saccharomyces cerevisiae were evaluated. During wort boiling, only a moderate decarboxylation of free phenolic acids and formation of desired and undesired vinyl aromatics were monitored due to the thermal treatment. In contrast, this reaction mainly occurred enzymatically catalyzed during fermentation with S. cerevisiae strain W68 with normal Por activity (phenolic off-flavor) resulting in a wheat beer eliciting the typical aroma requested by consumers due to high concentrations of 2-methoxy-4-vinylphenol (1790 mu g/L) and 4-vinylphenol (937 mu g/L). Unfortunately, also a high concentration of undesired styrene (28.3 mu g/L) was observed. Using a special S. cerevisiae strain without Por activity resulted in a significant styrene reduction (<LoQ), but also in low amounts of 2-methoxy-4-vinylphenol (158 mu g/L) and 4-vinylphenol (46.7 mu g/L), resulting in a less pronounced wheat beer aroma.