Release of hydroxycinnamic acids and formation of flavour-active volatile phenols during the beer production process

Among the flavour-active volatile phenols in beer, most of them originate from the raw materials used in the brewing process. Only some of them can be formed by yeast activity, namely 4-vinylguaiacol (4VG) and 4-vinylphenol (4VP). The presence of these volatile phenolic compounds is considered undesirable when present in excessive concentration in bottom-fermented pilsner beers, hence the term “phenolic off-flavour” (POF). It is attributed to beers with a strong medicinal, clove-like aroma. Despite being historically catalogued as an off-flavour, these compounds are known to be essential flavour contributors to the characteristic aroma of Belgian white beers (made with unmalted wheat), German Weizen beers (made with malted wheat) and Rauch beers. In recent years, volatile phenolic flavour compounds have been increasingly encountered in Belgian specialty beers. While some brewers wish for a clear phenolic note in (some of) their beers, others do not aim for their, sometimes overwhelming, presence in the taste pallet of beer. Little is known of the dynamics behind the release of their precursors during brewing, the interactive role of native barley enzymes underlying this release and the subsequent decarboxylation to the flavour-active compounds during wort fermentation. Also the causes of the temperature dependent decrease of 4VG during beer conservation need to be elucidated. The suitability of a simple and rapid isocratic RP-HPLC method with amperometric detection for the simultaneous detection and quantification of hydroxycinnamic acids and their corresponding aroma-active volatile phenols in wort and beer is developed and validated. The method was used to perform an extensive survey on the occurrence of hydroxycinnamic acids (HCA) and volatile phenols in a range of beer styles. Odour and flavour thresholds of 4VG determined in a diverse range of beer styles confirmed the contribution of 4VG to the overall flavour perception of many top-fermented specialty beers. HCA and volatile phenols are monitored through out the beer production process to identify realistic control points for the final volatile phenol level in beer. A large variability in HCA content between different barley malt varieties and their corresponding worts was observed. Differences were also found between free FA levels from identical malt varieties originating from different malt houses. This demonstrates the importance of selecting a suitable malt variety. It was shown that only a small part of the HCA in malt is transferred to wort during mashing, while the lion share remains in the spent grains. Free HCA in wort are both water-extracted and enzymatically released by cinnamoyl esterase activity. Esterase activities clearly differ between different barley malt varieties, as do other AX-degrading enzyme activities. The release of ferulic acid during mashing did not only depend on the esterase activity, but also on the amount of esterbound ferulic acid initially present in the wort and on the endoxylanase activity. Apart from the choice of a suitable barley malt variety, final HCA concentrations in wort are also seriously affected by brew house operations. A clear difference in temperature and pH dependence between the release of the water-extracted and enzymatically hydrolysed fraction was found. In contrast to the water-extracted fraction, the hydrolysis of esterbound FA is subject to close technological control. To contribute to the odour and taste pallet of specialty beers, HCA have to be decarboxylated to the corresponding volatile phenols. Concerning thermal decarboxylation in pilsner beer, the combined time of wort boiling, transfer, whirlpool and pasteurisation times can give rise to the 4VG concentrations observed in the survey. However, the high concentrations often encountered in blond and dark specialty beers must originate from the enzymatic decarboxylation of HCA bystatus: publishe