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Production of phenolic compounds

One of the major spoilage characteristics of Saccharomyces beer-spoiling yeasts and some non- Saccharomyces species (including Brettanomyces), is the production of phenolic compounds during fermentation. Although these represent desirable flavour attributes in many wheat beers, in the majority of instances the presence of these compounds signifies a serious quality defect. In Saccharomyces yeasts, the production of phenolic compounds is largely influenced by the PAD1 gene (often referred to as the phenolic-off-flavour or POF1 gene) which encodes for phenylacrylic acid decarboxylase (Clausen et al., 1994), while in Brettanomyces a similar function is performed by dbPAD (Godoy et al., 2014). This essentially regulates the decarboxylation of hydroxycinnamic acids such as ferulic acid, p-coumaric acid, and cinnamic acid to create 4-vinylguaiacol (4-VG), 4-vinylphenol (4-VP), and styrene, respectively (Schwarz et al., 2012). These impart characteristic phenolic (4-VG), clove-like (4-VP), and plastic aromas (styrene) to the beer. At the cellular level, the role of these enzymes is not fully understood. However, it is interesting that they may function to detoxify the cell, since overexpression of PAD1 in S. cerevisiae results in enhanced growth rate and ethanol productivity in the presence of the hydroxycinnamic acids (Larsson et al., 2001). In Brettanomyces species, vinylphenol reductase (VPR) further converts 4-VG and 4-VP to 4-ethylguaiacol and 4-ethylphenol (Heresztyn, 1986), which impart the smoky, leathery, medicinal, and clove like flavours often summarized as being ‘barnyard' or simply ‘Brett' characteristics. It has been argued that the physiological impact of VPR is to contribute towards redox balance within the cell, since NADH is used as a cofactor in the reduction of 4-VG and 4-VP to their ethyl derivatives. This hypothesis is supported by observations that VPR activity is enhanced under oxygen-limited conditions, increasing NAD+ availability (Curtin et al., 2013).

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