Yeasts vinyl phenols

Grape compounds which can enter the yeast cell either by diffusion of the undissociated lipophilic molecule or by carrier-mediated transport of the charged molecule across the cell membrane are potentially subject to biochemical transformations by enzymatic functions. A variety of biotransformation reactions of grape compounds that have flavour significance are known. One of the earlier studied biotransformations in yeast relates to the formation of volatile phenols from phenolic acids (Thurston and Tubb 1981). Grapes contain hydroxycinnamic acids, which are non-oxidatively decarboxylated by phenyl acryl decarboxylase to the vinyl phenols (Chatonnet et al. 1993 Clausen et al. 1994). 

Grapes contain several hydroxycinnamic acids, p-coumaric, caffeic, ferulic and sinapic acids, which exist as free acids and esterified with tartaric acid. Saccha-romyces species can take up free acids to produce the corresponding vinyl phenol catalysed by hydroxycinnamate decarboxylase (phenylacrylic acid decarboxylase Padlp) (Fig 8D.11) (Chatonnet et al. 1992b Chatonnet et al. 1993 Edlin et al. 1995). Vinyl phenols are unstable and highly reactive. Dekkera bruxellensis is one of few wine microorganisms that can further reduce vinyl phenols to highly stable ethyl phenols in wine. Vinyl phenols can also react with anthocyanins to form vinyl derivatives, a reaction that is favoured by fermentation yeast having hydroxycinnamate decarboxylase activity (Morata et al. 2006). 

More potent yeasts responsible for spoilages of wines with volatile phenols are of Brettanomyces/Dekkerra type that can produce the vinyl phenols and ethyl phenols of hydroxyl cinnamic acids. Ribdreau-Gayon et al. (2000) give an overview on this topic. Another source of volatile phenols is the oak wood of barrel and oak chips. These phenols are characterized by smoked and toasted aromas. Important volatile phenols in wine are depicted in Fig. 9C.4. 

Finally, oxidation of hydroxycinnamoyl tartaric acid (HCTA) into hydroxycinnamic acids (HCA), and the successive decarboxylation operated by Saccharomyces yeasts, or reduction and decarboxylation by Brettanomyces yeasts, induce off-flavours due to vinyl phenols and ethyl phenols formation, respectively. 

Ethyl phenols are a result of enzymatic activities linked to the decarboxylation of cinnamic acids and the subsequent reduction in vinyl phenols caused by the Brettanomyces/Dekkera yeast genus (Chatonnet et al., 1992), apart from very small quantities produced in peculiar... 

Phenolic acids are colorless in a dilute alcohol solution, but they may become yellow due to oxidation. From an organoleptic standpoint, these compounds have no particular flavor or odor. They are, however, precursors of the volatile phenols produced by the action of certain microorganisms (yeasts in the genus Brettanomyces and bacteria) (Section 8.3). Ethyl phenols, with animal odors, and ethyl gaiacols are found in red wines (Figure 6.4). In white wines, vinyl phenols, with an odor reminiscent of gouache paint, are accompanied by vinyl gaiacols. It has been clearly established that these compounds result from the... 

The vinyl-phenols in white wines are formed due to enzymic decarboxylation by yeast of two cinnamic acids (p-coumaric acid and ferulic acid) in must, producing vinyl-4-phenol and vinyl-4-guaiacol, respectively (Figure 8.6). 

For many years, it was assumed that the low vinyl-phenol concentrations in red wines were due to the fact that, after their formation by yeast, they were converted into the corresponding ethyl-phenols by lactobacilli during malolacdc fermentation (Dubois, 1983). This interpretation is no longer accepted (Section 8.4.4). Indeed, it is now known that lactic bacteria are not involved in the prodnction of ethyl-phenols or vinyl-phenols in red wines. 

The vinyl-phenol content of a white wine depends on the concentration of phenol acid precnrsors in the must, on the one hand, and the CD activity of the yeast strain responsible for alcoholic fermentation, on the other hand. 

The yeast strain also plays an essential role in determining the volatile phenol concentration in white wines. For many years now in the brewing industry (Goodey and Tubb, 1982), yeast strains have been selected for their low production of vinyl-phenols, as malt has a high phenol acid content. These are called Pof- (phenol off-flavor) strains. The selection of winemaking yeast has... 

Fig. 8.9. Impact of the strain of winemaking yeast on the vinyl-phenol concentration of white wine (Chatonnet et al., 1993b)...

Dubourdieu (1992) points out that wine yeasts Saccharomyces cerevisiae also contains the cinnamate decarboxylase and thus are capable of producing the vinyl phenol intermediate. However, flavonoid phenols (tannins) inhibit its activity hence, the formation of volatile phenols in red and rose wines is significantly less than that seen in white wine fermentations. Activity of cinnamate decarboxylase in the case of Brettanomyces and Dekkera, however, is not inhibited by polymeric phenols. 

Interactions between LAB and phenolic compounds are very complicated. Among them, the decarboxylation of hydroxycinnamic acids (caffeic, p-coumaric, and ferulic) is of particular interest in oenology (Silva et al. 2011). These acids are esterified to tartaric acid and freed by the cinnamoyl esterase by yeasts and less often by bacteria. The decarboxylation releases vinyl phenols, which are reduced to ethyl phenols producing off-flavors. The second reaction can be achieved by bacteria to maintain the redox balance however, it is mainly carried out by the spoilage yeast Brettanomyces bruxellensis, often present in wine. 

Vanbeneden, N., Gils, R, Delvaux, R, Delvaux, R R. (2008). Formation of 4-vinyl and 4-ethyl derivatives from hydroxycinnamic acids occurrence of volatile phenolic flavour compounds in beer and distribution of padl-activity among brewing yeasts. Food Chemistry, 107, 221-230. http //dx.doi.Org/10.1016/j.foodchem.2007.08.008. 

Phenolic (general term) Herbal, cloves, medicinal. - - Wild yeast. See specific phenolic notes. eugenol/4-vinyl guaiacol, etc. 

Vinyl guaiacol (4-vinylguaiacol 4VG) Spicy, clove, herbal, phenolic. 0.05-0.55 0.3 A typical wheat beer note (Coghe, Benoot, Delvaux, Vanderhaegen, Delvaux, 2004). Wild yeasts or specialty yeasts. 

Wheat beers utilize raw or malted wheat, rather than malted barley, as their primary raw material base. Fermentation may be spontaneous (natural flora of the brewery, Belgian Lambic beers [see below]) or with a top fermentation yeast strain. South German wheat beers (weissbier) utilize a top fermenting yeast strain that produces a characteristic phenolic flavor (4-vinyl-guaiacol, dove-like see Table 18.3 Anderson et al., 2000 McMurrough et al., 1996 Stewart Russell, 1998 Vanbeneden, Gils, Delvaux, Delvaux, 2008). Certain wild yeasts can produce phenohc off-flavors in nonwheat beer styles and as such would be regarded as contaminants. 

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