Yeast volatile 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). 

Grando, M. S., Versini, G., NicoUni, G., Mattivi, F. (1993) Selective use of wine yeast strains having different volatile phenols production. Vitis, 32, 43-50. 

Since yeast lees may adsorb some aroma compounds responsible of off-flavours in wines (volatile phenols), these components have been also proposed such as a cost-effective and efficient approach to remove or to decrease organoleptic defects in wine (Chassagne et al. 2005). 

Chassagne, D., Guilloux-Benatier, M., Alexandre, H., Voilley, A. (2005). Sorption of wine volatile phenols by yeast lees. Food Chem, 91, 39-44. 

Precursors. Both hydroxycinnamic acids and 4-vinylphenols can lead to the formation of hydroxyphenyl-pyranoanthocyanins. The main hydroxycinnamic acids present in wines are p-coumaric, caffeic, ferulic and sinapic acids. 4-Vinylphenol and 4-vinylguaiacol are volatile phenols associated with off flavors in wine (Eti6vant 1981) and arise from the decarboxylation of p-coumaric and ferulic acid, respectively, via the yeast cinnamate decarboxylase (CD) (Chatonnet et al. 1993). 

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. 

Volatiles phenols (VP) are secondary metabolites produced by yeasts, moulds and bacteria which affect the flavour of several fermented food commodities (Loureiro and Malfeito-Ferreira 2006). These molecules have been under study since the first detection in fermented grains (Steinke and Paulson, 1964). Later, Dubois and Brule (1970) reported their presence in wines and presently the importance of VP is mainly due to their role in the mediatic horse sweat taint in red wines. Available toxicological data suggest that VPs do not warrant concerns about acute or longterm effects (Rayne and Eggers 2007b). 

In juices, prevention should be based on (i) decreasing the release of free acids that is favoured by mould infections of grapes and by the decarboxylase activity of commercial enzyme preparations and (ii) avoiding the production of volatile phenols that is favoured by the uncontrolled activity of contamination yeasts growing in damaged grapes or in juices. Then, the main measures to be adopted are ... 

Heresztyn, T. (1986a). Metabolism of volatile phenolic compounds from hidroxycinnamic acids by Brettanomyces yeast. Arch. Microbiol., 146, 96-98. 

As early as 1964 it was recognized that 4-ethyl phenol and 4-ethyl guaiacol were produced by yeast and bacteria during fermentation by the decarboxylation of the hydroxyciimamic acids p-coumaric and fendic acid (88). Later it was reported that among yeast only Brettanomyces species possess the metabolic ability to enzymatically decarboxylate hydroxycinnamic acids to produce ethyl derivatives (29, 89). Heresztyn was the first to identify 4-ethyl phenol and 4t-ethyl guaiacol as the major volatile phenolic compounds formed by Brettanomyces yeast (84). ... 

Isovaleric acid (3-methyl butanoic acid) was found to be the dominant odorant in the "high Brett" wine as detected by CharmAnalysis. The odor described by the GCO sniffer was rancid the chemical identity of the odorant was confirmed by GC-MS. This acid is produced in wine by yeast as a metabolic byproduct of protein (99). Volatile phenolic compounds, such as 4-ethyl guaiacol, guaiacol, and 4-ethyl phenol, were also among the dominate odor active compounds in this wine however, the individual contribution by each of the three phenolics was half or less than the odor activity of isovaleric acid. 

Volatile phenols originate from hydroxycinnamic acids (ferulic, p-coumaric, or caffeic acid) by the action of hydroxycinnamate decarboxylase enzyme, which turn the hydroxycinnamics acid into vinylphe-nols (Albagnac, 1975 Grando et al., 1993). Then, these compounds are reduced to ethyl derivatives by vinylphenol reductase enzymes characteristic of species, such as Dekkera bruxellensis, Dekkera anomala, Pichia guillermondii, Candida versatilis, Candida halophila, and Candida mannitofaciens (Edlin et al., 1995 1998 Dias et al., 2003 Chatonnet et al., 1992 1995 1997 Dias et al., 2003), apart from very small quantities produced by some yeasts and lactic acid bacteria under peculiar growth conditions (Chatonnet et al., 1995 Barata et al., 2006 ... 

Along with changes in anthocyanin and tanin proportions, winemaking practices can bring additional components in wines through selection of fermentation yeasts. Enzymatic activities of particular yeast strain influence the release of volatile phenols, pyruvic acid and acetaldehyde and, therefore, control the relative proportions of the corresponding pyranoanthocyanins. 

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 cinnamate decarboxylase (CD) of Saccharomyces cerevisiae is highly specific. These yeasts are incapable of converting benzoic acids into volatile phenols. Only certain acids in the cinnamic series (phenyl-propenoic acids) may be decarboxylated by this microorganism. Among the cinnamic acids in grapes, only ferulic and p-coumaric acids are affected by the CD activity. Caffeic (4,5-dihydroxycinnamic) and sinapic (4-hydroxy-3,5-dimethoxycinnamic) acids are not decarboxylated by S. cerevisiae. Cinnamic acid and... 

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... 

Table 8.6. Comparative study of the synthesis of volatile phenols by lactic bacteria and yeasts in a model medium supplemented with hydroxycinnamic acids (5 mg/1). Culture at 25°C for 2 weeks under anaerobic conditions (Chatonnet et al., 1995)...

Some phenolic acids like caffeic acid, p-coumaric acid and ferulic acid can act as precursors of volatile phenols, which could contribute positively to wine aroma, when they are present at low concentrations associated descriptors are smoky, dove-like and leather (Table 1). Yeasts can conduct the decarboxylation of phenolic adds to volatile phenols, as well as esterase activities present in enzymatic preparations used in winemaking. During wine storage and ageing, volatile phenols may be further transformed. 

Besides the yeast conversion of phenolic adds into flavour-active volatile phenols that may have an adverse impact on beer aroma as discussed above, there is an interest in the contribution of phenolic acids to beer antioxidant activity (Piazzon et al 2010). Indeed, the use of hydrolytic enzymes (e.g. esterases) to release phenolic adds during mashing has been reported (Szwajgier, 2011). It must be cautioned that boosting the level of phenohc acids snch as fendic acid in the wort may inaease the content of volatile phenols in the finished product, and a flavour defect may ensue. 

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. 

Due to the labor-intensive nature of monitoring programs, alternatives have been sought. Enzyme-linked immunosorbent assay (ELISA) has been proposed (Kuniyuki et al., 1984). The advantage of ELISA is that viable yeasts need not be present. Unfortunately, the method is too sensitive for routine production applications and, at present, too costly. The volatile phenol, 4-ethylphenol, has been proposed as a marker for present/past growth of Brettanomyces and Dekkera in wine. For details regarding analysis, see Zoecklein et al. (1995). 

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. 

According to Ciani and Ferraro (1997), the lack of NAD under anero-bic conditions is not restored by glycerol production from dihydroxyace-tone phosphate (Fig. 1.8). In fact, glycolytic pathways are temporarily inhibited in Dekkera/Brettanomyces when these yeasts are introduced into an anaerobic environment (Wijsman et ak, 1984). Because of the lack of NAD, Dekkera/Brettanomyces WAX conduct a limited alcoholic fermentation with the production of primarily ethanol, not acetic acid (Ciani and Ferraro, 1997). Biochemically, another source of NAD during growth of these yeasts in red wines may be the formation of volatile phenols (Section 11.2.2). 

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