Ethyl-phenol Fermentation

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). 

Brettiness". "Volatile phenols can be considered as natural components in wines and beer, or as spoilage compounds when present in excessive amounts" (84). Brettanomyces species can produce both mousiness and, at low concentrations of volatile phenolics, also "a distinct aroma described variously as cider-like, spicy, dove-like or phenolic... formed toward the end of fermentation" (84). The ethyl phenols produced can exceed the sensory threshold 16-fold (85), producing wines, at high concentrations of volatile phenolics, with distinct "barnyard", "stable", and "animal" phenolic odors (85-87). Wines with high concentrations of phenolic odors are deemed "Bretty" by tasters. 

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). ... 

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. 

For many years (Dnbois, 1983), ethyl-phenols were thought to result from the metabolism of lactic bacteria. However, it was never possible to link their appearance to the completion of malolactic fermentation or to the storage of wines in the presence of lees containing bacteria (Di Stefano, 1985 Chatonnet et al, 1992b). 

Vanilla flavour is not only determined and characterised by the vanillin molecule, but also by many more phenolic compounds and vanillin derivatives. Two examples of molecules that recently obtained FEMA-GRAS status are vanillyl ethyl ether and vanillin 2,3-butanediol acetal (Scheme 13.11). Vanillin can be hydrogenated to form vanillyl alcohol, which is also used in vanilla flavours. Vanillyl alcohol can be reacted with ethanol to form vanillyl ethyl ether. Vanillin can also form an acetal with 2,3-butanediol (obtained by fermentation of sugars) catalysed byp-toluene sulfonic acid in toluene. 

Tyrosol (Figure 6.5) or j9-hydroxy-phenyl-ethyl alcohol may be included in this group of compounds (Ribereau-Gayon and Sapis, 1965). It is always present in both red and white wine (20-30 mg/1) and is formed during alcoholic fermentation from tyrosine (j -hydroxyphenyl-alanine), in turn synthesized by yeast. This compound, which remains at relatively constant concentrations throughout aging, is accompanied by other non-phenolic alcohols like tryptophol (0-1 mg/1) and phenyl-ethyl alcohol (10-75 mg/1). 

Conditions Vinyl-4-phenol (fig/1) Ethyl-4-phenol (figd) Fermentation time (days) Inhibition (%)... 

Heresztyn (1986) reports that fermentations utilizing B, intermedius and B, anomalus produced substantial amounts of the volatile phenols 4-ethyl... 

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