White wines vinyl-phenol concentrations

Fig. 8.4. Comparison of (a) vinyl-phenol concentrations in white wines and (b) the ethyl-phenol content in red wines (Chatonnet et al, 1992b, 1993b)...

Parameters on the Vinyl-Phenol Concentrations of White Wines... 

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

A considerable decrease in vinyl-phenol concentration, and the resnlting olfactory impact, occurs as white wine ages, especially dnring bottle aging. This is mainly dne to radical polymerization of the vinyl-phenols into odorless polyvinyl-phenols (Klaren De Witt et al., 1971). Vinyl-phenols may also be converted into ethoxy-ethyl-phenols with little odor by an addition reaction of ethanol in an acid medinm (Dugelay et al., 1995). 

The perception threshold of an odoriferous compound is conventionally considered to be the minimum concentration at which its presence in a model dilute alcohol solution is detectable by 50% of trained tasters. The recognition threshold of an odoriferous compound corresponds to its perception threshold in wine. The preference threshold of a compound is the concentration above which the overall aroma of a wine is affected. In the case of vinyl- and ethyl-phenols, the preference thresholds have been estimated at 720 p.g/1 for a 1/1 mixture of vinyl-4-phenol and vinyl-4-guaiacol in white wines and 420 fig/l for a 10/1 mixture of ethyl-4-phenol and ethyl-4-guaiacol in red wines. These values are relatively close to the recognition thresholds (in wine) for the same mixtures of vinyl- and ethyl-phenols. 

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... 

Other strains, like VLl, were selected for their low vinyl-phenol production. These compounds possess rather unpleasant pharmaceutical aromas. Above a certain concentration, they dull the aroma of dry white wines (Volume 2, Sections 8.4.2 and 8.4.3). These strains have low cinnamate decarboxylase activity. During alcoholic fermentation, this enzyme catalyzes the partial transformation of p-coumaric and ferulic acid found in juice into vinyl-4-phenol and vinyl-4-gaiacol. Since this enzyme is inhibited by phenolic compounds, only white wines can contain quantities of vinyl-phenols likely to affect their aroma. The use of strains with low cinnamate decarboxylase activity is recommended— particularly for white juices containing high concentrations of hydroxy cinnamic acid. 

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