## 3 Polyphenols Effects on VOCs Release
### Effects on Volatile Phenols  
Volatile phenols are a family of volatiles that comprise (i) volatile phenols formed during the fermentation process and released from grape-derived glycosides, (ii) volatile phenols formed during the fermentation process by the metabolism of hydroxycinnamic acids, precisely by yeasts of the genus _Brettanomyces/Dekera_, through the decarboxylation of _trans_ ferulic and _trans_\(p\)-coumaric acid, and (iii) volatile phenols extracted when storing wine in contact with toasted oak wood [70; 71; 72]. While some of them contribute positively to wines aroma complexity (i.e., guaiacol and eugenol), others (i.e., 4-ethylphenol, 4-vinylphenol, 4-ethylguaiacol, and 4-vinylguaiacol) might be involved in the appearance of unpleasant notes. In particular, while low concentrations of volatile phenols extracted when storing wine in contact with toasted oak wood by the metabolism of yeasts of the genus _Brettanomyces/Dekera_ (i.e., 4-ethylphenol and 4-ethylguaiacol) can contribute to wine aroma complexity, high concentrations of these two VOCs are indicative of Brett character, which is one of the most widespread red wine defects. However, depending on consumers' expectation of a particular wine, the presence of Brett character can be considered either negative or positive. For example, ethylphenols can be found at concentrations much higher than their detection thresholds in certain very expensive French red wines, where they may be considered part of the wine's style, as an expression of terrori and part of nature, rather than a fault [10]. Consequently, it is important for winemakers to manage the increase and/or the production of these VOCs and to understand which conditions favor their perception or otherwise. For this reason, the influence of polyphenols on the release of different volatile phenols (\(1.32\leq\text{log}P\leq 2.61\)) and in some cases on their sensory perception has been evaluated both in model solutions and real wines (Table 1). Except for guaiacol and eugenol, which are characterized by the lowest log\(P\) values and better volatilized in the presence of grape tannins at 0.5-1.5 g/L [69], the release of volatile phenols was essentially reduced by PPhs. Important results regarding the effects of polyphenols on the two volatile phenols 4-ethylphenol and 4-ethylguaiacol in model solutions have been understood from a recent study [73]. The authors showed that at increasing polyphenols concentration, a significant and linear decrease in the volatility of these two VOCs has been observed due to \(\pi\)-\(\pi\) interactions. Additionally, performing sensory tests, they showed that the unpleasant and characteristic \"phenolic\" taint, due to the presence of 4-ethylphenols [70; 74; 75], has been significantly higher in the trials with lower polyphenol content, highlighting a consistent and significant masking effect of polyphenols on the perception of the Brett character. This result may be of great interest in winemaking, since controlling the concentration and the sensory impact of these compounds in wine is an always current topic in geology. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \multicolumn{4}{c}{**Aromas Characteristics**} & \multicolumn{4}{c}{**Matrix Characteristics**} & \multicolumn{2}{c}{**Effects**} & \multicolumn{1}{c}{**Ref. **} \\ \hline
**Compound** & **Descriptors* Concentration** & **logP(a/w)* & **Type of Matrix** & **\%Ethanol** & **Added** & **Tannins** & **Effects on** & **Effects  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multicolumn{4}{c}{**Aromas Characteristics**} & \multicolumn{4}{c}{**Matrix Characteristics**} & \multicolumn{4}{c}{**Effects**} & \multicolumn{2}{c}{**Ref. **} \\ \hline
**Compound** & **Descriptors* Concentration** & **logP(a/w)* & **Type of Matrix** & \%Ethanol (p/e) & **Added** **Tannins** & **Tannin** & **Effects on** & **Effects on** & **Orthonasal** \\ \hline _ESTERS_ & & & & & & Skin & 0.5-3 g/L & \(\downarrow\) & \\
**Diethyl succinate** &