## 3 Polyphenols Effects on VOCs Release  
Different studies have investigated the effects of polyphenols on wine odor. In most of the studies, static or dynamic HS-SPME/GC-MS or FID analyses have been applied to investigate aroma release from the matrix. Some of them have combined chemical and sensory experiments to study and compare the two effects, while only a few studies performed GC-O (Gas Chromatography-Olfactometry) analysis, and no data regarding the use of electronic nose are reported in the literature.  
The different analytical methodologies were applied to model solutions and real wines with significant compositional differences or deodorized/reconstituted wines.  
These different approaches, notwithstanding their advantages and drawbacks, tested the behavior of several VOCs belonging to different chemical classes. Different results, sometimes contradictory and difficult to interpret, have been reported. To compare these results, they were summarized in Table 1, which was organized listing VOCs belonging to the same chemical class accordingly to their increasing hydrophobicity expressed as \(\text{log}\text{P}_{\text{octanol/water}}\):  
Considering that ethanol can affect PPhs structure/colloidal state and solubility, as well as VOCs solubility, release, and perception, in Table 1, we have also reported its concentration used in the studies. However, since in all relevant studies, ethanol levels ranged between 10 and 12% (_v/o_), it is difficult to hypothesize a significant effect on both PPhs and VOCs chemical characteristics. In the literature, differences in PPhs particle size and colloidal state [54,55], as well as in VOCs solubility, release, and sensory perception have always been observed at different ethanol levels when higher than 2% (_v/o_) [56], and references there in.  
As further information, in Table 1, we also specified the type of matrix as well as the nature and/or the content of PPhs that were tested in the different studies to consider different wine systems. Grape tannins and polyphenols extracted from grape skins or seeds are characterized by different properties. The highest concentration of tannins in grape berry derives from grape skins, which differ from seed tannins in terms of polymerization degree (DP) and amount of gallates [57]. The average DP for skin tannins is higher than the average DP for seed tannins, which tend to be in monomeric form rather than polymerized. In addition, real wines (i.e., white, young red, and old red wines) are widely different in terms of polyphenic characteristics. They differ in terms of total phenolic content, which can vary from around 200 mg/1 of gallic acid equivalents (GAEs) in white wines to 2000 mg/L in young-reds and 3500 mg/L or more in aged-red wines [10]. Moreover, older red wines are normally characterized by a decrease of several low molecular weight phenolic compounds and anthocyanins and higher concentrations of polymeric pigments, while younger wines have higher concentrations of anthocyanins and other phenolic compounds [58, 59, 60].  
Considering all the described frameworks, in the following paragraphs, we attempted to link and critically discuss the observed effects of polyphenols on VOCs release.