*Phenolic Compounds of Grapes and Wines: Key Compounds and Implications in Sensory… DOI: http://dx.doi.org/10.5772/intechopen.93127*

capability of binding with proteins. The mechanisms of tannin-protein interactions involved different types of interaction such as the hydrophobic interactions, which are the predominant mechanisms involving the hydrophobic nature of the condensed tannin carbon skeleton and the apolar regions of the proteins. Together with hydrophobic interactions, some hydrogen interactions also occur between the carbonyl function of proline and the hydroxyl functions of phenols as well as some ionic interactions. This mechanism and thus the final astringency of wine are influenced by many factors, such as the structure and amount of condensed tannins in the wine as well as the composition of the wine matrix [95]. Indeed, the intensity and the quality of the astringency perception are influenced by the concentration of condensed tannins [96], their degree of polymerization [97], their percentage of galloylation [98], and prodelphinidins [99]. The matrix, on the other hand, impacts the perception according to its acidity [100], its ethanol concentration [101], and the presence of macromolecules such as polysaccharides. Fontoin and co-workers demonstrated that the astringency sensation of grape-seed oligomer tannins decreased with increasing pH and the percentage of ethanol [100]. For example, Cabernet Sauvignon wines having a mDP between 2.0 and 4.0 were characterized as mellow and slight astringent. Meanwhile, a rougher sensation (tannic) was perceived for wines with a mDP higher than 4.0. [102]. The analytical determination of the proanthocyanidin content and the type of subunit that is dominant in tannin chains might be a valuable tool for astringency estimation during wine aging [103]. Astringency intensity is influenced by the source of proanthocyanidin (seed or skin) and by well-defined proanthocyanidin fractions (oligomeric or polymeric). Polymeric seed fraction was perceived more astringent than the monomeric/oligomeric one, whereas polymeric skin fraction was characterized less tannic than the monomeric/oligomeric skin fraction, indicating a negative relationship between astringency intensity and % of prodelphinidins [104]. The presence of epigallocatechin units in the proanthocyanidins has been shown to lower the "coarse" perception through the increase of the degree of B-ring trihydroxylation. Furthermore, seed fraction with a higher proportion of galloyl group and a lower mDP was perceived to be bitterer than the skin fraction [105]. Both the interflavanoid bonds and stereochemistry of subunits impact bitterness sensation: catechin-(4–6)-catechin dimer was bitterer than both catechin-(4–8)-catechin and catechin-(4–8)-epicatechin [91].

Moreover, astringency sensation perceived always reduces with the increasing saliva volume flow rate. A linear correlation was found between protein concentration and tannin-binding affinity. The saliva proteins, including PRP family (acidic, basic, and glycosylated), α-amylase, statherin, histatins, and mucins, show diversified ability to interact with tannins [106]. Some proteins are specifically bound to astringents. For instance, tannins and alum precipitated PRPs, while acid and alum precipitated mucins [107, 108].

Ellagitannins (hydrolyzable tannins) impart an oral sensation described as astringent at relatively low threshold concentrations spanning from 0.2 to 6.3 mmols by means of the half-tongue test in bottled water (pH 4.5). Due to their lower astringent taste thresholds, hydrolyzable tannins are usually perceived as more astringent than condensed tannins (1.1 μM for both castalagin and vescalagin, compared to 410 μM for catechin, 930 μM for epicatechin, 240 μM for procyanidin B1, 190 μM for procyanidin B2, and 200 μM for procyanidin B3) [66, 69, 109]. Among ellagitannins, the monomers grandinin and roburin E exhibited an astringent mouthfeel at the lowest taste thresholds (0.2 μM), whereas values for dimers ranged between 2.9 and 6.3 μM. Thus, the *C*-glycation of castalagin and vescalagin seems to favor the astringent sensation, while dimerization seems to reduce it [66].

*Chemistry and Biochemistry of Winemaking, Wine Stabilization and Aging*

and a stabilizing effect on color [77].

and benzoic acid derivatives [91].

sition and astringency during bottle aging [87].

**5. Sensory impact of phenolic compounds in wine**

the taste of bitterness are related to phenolic compounds.

part in oxidation reactions that may favor the polymerization reactions between flavanols and between flavanols and anthocyanins. Furthermore, they can directly react with these types of compounds giving rise to flavano-ellagitannins or anthocyano-ellagitannins [85]. The formation of flavano-ellagitannins and the *β*-1-*O*-ethylvescalagin in red wines aged in oak barrels has been reported. The ellagitannin concentrations fluctuated between 4 and 8 mg/L, being castalagin the ellagitannin with the highest concentration, followed by mongolicain A [86]. As a consequence, ellagitannins can modulate wine astringency and color through interactions with these compounds. Strong correlations have been observed between ellagitannin concentration and both antioxidant capacity and astringency sensation [5, 63, 78]. The amount of ellagitannins released into the wine depends on the content in the oak wood barrel, which in turn is dependent on several factors (Section 3). For instance, after 12-month aging with woods, the total ellagitannin level revealed a large diversity of concentrations ranging from 6.3 to 26.1 mg of ellagic acid/L wine. The wine with heavy toast woods and the wine with low toast woods presented, respectively, the less and the highest ellagitannin concentrations [78]. Storage with oak can also cause a decrease in anthocyanins, catechin, and epicatechin but an increase in total phenolic content

Besides the winemaking process, and oak wood aging, wine can be further exposed to oxygen during aging in the bottle, depending on the oxygen permeability of the closure. Because of the extremely low rates of oxygen ingress through a closure, this form of oxygen exposure has been referred to as nano-oxygenation. Oxygen transmission rates (OTR) of wine closures may vary widely depending on closure type and strongly influence the evolution of white and red phenolic compo-

"In-mouth" sensory properties of red wines encompass multiple interacting sensations such as acidity, sweetness, bitterness, retronasal aroma perception (flavor), viscosity, warmth, and astringency. Among these, the sensation of astringency and

Bitterness perception is a taste recognition mediated by taste buds present in the tongue papillae. Each taste bud, consisting of approximately 50–100 taste receptor cells, is innervated by multiple taste fibers that transmit nervous signals to the brain [88]. In humans, each bitter receptor cell contains approximately 25 bitter G protein-coupled receptors encoded by a TAS2R gene family. It was evidenced by Soares and colleagues [89] that different phenolic compounds activate distinguished combination of TAS2Rs: epicatechin stimulated three receptors whereas procyanidin trimer only one. In general, in wines containing a large number of polyphenols, the taste of bitterness is attributed to flavan-3-ols and their polymers, although it is also able to be elicited by some flavonols [90], hydroxycinnamates,

While bitterness is a gustatory sense recognized by nervous signals, astringency is an oral sensation involving dryness and puckering [92]. According to the American Chemical Society, astringency refers to "the complex of sensations due to shrinking, drawing or puckering of the epithelium as a result of exposure to substances such as alums or tannins" [93]. It has been classically postulated that tannins possess the ability to interact with salivary proteins, with or without precipitation of the corresponding complexes [94]. In fact, the name of "tannins," originating from "tanning" of leather, has been used over decades for their

**14**

When the same concentration of ellagitannins and skin and seed tannins was tested, the ellagitannins were perceived softer [104].

Interaction of ellagitannins with salivary proteins has been poorly investigated up to now, probably because of their lower wine content compared to condensed tannins. Even if perceived as more astringent, ellagitannins have been noted as poorer protein precipitants than condensed tannins [94]. Soares and co-workers [110] stated that ellagitannins act as multidentate ligands cross-linking different salivary protein units, via their galloyl moieties. It is noteworthy to mention that these units are responsible for the antioxidant ability of hydrolyzable tannins; thus when complexed with salivary proteins, the antioxidant capacity of ellagitannins may be significantly impaired. At higher concentration levels, the main eight oak ellagitannins have also been observed to provide the wine with a bitter taste [66].

Apart from tannins, other polyphenolic compounds present in wine have been related with the overall perception of astringency sensation or bitterness. Very recently, some works have provided evidence about the interaction of anthocyanins and pyranoanthocyanins with salivary proteins. Indeed, malvidin-3-*O*-glucoside, the major anthocyanin of wine, has demonstrated to interact with acidic prolinerich proteins (aPRPs) showing dissociation constants (KD) calculated by NMR of 1.88 mM [111] that can be compared to those obtained for procyanidins (dimers B1–4 and trimer C2) (between 0.4 and 8 mM) [112]. In addition, Paissoni and colleagues [113] tested the interaction with saliva proteins of the three representative of wine anthocyanins (glucosides, acetylated, and cinnamoylated) proving that cinnamoylated anthocyanins are the most reactive and also those that present the lowest perception threshold in wine model solutions. More recently, another work showed that pyranoanthocyanins (pyranomalvidin-3-glucoside, pyranolmalvidin-3-glucoside-catechol, and pyranomalvidin-3-glucoside-epicatechin) can also able to interact aPRPs with KD even lower (more affinity) than for anthocyanins (between 0.87 and 1.73 mM) [114].

Concerning bitter taste, malvidin-3-*O*-glucoside has also demonstrated to stimulate one member of the bitterness receptor family (TAS2R7) at micromolar levels (12.6 μM) [89]. With regard to flavanols, the addition of quercetin-3-*O*-glucoside (0.25–2 g/L) to white and red wines resulted in a noticeable increase of astringency and bitterness evaluated by sensory analysis. In general, wines were described as smooth-tasting before the flavonol addition and became more astringent, rough, green, dry, bitter, and persistent in presence of quercetin-3-*O*-glucoside [33].
