*2.1.2 Flavan-3-ols and proanthocyanidins (oligomeric proanthocyanidins or condensed tannins)*

Flavan-3-ols are monomeric flavonoids formed by a benzopyran unit (rings A and C) with an aromatic cycle (ring B) linked to the carbon C-2 of the pyranic cycle (ring C). The presence of two chiral centers on the molecule (C2 and C3) gives rise to four possible configurations for a single monomer. These monomeric structures may be joined together forming dimers, oligomers (3–1o units of flavan-3-ols), and polymers (more than 1o units of flavan-3-ols). All these more complex structures are so-called condensed tannins. If they are formed by (+)-catechin and (−)-epicatechin and their gallic esters, they are named procyanidins, while when they are

**5**

**Figure 2.**

*Chemical structures of major grape phenolic compounds.*

*Phenolic Compounds of Grapes and Wines: Key Compounds and Implications in Sensory…*

*DOI: http://dx.doi.org/10.5772/intechopen.93127*

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

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

non-*V. vinifera* species or hybrid red grapes [25].

Blanc, Riesling, and Chardonnay) can contain measurable traces of anthocyanins [10]. Several factors can influence the anthocyanin biosynthesis in grapes such as origin and type of the grape vine, degree of maturity, and weather conditions like

Regarding total anthocyanins, their quantities vary between 11.47 and 29.83 g/kg of dry matter (dm) in red grape skins [12] (**Table 1**). The principal individual anthocyanins in *Vitis vinifera* cultivars are the 3-*O*-monoglucosides (glucose linked through glucosidic bonds at the C3 positions of C ring) of delphinidin, cyanidin, petunidin, peonidin, and malvidin (**Figure 2**). Among these, malvidin-3-*O*-glucoside is generally the most abundant with values of 4.12–10.19 g/kg dm [14]. More recently, He and co-workers demonstrated for the first time the presence of pelargonidin-3-*O*-glucoside at trace levels on berry skins of Cabernet Sauvignon and Pinot noir cultivars [20]. Moreover, the monoglucoside forms can be acylated at the C6″ position of the glucose moiety with both aromatic (*p*-coumaric, caffeic, ferulic, and sinapic acid) or aliphatic acids (acetic, malic, malonic, oxalic, and succinic acid). The most common acylated anthocyanins in *V. vinifera* grape includes 3-*O*-(6″-*p*-coumaroyl)-glucosides, 3-*O*-(6″-acetyl)-glucosides, and 3-*O*-(6-caffeoyl)-glucosides of delphinidin, petunidin, peonidin, and malvidin [8, 12, 14, 21]. To go further, even anthocyanin dimers (malvidin-3-*O*-glucoside dimer and malvidin-3-*O*-glucoside-peonidin-3-*O*-glucoside) have been identified in grape skins [8, 22]. The presence of these acetylated forms is important for the color stabilization and intensity of wines [23]. The color intensity increases with the number of substituted groups on the B ring (di-oxygenated forms are redder, while tri-oxygenated are more purple) and with the replacement of hydroxyl by methoxyl groups (i.e., malvidin has the darkest color). Moreover, methoxylated anthocyanins (malvidin and peonidin) are more stable than hydroxylated ones to environmental and viticultural factors [24]. Additionally, anthocyanins can be found as 3,5-*O*-diglucosides or acylated 3,5-*O*-diglucosides, which are considered as marker compounds of

In general, anthocyanin concentration is maximized under nonirrigated conditions in all cultivars, but anthocyanin profile and relative distribution of individual anthocyanins among irrigation treatments are influenced principally by the cultivar. In fact, Cabernet Sauvignon, Merlot, Syrah, and Tempranillo are characterized by a major proportion of malvidin forms, while in Nebbiolo (Italian cultivar) peonidin-3-*O*-glucoside is the most prevalent anthocyanin [11]. Other varieties, for example, Pinot noir, red Chardonnay, and pink Sultana (white red-colored mutants), are not able to synthetize acetylated anthocyanins [26]. In consequence, the anthocyanins profile in grapes can be used as an authentication tool of varietal

*2.1.2 Flavan-3-ols and proanthocyanidins (oligomeric proanthocyanidins or* 

Flavan-3-ols are monomeric flavonoids formed by a benzopyran unit (rings A and C) with an aromatic cycle (ring B) linked to the carbon C-2 of the pyranic cycle (ring C). The presence of two chiral centers on the molecule (C2 and C3) gives rise to four possible configurations for a single monomer. These monomeric structures may be joined together forming dimers, oligomers (3–1o units of flavan-3-ols), and polymers (more than 1o units of flavan-3-ols). All these more complex structures are so-called condensed tannins. If they are formed by (+)-catechin and (−)-epicatechin and their gallic esters, they are named procyanidins, while when they are

temperature, water availability, or the light exposure and intensity [11].

**4**

wines [27].

*condensed tannins)*




**Figure 2.**

*Chemical structures of major grape phenolic compounds.*

constituted by (+)-gallocatechin and (−)-epigallocatechin and their galloylated derivatives, the term used is prodelphinidins [28].

They are located in all grape clusters solid parts (skins, seeds, stalks) and are responsible for the stabilization of wines' both color and sensory characteristics due to their astringent and bitter properties [29]. Five monomeric flavan-3-ols are commonly present in grapes (**Figure 2**): (+)-catechin and its stereoisomer (−)-epicatechin as the predominant ones in seeds (2.14–2.44 g/kg dm and 0.88–1.60 g/kg dm, respectively) (**Table 1**) (+)-gallocatechin, (−)-epigallocatechin, and (+)-catechin-3-*O*-gallate [13, 15–19].

As explained above, condensed tannins are oligomers of flavan-3-ol monomer units. These units can be linked by C-4 → C-6 or C-4 → C-8 bonds, so-called B-type proanthocyanidins. A-type condensed tannins are characterized by the presence of a second interflavonoid bond by C▬O oxidative coupling (C-2 → O-7 on the basic flavan-3-ol unit) [28]. B-type proanthocyanidins, and in particular, dimers as B1, B2, B3, and B4 or trimer C1 are mainly located in grape skins (0.01–0.86 g/kg dm) and, in a lower extent, in seeds (0.04–0.18 g/kg dm) [16–18]. On the contrary, complex procyanidins (n > 3) are more abundant in seeds (58–163 g/kg dm) than in skins (45–71 g/kg dm) (**Table 1**).

Tannins' structure is characterized by the nature of its constitutive extension and terminal units, its mean degree of polymerization (mDP; average number of units in the polymer), and its degree of galloylation (%G; percentage of subunits bearing gallic acid esters). In the case of skins, the percentage of (−)-epigallocatechin (EGC) or also called the percentage of prodelphinidins (%P) is also used for characterization purposes. Condensed tannins with different mDPs may have different organoleptic properties. Generally, astringency increases with tannin concentration, molecular size, and %G [29]. Polymerized procyanidins are increasingly reactive with proteins and, therefore, have a more important astringent character [30]. Proanthocyanidins' molecular size could also affect bitterness since monomers are considered to be more bitter than oligomers and polymers. Therefore, the estimation of both mDP and %G of procyanidins could be a useful parameter to evaluate the type of procyanidins present in a sample.

The quantity of flavan-3-ols and proanthocyanidins varies during ripening being higher at flowering and lower as the grapes maturate [31]. Both flavan-3-ols and proanthocyanidins are the major polyphenolic compounds in *V. vinifera* grapes. The greatest content is observed in seeds (62–168 g/kg dm) followed by skins (45–73 g/kg dm) (**Table 1**). This amount can be also variable depending on the grape variety and vintage [13].

Very recently, a new condensed tannin called "crown" proanthocyanidin tetramer has been isolated for the first time in grape skins of Cabernet Sauvignon cultivar. This tetramer is totally absent in seeds differentiating it from the rest of proanthocyanidins. This name "crown" is associated to an unusual macrocyclic carbon skeleton that has never been characterized before in the plant kingdom [32].

#### *2.1.3 Flavonols*

Flavonols constitute a group of flavonoids, which have the peculiarity to present a double bound between C2 and C3 and a hydroxyl group in C3. They vary in color from white to yellow and possess an important role in the color stabilization of young red wines, through copigmentation interaction with anthocyanins [3], and in the sensory perception of astringency and bitterness [33].

Conventionally, flavonols are present in berry skins of both white and colored grapes, and their total flavonoid content varies notably depending on cultivars and ripening stage [34]. In relation with cultivars, more quantities of flavonols have

**7**

*2.2.2 Stilbenes*

*Phenolic Compounds of Grapes and Wines: Key Compounds and Implications in Sensory…*

been reported, for example, in *V. vinifera* French varieties (Syrah, Petit Verdot, Cabernet Sauvignon, and Merlot), than with Spanish ones (Tempranillo, Garnacha, and Garnacha Tintorera) [35]. The total amount of flavonols in grapes varies from 1 to 80 mg/kg of fresh berry, being the red cultivars regularly richer than the white

Flavonols are found in grape berry skins in 3-*O*-glycoside forms. The main flavonols reported in red grapes are the dihydroxylated quercetin-3-*O*-glucoside and 3-*O*-glucuronide and the trihydroxylated myricetin 3-*O*-glucoside. In addition, other compounds such as kaempferol and the methylated isorhamnetin, laricitrin, and syringetin 3-*O*-glucosides have also been identified [35]. Furthermore, kaempferol and laricitin-3-*O*-galactosides, kampferol-3-*O*-glucuronide, and even quercetin and siringetin-3-*O*-(6″-acetyl)-glucoside have been identified in Cabernet Sauvignon grapes in lower quantities [37]. Interestingly and more recently, laricitrin-3-*O*-galactoside and syringetin-3-*O*-galactoside were reported in red grapes for the first time. With regard to white grapes cultivars, myricetin, laricitrin, and

**2.2 Non-flavonoid grape polyphenols: phenolic acids and stilbenes**

and *p*-hydroxybenzoic acids are also present [39, 40] (**Figure 2**).

*p*-coumaric, and ferulic acids, respectively) [18] (**Figure 2**).

Phenolic acids can be classified in two main groups: hydroxybenzoic acids (C6-C1) and hydroxycinnamic acids (C6-C3). This family is found in skins, pulp, and seeds of grapes, being generally most numerous in skins (0.2–8.2 g/kg dm) (**Table 1**). The quantities of total hydroxycinnamic or hydroxybenzoic acids in grape skins vary depending on cultivar and origin. For example, hydroxycinnamic acids are more predominant in *V. vinifera* East Asian or North American grapes than in European grapes in which these phenolic acids are only present in trace levels. However, the hydroxybenzoic acid amounts are similar between cultivars [38]. Individually, the most important hydroxybenzoic acids in grapes are gallic, vanillic, and syringic acids. Predominantly present in grape seeds, gallic acid is considered the most important phenolic acid (100–230 mg/kg dm), being the precursor of all hydrolyzable tannins [39]. In lower quantities, protocatechuic acid

Regarding hydroxycinnamic acids, they are principally located in skins, being *p*-coumaric, caffeic, ferulic, and sinapic acids the most significant. It should be reminded that *p*-coumaric and caffeic acids can be found esterified by the glucose of the anthocyanin monoglucosides forming their acylated derivates. In grapes (mainly white) and also in wines, hydroxycinnamic acids are mainly esterified with tartaric acid forming caftaric, *p*-coutaric, or fertaric acids (from caffeic,

Phenolic acids, and overall, hydroxycinnamic acids can act as copigments. Indeed, they are implicated on the formation of new more stable pigments (pyranoanthocyanins) in wine and, in consequence, are considered as color stabilizer agents of young red wines, through copigmentation with anthocyanins [3]. Moreover, they are also associated with the sensory perception of astringency and bitterness [41].

Stilbenes (1,2-diphenylethylene) are formed from two phenyl rings linked together by an ethylene bridge generating a C6▬C2▬C6 structure. The aromatic rings are generally substituted by different functions such as hydroxyl, methyl, methoxyl, prenyl, or geranyl groups. Moreover, monomeric units (resveratrol) can

*DOI: http://dx.doi.org/10.5772/intechopen.93127*

syringetin have not been identified [36].

ones [35, 36].

*2.2.1 Phenolic acids*

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

been reported, for example, in *V. vinifera* French varieties (Syrah, Petit Verdot, Cabernet Sauvignon, and Merlot), than with Spanish ones (Tempranillo, Garnacha, and Garnacha Tintorera) [35]. The total amount of flavonols in grapes varies from 1 to 80 mg/kg of fresh berry, being the red cultivars regularly richer than the white ones [35, 36].

Flavonols are found in grape berry skins in 3-*O*-glycoside forms. The main flavonols reported in red grapes are the dihydroxylated quercetin-3-*O*-glucoside and 3-*O*-glucuronide and the trihydroxylated myricetin 3-*O*-glucoside. In addition, other compounds such as kaempferol and the methylated isorhamnetin, laricitrin, and syringetin 3-*O*-glucosides have also been identified [35]. Furthermore, kaempferol and laricitin-3-*O*-galactosides, kampferol-3-*O*-glucuronide, and even quercetin and siringetin-3-*O*-(6″-acetyl)-glucoside have been identified in Cabernet Sauvignon grapes in lower quantities [37]. Interestingly and more recently, laricitrin-3-*O*-galactoside and syringetin-3-*O*-galactoside were reported in red grapes for the first time. With regard to white grapes cultivars, myricetin, laricitrin, and syringetin have not been identified [36].
