**3.1 Influence of botanical species**

Among the more of 150 oak species classified in the genus *Quercus*, the most frequently used in cooperage for winemaking are *Quercus robur* (pedunculate oak) and *Quercus petraea* (sessile oak), both growing in Europe, and *Quercus alba*, commonly known as American white oak, growing in the United States [5]. American oaks differ from European species not only because of their mechanical properties (higher density and resistance and lower porosity and permeability) [60] but also for the chemical composition of their phenolic fraction. Ellagitannin concentration is generally lower in *Q. alba* than European species, which in turn show a greater ellagitannin content in pedunculate oak than in sessile oak (**Table 2**) [53, 55–58].

#### **3.2 Influence of geographical origin**

Until recently, French and American oak forests have been the quintessential source of wood for cooperage. Meanwhile, over the last few years, a huge number of studies on pedunculate and sessile oaks from different European origins (Hungary, Poland, Russia, Romania, Slovenia, Spain, Ukraine, and Moldova, among others) confirm their prospective use for maturation of quality wines [60]. Oaks from these new European origins appear to present ellagitannin concentrations halfway between French and American oaks [5, 58].

#### **3.3 Influence of single-tree variability**

The width of oak wood rings ("grain") is of great importance for the choice of oak wood for barrel and cask making, since it influences the wood chemical composition and affects the contribution of oak aging to wine quality. The higher the grain size, the larger the amount of ellagitannins released and the faster that release [67]. Furthermore, the grain size also exerts an effect on the oxygen transfer ratio (OTR): the smaller the grain size, the greater the OTR, and the faster the wine maturation [68].

#### **3.4 Influence of cooperage operations**

Fresh wood cannot be directly used in winemaking, due to the great percentage of humidity (up to 70%), an excess of phenolic compounds, and a shortage of aromatic constituents. Oak wood conditioning in cooperage includes two stages that will determine the enological quality of wood. Both seasoning and toasting affect

**11**

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

the structure and final chemical composition of the wood that is going to be in

Seasoning allows, not only reduction of humidity in wood, but also fiber contraction and wood maturation. During this process, an important decline of ellagitannin content is observed due to different physical, chemical, and biochemical mechanisms involved: stave leaching by rain, hydrolytic oxidative degradation, polymerization, and fungal enzymatic activity [56]. These phenomena occur particularly in the surface of wood and, in a lesser degree, but uniformly, in the inner wood [53]. Among the different oak wood seasoning methods, natural seasoning in open air seems to be more effective than mixed and artificial methods in reducing

Toasting also induces an important modification of wood chemical composition, including an additional decline of the ellagitannin content. During toasting, castalagin is mainly oxidized in dehydrocastalagin, whereas its diastereomer vescalagin is reduced in deoxyvescalagin [69]. Similar deoxy- and dehydro-derivates have been

Ellagitannin degradation, and in turn their sensory impact on wines, may be modulated by changing the toasting thermal profile (temperature and length) [5]. In this sense, an ever-widening range of toasting levels is available at cooperages for all oak wood products. The higher the toasting level, the greater the ellagitannin decomposition, via dimerization and hydrolysis reactions, as well as formation of

Recently, new compounds that showed [M-H]-ion peak at *m/z* 1055.0631 (compound A) and at *m/z* 1011.0756 (compound B) have been identified as a result of thermodegradation of ellagitannins. The A compound corresponds to castacrenin E which is the oxidation product of castacrenin D, a vescalagin with an additional aromatic ring (gallic acid) to the C-1 through a C▬C bond. The levels of these compounds, found under experimental conditions and further searched in commercial oak wood, are dependent on both oak wood size and

But, the extraction of oak phenolics into wine depends not only on the pool of potential extractible compounds originally present in wood, determined by the abovementioned factors, but also on the wine matrix, the aging length, and the

During the grape ripening phase, the physiological and biochemical changes determine grape quality. The first period of grapes growth consists mostly of cell division and expansion, followed by a rapid growth phase during which the berry is formed and the seed embryos are produced. In this period, several compounds are accumulated in the berries, especially the tartaric and malic acids, conferring the acidity of the future wine. During the first growth period, several polyphenolic compounds increased like hydroxycinnamic acids in grapes' pulp and skin and tannins and catechins in the skin and seed. The most important changes in grapes composition happen during the second growth phase (the ripening stage). Grapes switch from small, hard, and acidic berries to larger, softer, sweeter, less acidic, flavored, and colored ones. The majority of the solutes accumulated during the first growing phase remain at harvest. During the second period, the malic acid is metabolized and used as an energy source, its proportion decreasing toward the tartaric acid concentration, which remains almost unchanged. In general, the chemical

**4. From grapes to wine aging: phenolic compound evolution**

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

the excess of ellagitannins [56].

toasting method [70].

observed for roburins A and D, respectively.

copolymers with cell-wall components [64].

exposed wood area to wine volume ratio.

contact with wine.

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

the structure and final chemical composition of the wood that is going to be in contact with wine.

Seasoning allows, not only reduction of humidity in wood, but also fiber contraction and wood maturation. During this process, an important decline of ellagitannin content is observed due to different physical, chemical, and biochemical mechanisms involved: stave leaching by rain, hydrolytic oxidative degradation, polymerization, and fungal enzymatic activity [56]. These phenomena occur particularly in the surface of wood and, in a lesser degree, but uniformly, in the inner wood [53]. Among the different oak wood seasoning methods, natural seasoning in open air seems to be more effective than mixed and artificial methods in reducing the excess of ellagitannins [56].

Toasting also induces an important modification of wood chemical composition, including an additional decline of the ellagitannin content. During toasting, castalagin is mainly oxidized in dehydrocastalagin, whereas its diastereomer vescalagin is reduced in deoxyvescalagin [69]. Similar deoxy- and dehydro-derivates have been observed for roburins A and D, respectively.

Ellagitannin degradation, and in turn their sensory impact on wines, may be modulated by changing the toasting thermal profile (temperature and length) [5]. In this sense, an ever-widening range of toasting levels is available at cooperages for all oak wood products. The higher the toasting level, the greater the ellagitannin decomposition, via dimerization and hydrolysis reactions, as well as formation of copolymers with cell-wall components [64].

Recently, new compounds that showed [M-H]-ion peak at *m/z* 1055.0631 (compound A) and at *m/z* 1011.0756 (compound B) have been identified as a result of thermodegradation of ellagitannins. The A compound corresponds to castacrenin E which is the oxidation product of castacrenin D, a vescalagin with an additional aromatic ring (gallic acid) to the C-1 through a C▬C bond. The levels of these compounds, found under experimental conditions and further searched in commercial oak wood, are dependent on both oak wood size and toasting method [70].

But, the extraction of oak phenolics into wine depends not only on the pool of potential extractible compounds originally present in wood, determined by the abovementioned factors, but also on the wine matrix, the aging length, and the exposed wood area to wine volume ratio.
