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

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

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

seasoning and toasting periods.

**3.1 Influence of botanical species**

**3.2 Influence of geographical origin**

between French and American oaks [5, 58].

**3.3 Influence of single-tree variability**

**3.4 Influence of cooperage operations**

the wine maturation [68].

largely predominate in oak wood, representing from 40 to 65% by weight of total ellagitannins [53–58] (**Table 2**). Six additional ellagitannins have been identified in oak wood: the lyxose/xylose-bearing monomers grandinin and roburin E, the dimers roburins A and D, and the lyxose/xylose-bearing dimers roburins B and C [64]. Since ellagitannins are very soluble in wines and spirits, with a high reactivity, their levels in oak-aged beverages are much lower than what could be expected. When comparing both main monomers, vescalagin presents a more polar configuration that confers it a lower stability in hydro-alcoholic solutions [65]. From a sensory point of view, their level and profile may affect the astringency and bitterness of wine [66]. The level of ellagitannins in oak heartwood depends on the botanical species, the geographical origin, the single-tree variability, the sampling position in the tree, the grain, and the processing of wood in cooperage, notably the type and length of both

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].

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

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

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

**10**

composition of the final product is much more complex than in the raw material, due to the formation of new compounds [71].

## **4.1 Phenolic compound changes during winemaking**

Winemaking techniques involve the extraction of juice from ripe grapes, fermentation with yeast, and changes in polyphenolic composition that occur due to the participation of these compounds in various reactions such as copigmentation, cycloaddition, polymerization, and oxidation. These reactions begin after grape crushing, followed by fermentation and aging, contributing to the sensory properties of wines, mainly color and mouthfeel sensation. The total extractable phenolic content in grapes is encountered in seeds (60–70%), in the skin (28–35%), and in the pulp (about 10% or less). In the seeds, the phenolic content may range between 5 and 8%, by weight [72].

The understanding of the relationship between the quality of a particular wine and its phenolic composition remains one of the major challenges in enological research. For example, the anthocyanin fingerprints of varietal wines are proposed as an analytical tool for authenticity certification [27]. Patterns of some classes of flavonoids are under strict genetic control, and their distribution varies considerably among different grape cultivars [73, 74].

Several factors impact the wine phenolic composition, including the "terroir," the grape variety and its degree of maturation before harvesting, or the winemaking process with its specific conditions of fermentation or aging [75]. Certain technological procedures, such as addition of sulfur dioxide (SO2) and/or ascorbic acid prior to crushing the grapes, maceration, yeast strain utilization and alcoholic fermentation, oxidation, or adsorption, can also influence the levels of phenolics during the winemaking process [76]. The addition of SO2 and pectolytic enzymes before fermentation caused an increase in color intensity, color stability, total phenolics, anthocyanins, catechin, and epicatechin in a red Italian wine [77].

In white grape musts, the predominant phenolic compounds are hydroxycinnamic tartaric acid esters as catechins and proanthocyanidins which are found mainly in their skins. The must fermentation of red wines is realized in the presence of both grape skins and seeds. During this process, phenolic compounds such as anthocyanins are subjected to various reactions, such as enzymatic oxidation, nucleophilic substitution, degradation, and cycloaddition of the carbonyl compounds leading to the formation of vitisins (A and B). These pyranoanthocyanins in red wine are mainly orange pigments. Moreover, the red wine color evolution and stabilization are mainly induced by the formation of polymerized pigments. The acidic hydrolysis of proanthocyanidins leads to the formation of flavan-3-ol unit or tannin oligomers with a carbocation in C4 position which can be attacks by positions C6 and C8 of another proanthocyanidins or an anthocyanin. This reaction will induce the modification of the condensed tannin polymerization degree or the formation of the polymerized pigment. These newly formed purple pigments induce the color modification of the young red wine. Moreover, polymerization through acetaldehyde between two condensed tannins or between condensed tannins and anthocyanins also occurs during the winemaking process and aging. The formation of these ethyl bridge compounds will also produce modification in the organoleptic properties of the wine and the color stabilization since the ethyl-bridged anthocyanin-tannin compounds also exhibit purple color [72]. During the red wine maturation in bottles, all these newly formed purple polymerized pigments will undergo slight oxidative reaction to slowly form some more orange pigments which together with pyranoanthocyanins are forming the color of old wine.

**13**

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

The winemaking process involves the alcoholic fermentation of must, often followed by malolactic fermentation (MLF). When MLF is completed, the wine is subjected to different clarification and stabilization treatments and/or is stored in oak barrels for aging for a variable period of time. MLF and aging in oak barrels are two enological processes which modify the composition and sensory characteristics of the wines [5, 78–80]. When oak wood derivatives like chips are added after fermentation, wines seem to have a greater aging potential compared to the wines fermented with chips due to their higher ellagitannin content and enhanced condensation reactions. On the other hand, color stabilization and tannin polymerization occur faster when chips are added during fermentation, resulting in shorter aging periods suitable for early consumed wines [81]. MLF in tanks may simplify the control of the process; however, the use of oak wood during the MLF stage affects the chemical and sensory attributes of wines. In red wines, MLF container plays an important role on proanthocyanidin and anthocyanin concentration and evolution as oxygen in small quantities favors polymerization reactions among anthocyanins and tannins. Wines performing MLF in tanks present a higher total proanthocyanidin concentration (5.8 g/L wine) than that of those which accomplished MLF in medium-toasted barrels (4.9 g/L wine). The major wine glucosidic anthocyanin, malvidin, showed as well greater levels in wines carrying out their MLF (33 and 26 mg/L wine, respectively, for tank and barrel MLF). Regarding ellagitannin concentration, their content is strongly influenced by both barrel toasting and MLF container. For instance, in the case of medium-toasting barrels, castalagin was found at concentrations twofold times higher (19 mg/L wine) when MLF was performed in barrels [79]. Concerning sensory results, the MLF strengthens the organoleptic preference of wine when it takes place in barrels [79, 80, 82]. In white wines, total ellagitannin concentration varied from 7.8 to 17.4 mg castalagin equivalents/L for wines performing MLF in tanks and barrels, respectively [83].

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

**4.2 Using oak wood during winemaking**

**4.3 Evolution of phenolic compounds during aging**

The phenolic composition of wine changes along the wine aging process reflects in the color and astringency level of the final product. From 1978 to 2005 vintage for Cabernet Sauvignon wine, phenolic compounds, total tannins, and total anthocyanins varied from 1735 to 2903 mg/L, from 1.3 to 2.2 g/L, and from 15 to 123 mg/L, respectively [29]. In general, the relative anthocyanin content decreases upon aging although this chemical modification is associated with a very clear change in color. This characteristic is often used as a quality standard for aged wines. One of the main factors responsible for anthocyanin loss is the storage temperature [84]. The majority of red wines aged are in contact with oak wood, whether in form of barrels or in form of oak wood derivatives. As a consequence, their phenolic composition changes due to the addition of oak wood extracted compounds. These compounds include hydrolyzable tannins (*C*-glucosidic ellagitannins), aromatic carboxylic acids, and several aldehydes. Regarding wine-air interactions, barrel structure allows a controlled entrance of oxygen, which is essential to the polymerization and the slight oxidative reactions between different types of flavonoids, leading to a modification of the organoleptic properties of the wine. Indeed, wood can affect wine composition and, consequently, organoleptic properties through different mechanisms. On the one hand, wine compounds can be adsorbed onto wood surface. On the other hand, compounds, such as ellagitannins, can be extracted from wood to the wine due to the hydro-alcoholic nature of the latter. Ellagitannins can take

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