**3. Key phenolic compounds in oak wood**

Oak wood is the preferred material for the manufacture of barrels, casks, or whatever derived wood product (chips, blocks, winewoods, tankstaves, etc.) used during fermentation and/or aging of wines. Resistance, flexibility, easy handling, and low permeability make oak wood particularly suitable for wine maturation and storage, in relation to mechanical, physical, and chemical properties provided by other woods [48].

Regardless of the species, oak heartwood is basically composed by cellulose, hemicelluloses, and lignin, representing approximately 90% of dry wood and acting as key structural polymers of wood matrix. The remaining 10% of dry wood corresponds to an extractable fraction, mainly consisting of phenolic compounds but also presenting low molecular weight compounds and volatile compounds. Lignans, coumarins, phenolic acids, and phenolic aldehydes may be found in the oak wood phenolic fraction, but hydrolyzable tannins are the major constituents [49].

Depending on the release of gallic or ellagic acid under acidic conditions, hydrolyzable tannins may be classified, respectively, as either gallotannins or ellagitannins [50]. Gallotannins are the simplest hydrolyzable tannins with a structure consisting of polygalloyl esters of glucose. The oxidative coupling of their galloyl groups converts gallotannins to the related ellagitannins [51].

Ellagitannins are the major nonvolatile extractives from oak heartwood (**Figure 3**). These oak phenolics have a specific structure, consisting of an glucose open-chain esterified at positions 4 and 6 by a hexahydroxydiphenoyl unit (HHDP) and a nonahydroxyterphenoyl unit (NHTP) esterified at positions 2, 3, and 5 with a *C*-glycosidic bond between the carbon of the glucose and position 2 of trihydroxyphenoyl unit [52]. Among ellagitannins, the monomers castalagin and vescalagin

**9**

**Treatment**

**Species**

**Monomers**

**Castalagin**

5.82–27.50

1.76–23.90

0.69–13.70

1.00–10.40

0.39–6.64

0.95–5.34

0.44–4.67

0.47–6.29

13.47–91.02

[54, 56–59]

Untreated wood

*Quercus robur* *Quercus petraea* *Quercus alba*

> Seasoned wood

*Quercus robur* *Quercus petraea* *Quercus* 

0.41–4.45

0.15–6.44

0.15–2.30

0.07–1.81

0.03–0.40

0.02–0.84

0.02–0.79

0.01–0.20

0.89–8.93

*alba*

Toasted

*Quercus* 

3.60–5.44

0.89–1.15

1.80–2.39

0.28–0.66

0.41–0.47

0.45–0.50

0.17–0.20

0.12–0.19

7.72–11.00

[53, 60,

62, 63]

*robur*

*Quercus* 

1.75–3.79

0.36–1.46

0.09–2.23

0.13–0.59

0.20–0.50

0.16–0.44

0.10–0.22

0.03–0.17

3.53–8.96

*petraea*

*Quercus* 

0.21–0.44

0.08–5.28

0.04

0.02

ND

0.04

ND

0.01

0.55–5.72

*alba*

*All values are displayed in mg/g of wood. ND, not detected.*

**Table 2.** *Ellagitannin concentration in untreated, seasoned, and toasted oak wood from the main* Quercus *species used in cooperage.*

wood

0.90–15.91

0.39–11.76

0.39–7.98

0.12–5.80

0.05–2.73

0.02–2.58

0.04–1.84

0.01–1.97

1.98–39.77

1.43–13.00

0.85–10.60

0.77–9.20

0.36–6.15

0.09–1.91

0.18–2.52

0.09–0.48

0.14–1.72

3.93–43.97

[53, 55, 56, 60–62]

1.19–5.29

0.70–2.83

0.29–1.38

0.38–2.17

0.31–0.70

0.08–0.67

0.13–0.83

0.04–0.54

3.48–13.14

1.63–11.76

1.05–8.01

0.78–4.44

0.76–5.47

0.13–2.05

0.17–3.27

0.29–2.41

0.06–2.25

5.87–34.41

**Vescalagin**

**Roburin E**

**Grandinin**

**Roburin A**

**Roburin D**

**Roburin B**

**Roburin C**

**Pentosylated monomers**

**Dimers**

**Pentosylated dimers**

**Total ellagitannins**

**References**

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

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

**Figure 3.** *Chemical structure of main ellagitannins present in oak wood.*


**Table 2.** *Ellagitannin concentration in untreated, seasoned, and toasted oak wood from the main* Quercus *species used in cooperage.*
