**4.2 Chemical composition**

Cork performance depends on structure and chemistry [27], although the impacts of variation are far from being well established, for example, it is believed that the cell wall chemical variation is related to contents in suberin (23.1–54.2%) and lignin (17.1–36.4%) and the suberin-to-lignin ratio plays a determining role in properties, namely, in compression [28–30].

**Table 2** summarizes the chemical composition of cork and the range of its natural variability [29]. Cork is mainly composed of suberin, representing on average 43% of the total dry mass. Suberin is a polyester macromolecule obtained by polymerization of linear long-chain fatty acids and hydroxy acids and glycerol, which are assembled and develop spatially as flexible ribbon-like structures. Suberin is, therefore, the component responsible for the elastic properties of cork and allows the bending and compression of cell walls [3, 27].

**229**

**Table 2.**

*deviation) [29].*

*Cork and Cork Stoppers: Quality and Performance DOI: http://dx.doi.org/10.5772/intechopen.92561*

Monosaccharide composition, % of neutral sugars

Lignin is an aromatic cross-linked polymer that represents on average 22% of the cork dry mass and is responsible for the structural rigidity of the cells and their resistance to compression [29]. Cork lignin is mostly constituted of G units (guaiacyl units) with an important proportion of H units (hydroxyphenyl units) and minor contents of S units (syringyl units) (H:G:S 1:2.5:0.3, S/G 0.12) [31].

*Chemical composition of cork, in % of oven-dried initial material (mean of 58 samples and standard* 

Glucose 46.1 3.6 Xylose 25.1 3.7 Arabinose 18.0 3.0 Mannose 3.0 2.8 Galactose 7.3 1.2 Rhamnose 0.5 0.5

**Chemical parameter (% o.d. cork) Mean Std.** Extractives, total 16.2 3.9 Dichloromethane 5.8 0.8 Ethanol 5.9 3.0 Water 4.5 1.6 Suberin, total 42.8 6.2 Long-chain lipids 41.0 5.2 Glycerol 3.8 0.6 Lignin, total 22.0 3.3 Klason lignin 21.1 3.3 Acid soluble lignin 0.9 0.2

The cellulose and hemicelluloses are less representative and amount up to 16.2% of the cell wall structural components [31]. Hemicelluloses are mainly composed of arabinoxylans with a significant proportion of galactose including uronic acids. The suberin-to-lignin ratio is 2.0, and the cellulose-to-hemicellulose ratio, determined

Cork contains a substantial proportion of nonstructural compounds that may be removed by solubilization with suitable solvents without impairing the core properties of cork. These so-called extractives represent on average 16.2% of cork: 5.8% are nonpolar compounds (e.g., lipids and terpenes), and 10.4% are polar compounds of

The stoppers are punched out from cork strips so that their cylindrical axis is parallel to the axial direction of cork. Therefore, the surface of the cork stoppers is not homogeneous relative to the section of cork: (a) the circular tops correspond to transverse sections with the lenticular channels crossing the surface as thin rectangular channels perpendicular to the growth rings and (b) the lateral surface of the body ranges from regions corresponding to tangential and radial sections of cork (**Figure 4**). The lenticular channels appear differently shaped in these two sections: in the radial section, they look like elongated rectangular channels, and in the tangential section, they have an approximately circular to elliptical form [3].

by the ratio of glucose to other sugars, is 1:1.2 [27, 29, 31].

**4.3 Looking at the surface and inside of a cork stopper**

phenolic and polyphenolic nature [29].


#### **Table 2.**

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

**Material Natural suberized lignocellulosic composite**

Density 120–170 kg m<sup>−</sup><sup>3</sup> Type of cells Closed Mean edges/face *n* = 6 Mean faces/cell *f* = 14 Individual cell shape Hexagonal prism Symmetry of structure Axisymmetric Cell thickness 1–1.5 μm Fraction of solid material 10% Largest principal cell dimension 40 μm Smallest principal cell dimension 20 μm Intermediate principal cell dimension 30 μm Shape anisotropy ratios R13 = 1.5–1.7, R12 = 1–1.1 Other specific features Growth rings, lenticular channels

from the outside to the inner tissue and are loosely filled with a dark brown colored, unsuberified material, usually conspicuous to visual observation [20]. The lenticular channels appear differently shaped in the three sections of cork (**Figure 1**): (a) in the transverse and radial sections, they are thin elongated rectangular channels oriented radially, and (b) in the tangential section, they present circular to elliptical form.

*Fluorescence image of the transverse section of a cork piece showing the different cork rings and a scanning* 

Cork performance depends on structure and chemistry [27], although the impacts of variation are far from being well established, for example, it is believed that the cell wall chemical variation is related to contents in suberin (23.1–54.2%) and lignin (17.1–36.4%) and the suberin-to-lignin ratio plays a determining role in

**Table 2** summarizes the chemical composition of cork and the range of its natural variability [29]. Cork is mainly composed of suberin, representing on average 43% of the total dry mass. Suberin is a polyester macromolecule obtained by polymerization of linear long-chain fatty acids and hydroxy acids and glycerol, which are assembled and develop spatially as flexible ribbon-like structures. Suberin is, therefore, the component responsible for the elastic properties of cork and allows

This feature contributes to increase the structural anisotropy of cork.

*electron micrograph detailing the transition between latecork and earlycork.*

**228**

**4.2 Chemical composition**

**Figure 3.**

**Table 1.**

*Main characteristics of cork structure [3].*

properties, namely, in compression [28–30].

the bending and compression of cell walls [3, 27].

*Chemical composition of cork, in % of oven-dried initial material (mean of 58 samples and standard deviation) [29].*

Lignin is an aromatic cross-linked polymer that represents on average 22% of the cork dry mass and is responsible for the structural rigidity of the cells and their resistance to compression [29]. Cork lignin is mostly constituted of G units (guaiacyl units) with an important proportion of H units (hydroxyphenyl units) and minor contents of S units (syringyl units) (H:G:S 1:2.5:0.3, S/G 0.12) [31].

The cellulose and hemicelluloses are less representative and amount up to 16.2% of the cell wall structural components [31]. Hemicelluloses are mainly composed of arabinoxylans with a significant proportion of galactose including uronic acids. The suberin-to-lignin ratio is 2.0, and the cellulose-to-hemicellulose ratio, determined by the ratio of glucose to other sugars, is 1:1.2 [27, 29, 31].

Cork contains a substantial proportion of nonstructural compounds that may be removed by solubilization with suitable solvents without impairing the core properties of cork. These so-called extractives represent on average 16.2% of cork: 5.8% are nonpolar compounds (e.g., lipids and terpenes), and 10.4% are polar compounds of phenolic and polyphenolic nature [29].
