**4. What matters for corking**

The most valuable products in the cork industry are the natural cork stoppers, and therefore, the value of the cork planks as raw material is related to their suitability to produce cork stoppers. Only the cork obtained after the second stripping is used, but the plank thickness that is related to the cork annual growth is a major parameter to establish the technological quality required to produce natural cork stoppers [9, 10]. Given its importance, the cork plank thickness is normalized by caliper classes (NP 298:1993 and ISO 1219:1998). Having the suitable thickness, the plank quality is given mostly by the cork porosity [20, 21] that also determines the suitability and quality yields of the products [22–24]. Porosity quantification by surface image analysis is the basis of cork stoppers classification into quality grades [24, 25].

### **4.1 Structure**

Cork is a cellular material with a compact three-dimensional structure of closed prismatic, on average hexagonal, cells that are assembled base to base creating

**227**

**Figure 2.**

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

cell volume is on average 1.7 × 10<sup>−</sup><sup>5</sup>

[19, 26].

respectively) [26].

rows that are aligned in the radial direction in the tree and disposed of in parallel, forming a honeycomb-type structure [19, 26]. The structure of cork observed by scanning electron microscopy in the three principal sections is shown in **Figure 2**. In adjacent rows, the prism bases of neighbor cells most often lay in staggered positions. **Table 1** summarizes the main structural features of cork. The individual

total volume. The cork cell walls, especially those that constitute the lateral prism faces, show ab initio some bending and undulations of varying intensity that can attain strong corrugation derived from constraints during cork growth in the tree

Cork cell walls are composed of a suberinic secondary wall and are flexible enough to undulate or corrugate with variable intensity under compression without fracture. The latecork cells are more rigid and less compressible than the earlycork cells due to their thick walls and small size (**Figure 3**). When the meristematic activity starts in spring, usually in early April, the initial cork cells formed are compressed against the existing cork layers causing the undulation of the cell walls [3]. As referred previously, the cork tissue is not completely homogeneous, and the cellular structure contains discontinuities that influence several properties of the material and the in-use performance of cork products and are thereby closely associated with the commercial value of raw cork and of cork products [3]. The occurrence of lenticular channels crossing radially the cork tissue is one of the most important features of cork heterogeneity: they cross the cork layers

*Schematic representation of the cellular structure of cork with scanning electron micrographs of sections of reproduction cork: (a) tangential, (b) radial, and (c) transverse sections (adapted from Ref. [3]).*

The formation of cork rings (**Figure 1a**) that give cork a layered structure is the consequence of the biological annual growth rhythm: in the main growth period from April to July, the cells formed are bigger (i.e., with a larger prism height) and have thinner walls than those formed in high summer and autumn, at the end of the growth period, which are smaller and thicker walled (earlycork and latecork cells,

, and the solid cell wall content is 10% of the

mm3
