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THE EFFECT of CHEMICAL TREATMENTS on the CELLULAR STRUCTURE of CORK Key Words IAWA Bulletin n. s., Vol. 9 (4), 1988: 337-345 THE EFFECT OF CHEMICAL TREATMENTS ON THE CELLULAR STRUCTURE OF CORK by Helena Pereira* and A. Velez Marques** Summary The effect of chemical treatments on the pressibility (Ross & Krahmer 1971). An ob­ cellular structure of cork from Quercus suber vious example is given by the cork of the L. was studied using SEM. Treatments in­ cork-oak (Quercus suber L.), as it forms a cluded successive solvent extractions with thick layer of several centimetres with out­ petroleum ether, ethanol and water; desuber­ standing properties of impermeability to liq­ inisation with NaOCH3 in methanol; and de­ uids, compressibility and resilience, and en­ lignification with HNOJ in CH3COOH. ergy absorbing capacity (Gibson et al. 1981). Solvents swell cork anisotropically, with The cellular structure of Q. subercork has considerably larger dimensional increases in been shown to be regular and homogeneous the radial direction, but removal of extractives with rare intercellular voids apart from the does not change its cellular structure. The presence of lenticular channels. The cells are properties of cork are affected by the chemi­ approximately prismatic, mostly pentagonal cal treatments that react with and remove the and hexagonal , stacked in rows in the radial main cell wall structural components. The direction of the tree, and have corrugations in removal of suberin leaves holed cell walls the radial and transverse walls (Pereira et ai. and reveals a loose ribbonlike network; lignin 1987). These structural characteristics are acts as a supporting framework and deligni­ generally invoked when explaining some of fied cell walls collapse. In both cases, cork the properties of cork (Gibson et ai. 1981, looses its low density, compressibility and Rosa & Fortes 1987a, b). However, it may resilience. be assumed that chemical composition and Key words: Quercus suber; chemical treat­ wall topochemistry will also contribute to es­ ments; cell walls; suberin; lignin; solvent tablish these properties. extractions. The role of the different cell wall compo­ nents is already well understood in relation to Introduction the properties of wood. However, cork cells Cork cells in tree barks are thin-walled differ from wood cells in their chemical com­ cells, unpitted or with very infrequent pits. position, with their most typical chemical fea­ They are observed in sections with an ap­ ture being the presence of suberin and waxes. proximate polygonal form and they show In Q. suber, cork is made up mainly of sub­ corrugations or wrinkling of the cell walls erin and lignin (respectively, 40% and 22%); (Esau 1965; Krahmer & Wellons 1973; Ho­ cellulose only represents approximately 9% ward 1977). Cork cells have physical and of cork (Pereira 1988a). The comparison of mechanical properties that differ from the these values with the average chemical com­ other bark and wood cells, namely in relation position of wood shows a similar lignifica­ to density, dimensional variation and com- tion but very different cellulose contents, * Departamento de Engenharia Florestal, Instituto Superior de Agronomia, Universidade Tee­ nica de Lisboa, 1399 Lisboa Codex, Portugal. ** Departamento de Qufmica Organica, Instituto Superior de Engenharia de Lisboa, 1900 Lis­ boa, Portugal. Downloaded from Brill.com10/07/2021 05:40:44PM via free access 338 IAWA Bulletin n.s ., Vol. 9 (4),1988 suggesting that the role of main cell wall After each treatment the cubes were air­ component played by cellulose in wood is dried, and dimensional variation and mass taken by suberin in the case of cork. yield were measured. For the solvent extrac­ Some aspects of the role of the different tions, the absorption of solvent was cal­ chemical components in establishing cell culated from the mass increase after each ex­ structure were investigated by observing cork traction and the extracted material was gravi­ samples of Q. suberafter the removal of cell metrically determined in the solvent. wall components with different chemical Samples for observation in the scanning treatments. This paper reports the results ob­ electron microscope were cut from the cubes tained by using scanning electron microscopy and coated with gold (approximately 200 A on the chemically treated cork samples. thickness). The surface and the interior ofthe cubes were observed in radial, transverse and tangential surfaces. Material and Methods Reproduction cork from Quercus suberL. Results and Discussion with an average thickness of 2.4 cm and of The chemical treatments of the cork cubes good quality (low porosity and absence of under the conditions used in this study defects) was used in this study . Samples with should only affect the superficial layers of the approximate dimensions of 2 x 2 x 2 em were samples, since reactants and reaction times prepared by cutting cubes from the same cork were similar to those used when attacking plank. All the dried phelloderm and phloem cork samples of much smaller dimension, tissues from the outer surface of cork were e. g. 40-60 mesh fractions, as in the course removed prior to the cutting. The cubes were of chemical analysis (Pereira 1988a). This measured, and weighed and subjected to the was in fact the case, as seen macroscopically following chemical treatments: by the extent of penetration of reagents and 1. Successive solvent extractions, in soxhlet, the colour change of the cork, and sub­ with petroleum ether, ethanol and water sequently observed in the scanning electron during 10 h for each solvent. microscope. This allowed the comparison 2. Desuberinisation of the pre-extracted cork between chemically changed and unchanged cubes with 3% NaOCH3 in methanol in cells, respectively at the surface and in the reflux for 6 h. interior of each cork cube. 3. Treatment of the pre-extracted cork cubes The results of the successive solvent ex­ with a solution of HN03 and CH3COOH tractions are shown in Table 1 in relation to (aqueous solution of 90 ml HN03 and dimensional variations and solvent absorp­ 732 ml CH3COOH per liter) in reflux for tion. Absorption of solvent and swelling of 1 h. the cork cubes occurred in variable extents Table 1. Successive solvent extraction of cork cubes with petroleum ether, ethanol and water . % of initial cork weight % of initial linear dimension Extracted Weight Dimensional swelling material increase* Radial Tangential Axial Petroleum ether 0.2 3.0 2.5 0.0 0.3 Ethanol 0.9 71.2 4.8 1.8 2.3 Water 4.0 70.4 9.5 1.9 2.5 * After extraction, before drying . Downloaded from Brill.com10/07/2021 05:40:44PM via free access Pereira & Velez Marques - Chemical treatment of cork 339 Fig. 1. SEM photographs of the surface of cork cubes after solvent extraction . - a: Petroleum ether extracted; transverse section showing the general structure of the cork tissue, namely the corrugations of cell walls and late cork cells at the end of an annual growth ring (arrow); bar = 100 urn. - b: Water extracted; radial section showing the flattening of cell wall corrugations; bar = 20 urn, - c: Petroleum ether extracted; tangential section showing cell wall voids and lamellation (arrows); bar = 10 urn. with the different solvents. Swelling was ob­ of swelling with much larger increases in the served after each solvent treatment but the ef­ radial direction. This is probably the result of fect of petroleum ether was smaller than that the cell geometry and corresponds to the ef­ of ethanol or water: the increase in volume fect of flattening of the cell wall corrugations; corresponded to 2.8% when using petroleum this will cause a larger increase in the cellu­ ether, 9.1% after the ethanol treatment and lar prism height and therefore an enhanced 14.4% with the final extraction with water. swelling in the radial direction. Petroleum The dimensional variations show anisotropy ether was not significantly absorbed by cork, Downloaded from Brill.com10/07/2021 05:40:44PM via free access 340 IAWA Bulletin n.s., Vol. 9 (4),1988 Downloaded from Brill.com10/07/2021 05:40:44PM via free access Pereira & Velez Marques - Chemical treatment of cork 341 Fig. 2. SEM photographs of the pre-extracted cork cubes after desuberinisation. - a: The sur­ face of the sample in radial section, showing transition between two annual growth rings with failure between growth rings; bar = 100 11m. - b: Radial section of the interior of one sample, from the surface (arrow) inwards; bar = 100 11m. - c: As b, showing the transition between the area of chemical attack and the interior of sample; bar = 20 11m. - d: Radial section; holed cell walls at the surface of sample; bar = 111m. - e: Transverse section showing fragments of terti­ ary walls; bar = 20 11m. - f: Detail of e, showing a loose intercrossed cell wall structure; bar = 5 11m. - g: Cells where desuberinisation is beginning; bar = 11m. - h: Amorphous material in the desuberinised region; bar =211m. as was the case with ethanol and water. Ab­ traction (Fig. la, b). The removal of extrac­ sorption of the latter accounted for approxi­ tives does not alter the physical aspect of the mately 70% weight variation in the samples. cell wall and its thickness remains un­ Absorption into the cork cell wall seems to changed. However, extraction of waxes by require a polar and H-bonding solvent such petroleum ether in some cases causes voids in as ethanol or water. A similar effect in di­ the cell wall and seems to facilitate the sepa­ mensional variations and solvent swelling ration of the cell wall into lamellae, as ob­ characteristics has been reported for cork served in Figure 1c both in transverse and cells in Douglas-fir bark (Krahmer & Wel­ tangential sections of the cell wall. lons 1973). This is in agreement with the localisation The SEM observations of the extracted of soluble waxes between the suberin lamel­ samples did not show remarkable differences lae in the secondary wall of cork, as pro­ in the structure of cork after the solvent ex­ posed by Sitte (1962, 1975).
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