Holzforschung 2015; aop *Corresponding author: Vanda Oliveira, Universidade de Lisboa, Instituto Superior de Agronomia, Centro de Estudos Florestais (CEF), Tapada da Ajuda, P-1349-017 Lisboa, Portugal, e-mail: vandaoliveira@isa.ulisboa.pt. http://orcid.org/0000-0003-3461-7028 Jan Van den Bulcke and Joris Van Acker: UGCT-Woodlab-UGent, Faculty of Bioscience Engineering, Department of Forest and Water Management, Ghent University, Coupure Links 653, 9000 Ghent, Belgium Thomas de Schryver: UGCT, Centre for X-ray Tomography, Department of Physics and Astronomy, University Ghent, Proeftuinstraat 86, 9000 Ghent, Belgium Helena Pereira: Universidade de Lisboa, Instituto Superior de Agronomia, Centro de Estudos Florestais (CEF), Tapada da Ajuda, P-1349-017 Lisboa, Portugal. http://orcid.org/0000-0002-5393-4443 Vanda Oliveira*, Jan Van den Bulcke, Joris Van Acker, Thomas de Schryver and Helena Pereira Cork structural discontinuities studied with X-ray microtomography Abstract: Cork is a natural cellular material with a rather unique set of properties, and its best known application is as stopper for wine bottles. The cork tissue contains struc- tural discontinuities, for example, lenticular channels (LCh), that influence the in-use performance of cork prod- ucts. X-ray microtomography, in combination with image analysis, has been used for cork characterisation and pro- vided new insights into the three-dimensional location of discontinuities, which are hidden for a visual inspection. It was demonstrated that the presence of LCh is positively correlated with cork density, and the void fraction of LCh in the lower part of a cork stopper is strongly related to the oxygen ingress in the bottle during the first month after bottling. The results contribute to better understanding the natural variation of cork properties. Keywords: cork, image processing, lenticular channels, natural cork stoppers, structural discontinuities, X-ray microtomography DOI 10.1515/hf-2014-0245 Received September 9, 2014; accepted December 12, 2014; previously published online xx Introduction Cork is produced from the bark of Quercus suber L. Its properties as a lightweight material are unique. For example, it is viscoelastic and allows large deformation under compression without fracture, largely imperme- able to water and other liquids and gases, a thermal and electric insulator, and an acoustic and vibration absorber (Fortes et al. 2004; Pereira 2007). Cork is best known as a successful bottle closure since ancient times (Pereira 2007) and is still the most popular closure, especially for long-term aging of red wines (Phillips 2014). The cork tissue has a honeycomb structure with a considerable regularity in the cellular arrangement: the cells are closed and noncommunicating, with thin walls that surround an air-filled lumen; the cell volume is, on average, 1.7 × 10 -5 mm 3 , and its solid content is around 13.5%. However, the tissue contains discontinuities that influence the in-use performance of cork products and are thereby closely associated with the commercial value of raw cork and of cork products (Pereira 2007). Lenticular channels (LChs) are the most important features in this context: they cross the cork layers from the outside to the inner tissue and are loosely filled with a dark brown, unsuberified material, usually conspicuous to visual observation (Pereira et al. 1996; Oliveira et al. 2012). Other discontinuities can have an accidental occurrence, such as the galleries made by the larva of Coroebus undatus or by the ant Crematogaster scutellaris (Gonzalez-Adrados and Pereira 1996; Pereira 2007). X-ray imaging in its two-dimensional (2D) form is widely used in wood science for microdensitometry studies (Ikonen et al. 2008; Boden et al. 2010; Helama et al. 2012; Knapic et al. 2014). X-ray tomography is a non- destructive 3D imaging technique that allows the study of interior structures of an object (Landis and Keane 2010). Industrial X-ray tomography scanners are successful for determination of wood properties in terms of internal log features such as pith, growth rings, heartwood and sapwood, knots, and decay (Wei et al. 2011; Longuetaud et al. 2012). X-ray microtomography (XRMT) has a higher spatial resolution and has been proved to be of value in different fields such as biomedicine (Kim et al. 2014), geoscience (Cnudde and Boone 2013), and material science (Dewanck- ele et al. 2012; Česen et al. 2013). In wood science, 3D XRMT was employed to study the shrinkage behaviour of cells (Taylor et al. 2013), the analysis of coatings (Van den Bulcke et al. 2010; Bessières et al. 2013), the compressive Authenticated | vandaoliveira@isa.ulisboa.pt author's copy Download Date | 1/21/15 11:10 AM