The cork viewed from the inside
Graphical abstract
Introduction
Cork was one of the first materials put under the microscope. The first depiction dates back to the years 1660, when Robert Hooke drew the scheme of its very characteristic cellular organization, giving the term cell to the basic biological unit (Hooke, 1664). More recently, Gibson et al. (1981) described the geometry of cork cells in three different sections: axial, radial and tangential (Fig. 1).
Cork was the prime candidate for sealing of beverages from amphorae at the Romanian age up to wine bottle with a marked increase since the industrialization of the glass processing in the 19th (Karbowiak et al., 2010). Nowadays, it is also used in a large range of applications such as floor covering, activated carbon, acoustic and thermal insulation (Gil, 2009, Silva et al., 2005). In its use as sealing, cork is sorted in different classes according to a main characteristic: the proportion of lenticular channels. These defects can be considered as the macroporosity. The more lenticels there are, the worst cork quality is. The knowledge of the structure is important to better understand the mass transfer properties in its use as a sealing material (Giunchi et al., 2008, Karbowiak et al., 2010, Lequin et al., 2010). Cork is generally sorted visually by hand or by optical analysis (Pereira et al., 1996, Prades et al., 2010) as a function of its overall outside general aspect, considering the defects viewed from the outside are a good estimation of the inside. Some other technics such as X-ray or Terahertz imaging were recently used to get a better understanding of the inner structure of cork samples or to perform 3D reconstructed images by tomography (Brunetti et al., 2002, Donepudi et al., 2010, Teti et al., 2011). Neutron imaging is another non-destructive and non-invasive method which allows characterizing materials structure and defects at the microscopic length scale (Lehmann et al., 2011). Whereas light can only probe the inside of transparent materials, neutrons and X-rays penetrate most materials to depths of several centimeters. X-rays are scattered by atomic electrons whereas neutrons are scattered by atomic nuclei. This results in a number of differences between the two methods, perhaps the most important being in the scattering from light elements. For example the hydrogen nucleus scatters neutrons strongly and aluminium only slightly. These last properties allow the observation of defects or fluid migration in biological materials (if one plays with the thickness of the samples). Structural studies inside usually opaque materials or during processes can also be easily performed (Tanoi et al., 2009). Classically, neutron imaging has been used for quality control purposes in industries that require precision machining such as aircraft, motor engineering, metallurgy or material science (Boillat et al., 2010, Kardjilov et al., 2009, Warren et al., 2013). However, to study the microstructure of complex materials with a higher resolution (few μm), X-ray and neutron imaging experiments have to be performed in large scale facilities.
Whatever the technique used, cork stoppers are grading in different classes. Generally there are 7 or 8 qualities (Ferreira et al., 2000; Benkirane et al., 2001; Fédération française des syndicats du liège, 2006; Natural cork users group, 2007), but there is no well-defined standard (Pizzurro et al., 2010). Class 0 represents the best quality while 6 or 7 (depending on the classification used) is the poorest. The aim of this study is to identify and quantify defects present in different classes of cork stoppers with two techniques: digital photography and neutron imaging (radiography and tomography).
Section snippets
Cork stoppers
Raw natural cork stoppers, from Quercus suber L. oak trees in the Mora (Portugal) production area, were supplied by Bouchons Trescases S.A. (Boulou, France). Two qualities of cork stoppers were chosen: high quality (class 0) and lower quality (class 4). Stoppers were neither washed nor surface treated (with paraffin or silicone) prior to use. Cork stoppers of 24 mm diameter were cut with a cutting machine, Mecatome T201 with resinoid cut-off wheels of 180 mm diameter and 0.5 mm thickness (Presi
Cork surface image analysis
Two pictures of cork wafers from class 0 and 4 are shown in Fig. 2a and c. These digital photographs (cropped around the region of interest) clearly display the cork surface of the 24 mm diameter axial section for a representative sample of the class 0 (Fig. 2a) and another one for the class 4 (Fig. 2c). Also displayed in this Fig. 2 is the resulting image from surface analysis performed after binarization and image treatment (Fig. 2b and d). In this axial view of cork, lenticels appear as
Conclusions
In this work, the defects of cork stoppers from two classes, represented by their lenticular channels, were analyzed by conventional digital photography as well as by neutron imaging. These technics allows identifying and quantifying the defects from the surface and from the inside of the material, respectively. In most of cases, the neutron radiography or the neutron tomography take more defects into account than the photography analysis. Comparing the two qualities of stoppers, photography
Acknowledgments
We gratefully acknowledge the Bureau Interprofessionnel des Vins de Bourgogne, the Comité Interprofessionnel du Vin de Champagne and the Regional Council of Burgundy for their financial supports and the Trescases Company for providing cork stoppers.
References (29)
- et al.
Cork quality estimation by using Compton tomography
Nucl. Instrum. Methods Phys. Res., Sect. B
(2002) - et al.
Analysis of mechanical properties of cork stoppers and synthetic closures used for wine bottling
J. Food Eng.
(2008) - et al.
New trends in neutron imaging
Nucl. Instrum. Methods Phys. Res. Sec. A – Accelerat. Spectrom. Detect. Assoc. Equip.
(2009) - et al.
How to organize a neutron imaging user lab? 13 years of experience at PSI, CH
Nucl. Instrum. Methods Phys. Res. Sect. A – Accelerat. Spectrom. Detect. Assoc. Equip.
(2011) - et al.
Differences of raw cork quality in productive cork oak woods in Sicily in relation to stand density
For. Ecol. Manage.
(2010) - et al.
Dehydration process of fish analyzed by neutron beam imaging
Nucl. Instrum. Meth. Phys. Res. Sect. A – Accel. Spectrom. Detect. Assoc. Equip.
(2009) Influence sylvicole dans la production de liège de qualité
- et al.
Possibilité de contrôle automatique de la qualité du liège par vision artificielle
Annal. For. Sci.
(2001) - et al.
Neutron imaging resolution improvements optimized for fuel cell applications
Electrochem. Solid State Lett.
(2010) - et al.
Influence of weather on cork-ring width
Tree Physiol.
(2000)
Influence of vision systems, black and white, colored and visual digitalization, in natural cork stopper quality estimation
J. Sci. Food Agric.
Cork embedded internal features and contrast mechanisms with dei using 18, 20, 30, 36, and 40 keV synchrotron X-rays
Res. Nondestr. Eval.
Characterization of cork growth and quality in one region of production
Ann. For. Sci.
Cited by (28)
Characterization techniques comparison towards a better understanding of different cork-based stoppers types
2022, Journal of Food EngineeringCitation Excerpt :The morphology of all the cork-based closures was evaluated using a Scanning Electron Microscopy (SEM) microscope (TM 3000, Hitachi) with an acceleration voltage of 15 kV (1000 × magnification). Three directions of each sample were analyzed: axial (A), tangential (T), and radial (R), following the schematic representation presented in the work conducted by Lagorce-Tachon et al. (2015a,b). First, the samples were dried at 40 °C for 24 h in an oven (De Leo, Brazil), cut with a razor blade, and placed on aluminum stubs with carbon double-sided tape for visualization.
Reinforcement of cellular materials with short fibres: Application to a bio-based cork multi-scale foam
2020, Mechanics of MaterialsCitation Excerpt :Each bead has a specific cellular orientation (Le Barbenchon et al., 2019). Cork cells have variable dimensions (Lagorce-Tachon et al., 2015b) but also variable chemical compositions (Pereira, 2013). In order to observe the structure of cells and interfaces, SEM samples were cut with a razor blade, which was replaced for each cut.
Time-resolved porosity changes at cement-clay interfaces derived from neutron imaging
2020, Cement and Concrete ResearchMulti-scale foam : 3D structure/compressive behaviour relationship of agglomerated cork
2019, MaterialiaCitation Excerpt :SEM observations of natural cork give information concerning the cell specific geometry [2] and surface distribution of each direction [13]. Concerning the spatial organisation, only the macroscopic defects of natural cork, like lenticular channel (also called lenticels) or ants galeries, were investigated by using either neutron tomography [14] (for a spatial resolution of 200 µm), Compton tomography [15] or X-ray tomography [16,17] (spatial resolution of 2 mm and 50 µm). Those techniques were used to study the correlation between cork quality (given by surface analysis) and internal defects of cork stoppers.
Morphology, static and fatigue behavior of a natural UD composite: The date palm petiole ‘wood’
2018, Composite StructuresCitation Excerpt :The huge difference of the climate between north Algeria (Mediterranean climate) and south Algeria (semi-arid climate) explains this noticeable discrepancy. The density of date palm wood is slightly higher than that of cork (135 kg m−3 to 140 kg m−3 [24]) and balsa (176 kg m−3 [25]). External shape of one palm petiole is presented on Fig. 4(a) and (b).
Evidence for moderate losses of dissolved CO<inf>2</inf> during aging on lees of a champagne prestige cuvee
2018, Journal of Food Engineering