Elsevier

Journal of Food Engineering

Volume 149, March 2015, Pages 214-221
Journal of Food Engineering

The cork viewed from the inside

https://doi.org/10.1016/j.jfoodeng.2014.10.023Get rights and content

Highlights

  • First time neutron imaging is applied to cork.

  • Comparison of conventional digital photography with neutron imaging for defect detection.

  • Analysis of the porosity in a full cork stopper by tomography.

  • Inter-individual variability from stoppers to stoppers is as important as the intra variability within a single stopper.

Abstract

Cork is the natural material stripped from the outer bark of cork oak. It is still the most used stopper to seal wine bottles and to preserve wine during storage. Cork stoppers are sorted in different classes according to apparent defects, named lenticels, which can be related to the cork macroporosity. The more lenticels there are, the worst cork quality is. The present work aims at investigating defects analysis of cork stoppers from two classes by comparing images recorded by digital photography and neutron imaging. Surface analysis of defects obtained from photography leads to more surface defects in class 4 (6.7%) than in class 0 (4.1%). Neutron radiography and tomography are powerful methods that really show the defects inside the material. From neutron radiography and tomography, class 4 contains 7.5% of volume defects and class 0 5.9%. Moreover, tomography also allows observing defects distribution along the whole stopper and possible interconnectivity.

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)

  • A. Costa et al.

    Influence of vision systems, black and white, colored and visual digitalization, in natural cork stopper quality estimation

    J. Sci. Food Agric.

    (2007)
  • V.R. Donepudi et al.

    Cork embedded internal features and contrast mechanisms with dei using 18, 20, 30, 36, and 40 keV synchrotron X-rays

    Res. Nondestr. Eval.

    (2010)
  • Fédération française des syndicats du liège. 2006. Charte des bouchonniers...
  • A. Ferreira et al.

    Characterization of cork growth and quality in one region of production

    Ann. For. Sci.

    (2000)
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