Characterization of colmated wine cork stoppers Dijana Todorovic Dissertação para obtenção do Grau de Mestre em Vinifera EuroMaster – European Master of Sciences of Viticulture and Oenology Orientadora: Professor Helena Pereira Co-orientadora: Doutor Sofia Knapic Júri: Presidente: Jorge Ricardo da Silva, Professor UTL/ISA Vogais: - Pierre-Louis Teissedre, Professor Universidade Bordeaux Segalen - Helena Pereira, Professor, UTL/ISA - Sofia Knapic, Investigador UTL/ISA Lisboa, 2012 Content I List of figures…………………….………………………………………………………………………4 II List of tables…………………………………………………………………………………………….6 III Aknowledgments….…………………………………………………………………………………...7 IV Abstract…………………………………………………………………………………………………8 V Resumo…………………………………………………………………………………………………9 1.Introduction………………………………………………………………………………………….10 1.1 The cork oak….…………………………………………………….…………………………...10 1.2 Formation, structure and cork processing..………………………………………………….15 1.3 Production of cork stoppers…...……………………………………………………………....18 1.3.1 Cork stripping…….……………………………………………………………………….18 1.3.2 Post-harvest processing.………….…………………………….……………………….20 1.3.3 Industrial processing….………………...…………………………….………………….21 1.3.3.1 Boiling….…………………………………………………………………………...23 1.3.3.2 Stabilization.……………………………………………………………………….25 1.3.3.3 Selection and trimming……..…………………………………………………….25 1.3.3.4 Punching cork stoppers…………………………………………………………..26 1.3.3.5 Washing and drying...…………………………………………………………….27 1.3.3.6 Classification...….…………………………………………………………………28 1.3.3.7 Surface treatment………………………………………………………………..29 1.3.3.8 Colmation process...…….………………………………………………………. .29 2. Cork properties………………………………………………………………………………………30 2.1 Macroscopic appearance………………………………………………………………………30 2.2 Cellular structure of cork……………………………………………………………………….34 2.3 Mechanical properties………………………………………………………………………….38 2 3. Material and methods………………………………………………………………………………..41 3.1. Material…………………………………………………………………………………………..41 3.1.1 Image acquisition and image analysis…………………………………………………42 3.1.2 Image data analysis……..…………………………………………………………….…44 3.2 Density and water absorption test………...…………………………………………………...45 3.2.1 Density…………………………………………………………………………………….45 3.2.2 Water absorption…………………………………………………………………………46 3.3. Mechanical testing……………………………………………………………………………...46 3.3.1 Compression test………………………………………………………………………..47 3.3.2 Data processing….…….’……………………………………………………………….48 3.4. Scanning Electronic Microscope …………………………………………………………..…50 4. Results and discussion………………………………………………………………………………51 4.1 Characterization of cork stoppers quality features using image analysis…………………51 4.2 Density and absorption test...………………………………………………………………….56 4.2.1 Density…………………………………………………………………………………….56 4.2.2 Absorption test…………………………………………………………………………....57 4.3. Mechanical test…………………………………………………………………………………59 4.4 SEM images analysis…………….…………………………………………………………….61 5. Conclusion……………………………………………………………………………………………64 VI Bibliography…..………………………………………………………………………………………66 3 LIST OF FIGURES Figure 1. Cork oak: leaves, branch with cork and trunk one year after stripping............................. 11 Figure 2. Approximate cork oak world distribution area ………………………................................. 12 Figure 3. Chart of structure of cork export per product type in value............................................... 14 Figure 4. Phellogen cell division and cork formation....................................................................... 15 Figure 5. External cork layer with longitudinal cracks due to tangential growth stress........................................................................................................................................... 16 Figure 6. Cork stripping photo: axe penetrates a cork plank; axe used as a lever in separation of plank; pulling the plank; marked cork tree after extraction................................................ 19 Figure 7. Scheme of counting rings for determination of cork age……………………………………. 20 Figure 8. Pile of cork planks and pallets with planks in the industrial yard....................................... 21 Figure 9. Scheme of flow of cork material in the industry…………………………………………… 22 Figure10. Boiling operation of planks in closed autoclaves…………………………………………. 24 Figure11. Preparation of final cork plank: trimming of the plank …………………………………….. 25 Figure12. Cutting cork strips with automatic functioning................................................................... 26 Figure13. Cork strip after punching out a stoppers…………………………………………….............. 27 Figure14. Automatic sorting machine based on vision technics……………………………………….. 29 Figure15. Lenticular channels in transversal, radial and tangential cork section…………………….. 31 Figure16. Image of body cylindrical surface for cork stoppers of the two quality classes, best and lower quality class............................................................................................................. 32 Figure17. Sections of the cork structure ……………………………………………….…………........... 35 Figure18. SEM micrograph of cork structure in radial and tangential direction……………………. 36 Figure19. SEM micrograph of tangential section in cork with cells corrugation................................ 36 4 Figure20. Example of stress-strain curve in compression of cork…………………………………… 39 Figure21. Image of cylindrical surface and top for Superior and Inferior quality class……………. 42 Figure22. Cork stoppers in acquisition images with marked ROI on a second stopper………….. 43 Figure23. Frame with the stoppers, detecting the pores on the second stopper………………….. 44 Figure24. Universal testing machine, equipment for mechanical tests of materials……………… 47 Figure25. Image of the cork stopper during the horizontal compression test performed on body surface (transversal compression) …………………………………………………………. 48 Figure26. Image of software processing data obtained in mechanical test………………………… 48 Figure27. SEM equipment……………………………………………………………………………….. 50 Figure28. Surface of body and tops of the Inferior, Superior and Colmated class of stoppers............................................................................................................................. 55 Figure29. Box plots of porosity coefficient, largest pore area and number of pores for Superior and Inferior quality class.................................................................................................... 56 Figure30. Force-stroke curves for the axial compression for the Colmated and Inferior stoppers.. 60 Figure 31 Force-stroke curves for the radial compression for the Colmated and Inferior stoppers.. 60 Figure 32 Partial section of colmated stopper………………………………………………………….... 62 Figure 33 SEM images of colmated pores of a cork stopper: the pore filling material (upper images), the irregular structure of the cork granules used in the filling (middle images) and the polymer binding material (bottom images)………………………………………… 63 5 TABLES Table 1. Montado world distribution area in hectares and corresponding percentages….. 13 Table 2. Average porosity parameters for Inferior and Superior class of stoppers, bodies and tops………………………………………………………………………………… 51 Table 3. Average, maximum and minimum values of porosity variables of Inferior class.. 53 Table 4. Average, maximum and minimum values of porosity variables in Superior class 53 Table 5. Densities with standard deviation for Inferior and Colmated group of stopper 57 Table 6. Increase of mass due to water absorption and volume swelling of the two groups of cork stoppers after 72h on 60°C........................................................... 58 Table 7. Young’s modulus and standard deviation for the axial compression for the Colmated and Inferior group of stoppers............................................................. 59 Table 8 Maximum, minimum and mean force and standard deviation for the given deformation of 1mm and 2mm, in radial compression for the Inferior and Colmated group of stoppers………………………………………………………….. 61 6 AKNOWLEDGMENTS I want to express my gratitude to: Professor Helena Pereira, for her guidance as supervisor, time and advices on my way in making this work Doctor Sofia Knapic, my co-supervisor, for her time, patience and willingness to introduce me to the cork world Professor Olga Laureano, Instituto Superior de Agronomia, for being team leader during all this time of Master in Portugal Laboratorio Nacional de Engenharia Civil (LNEC) and Doctor Jose Saparoti in behalf, for helping in the realization of the part of practical analysis Foresty department stuff, PhD students, technicians and all the others, for being nice hosts and making me company All the Enology and Viticulture classmates for being there, encouraging and supporting me My family for tireless support, even from distance 7 ABSTRACT The objective of this work is to describe and compare colmated and non-colmated stoppers cork stoppers, regarding their differences in appearance (image analysis), structure (analysis with scanning electron microscopy- SEM), and mechanical behavior (compression test). For this study 75 natural cork stoppers were used and divided equally in 3 groups: (i) stoppers of superior class; (ii) stoppers of inferior class to be colmated; (iii) colmated stoppers. Image analysis techniques were applied on the surfaces of superior and inferior (pre-colmated) class of stoppers, to analyze their porosity. Porosity features showed differences between two classes: higher values of all features in the inferior class and lower in the superior quality