The porosity of natural cork stoppers

Natural cork stoppers are the premium product of the cork industry with worldwide recognition of quality and performance as wine sealant. The occurrence of negative evolution of bottled wines has often been attributed to cork stoppers as a result of their intrinsic variability. One of the main characteristics of cork stoppers is their surface porosity which is the determining factor for their commercial quality grading. Although several studies have analysed the porosity of cork stoppers, the results have been limited to rather small samplings and the anisotropy of porosity was not studied in detail. In this study, the porosity of 600 natural cork stoppers was characterized by image analysis on the lateral surface and tops. Porosity coefficient was 2.4%, 4.0% and 5.6%, respectively for premium, good and standard stoppers. Tops (transverse section) have a lower porosity coefficient and much less pores per unit area (about 60%) in relation to the lateral surface. Moreover, pores in the tops are more elongated than in the lateral surface. The detailed observation of the lateral surface of stoppers showed its heterogeneity in relation to porosity features. The tangential regions present higher porosity (3.1%, 4.9% and 6.2% for, respectively, premium, good and standard quality) while radial regions had larger pores and higher largest pore area. The shapetype variables presented mean values similar for the entire lateral surface, with slightly higher shape factor in the tangential section than in the radial section. The anisotropy of porosity features shown by cork stoppers is a consequence of the biological development of cork and of the method of production i.e. the cutting orientation. Understanding this variation is a relevant component in the analysis of stoppers performance as wine sealants and can lead to improvements in cork quality classification algorithms used in the automated systems

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