Definition: cork stoppers - permeability

Professor Ribéreau-Gayon wrote in 1933 that oxygen diffuses into wine after bottling. It was generally accepted that natural cork was permeable to oxygen, allowing micro-quantities to pass into the bottle. "However, there were few publications on oxygen diffusion during bottle ageing or on the influence of different closure systems on this subject," points out Yves Glories, former dean of the Faculty of Oenology in Bordeaux, who, together with two other researchers, Paulo Lopes and Cédric Saucier, recently published a study on oxygen transmission rates through different types of corks, measured using a coloured reagent dissolved in bottles of water that were then stored horizontally.
Published in the Journal of Agricultural and Food Chemistry, this work "confirms that cork stoppers allow a small amount of oxygen to penetrate," says the Amorim group, which supported the research. The world leader in cork closures supplied the traditional and technological cork stoppers tested in the study: premium natural cork stoppers of various diameters, sealed stoppers, Agglomerate, Twin Top and Neutrocork. The synthetic Supremecorq (moulded) and Nomacorc (extruded) stoppers were also included in the panel.
All the stoppers allowed oxygen to pass through, but according to the researchers, the synthetic stoppers showed "substantial permeability": for Supremecorq, the limits of the oxygen quantification instruments were reached after less than 140 days of storage, with Nomacorc reaching the limits after 290 days. This is not the case for traditional cork stoppers, which, while exhibiting highly variable transmission rates, remain within values ranging from 0.24 to 0.50 mg of oxygen per litre per month (between 30 and 365 days of storage), compared to 1.5 for Supremecorq and 0.85 for Nomarcorc. As for technological corks, they have shown very low diffusion rates: 0.01 mg/O2/l/m for agglomerated cork, 0.02 for Twin Top and 0.10 for Neutrocork.
Significant oxygen diffusion in the first month:
"These data suggest that the material and manufacturing process of the corks may be responsible for the different oxygen diffusion rates obtained after the first month of storage," the researchers point out. During the first month of storage, most corks showed transmission rates of between 2 and 4 mg/O2/l/m, with the exception of Neutrocork (1.4) and Supremecorq (over 4). According to the authors of the study, "it is possible that at the beginning, the oxygen inside the cork diffuses outside the cork into the bottle, which could explain the high transmission rate during the first month for most of the corks tested. We then observed a slow diffusion, probably linked to the diffusion of atmospheric oxygen through the cork. This phenomenon will certainly be influenced by the characteristics of the cork material."
Matching the cork to the type of wine:
The variability in oxygen transmission rates through different types of corks, highlighted by the Bordeaux researchers, further demonstrates the crucial importance of choosing the right cork, if proof were needed. However, the authors admit that the exact mechanism of oxygen diffusion inside the bottle remains unknown. The authors, who used bottles filled with a solution of purified water and a coloured reagent (indigo carmine) for the study, are currently investigating the "real" ageing of red and white wines sealed with different closures (natural corks, synthetic corks and capsules). The objectives are to simultaneously study the influence of microquantities of oxygen during bottle ageing and the development of the wine's organoleptic properties and, ultimately, to adapt the closure to the type of wine.
Published on 23/02/2006 - Ligérienne de presse - Source: Milfeuille Presse / TV Agri.