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New life in oxygen debate
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New research by the University of Bordeaux 2 has confirmed what winemakers have long believed — that corks allow a tiny amount of oxygen to enter a bottle of wine. The findings reveal a less than three-fold variation in the oxygen transmission rates (OTR) of natural cork stoppers, contradicting recent claims of 1000-fold variability, according to the latest edition of Bark to Bottle, Amorim’s newsletter. The Bordeaux team found that natural cork stoppers had transmission rates of between 0.24 and 0.50 mg of oxygen per litre per month (equivalent to 0.002 and 0.004 cc per day). OTR for Amorim’s technical corks, Twin Top® and Neutrocork®, were 0.02 and 0.10 mg/L per month (0.0002 and 0.001 cc per day) respectively. Synthetic stoppers showed considerably higher OTR than cork closures and all reached the equipment measurement limit before the end of the 12-month trial. The findings have just appeared in the peer-reviewed Journal of Agricultural and Food Chemistry. Paulo Lopes, Cédric Saucier and Yves Glories from the Oenology Faculty of the Université Victor Segalen Bordeaux 2 measured OTR in Reference 1 natural corks (22, 24 and 26 mm diameters), Twin Top®, Neutrocork®, a colmated and an agglomerate cork, and two leading brands of extruded and moulded plastic stoppers. The trial did not include screw caps, however a further study with screw caps is scheduled. Lopes and his colleagues used a non-destructive method that measured the colour of an indicator solution in bottles stored lying down. The work was supported by Amorim as part of its research and development program. The Bordeaux results agree with those from the Australian Wine Research Institute’s (AWRI) long-term closure trial, which used analytical and sensory methods to assess the extent of oxidation in wine bottled under different closures. After 63 months, the AWRI figures showed that Reference 2 corks were only two to three times more variable than screw caps. Amorim Twin Top® closures were as consistent as screw caps. Most synthetic closures showed evidence of oxidation after 24 months. A recent report published by the Australian Closure Fund (ACF) quoting 1000-fold variation in the oxygen permeability of natural cork stoppers is based on the Mocon method. This is an established method for measuring oxygen permeability in ‘dry packages’. It is not a reliable indicator of cork’s permeability in wines stored under appropriate cellaring conditions, that is on their side or inverted.