Natural corks appear to mimic aspects of the winemaking practice of micro-oxygenation, according to new research.
Micro-oxygenation (MO) is a technique used by winemakers to soften tannins, improve mouthfeel and colour and remove undesirable reductive characters.
Because a cork will slowly allow very small amounts of oxygen into the wine after bottling, it has long been held that this property helps the wine to develop and age gracefully, apparently for the same reasons as those put forward in favour of MO. However, until now, researchers have not had a reliable method for measuring the amount of oxygen that enters the wine after bottling and for comparing different types of closures.
With support from Amorim, researchers at the University of Bordeaux have developed a non-destructive method for measuring oxygen ingress into bottled wine. The method depends on the colorimetric measurement of a dye that becomes colourless as it oxidises.
The Bordeaux team recently published the results of three years of their measurements, comparing the oxygen transmission rates (OTRs) of cork stoppers with both synthetic stoppers and screwcaps.
The picture that has emerged puts natural cork into an intermediate position in terms of oxygen transmission rates. The natural corks allowed between 0.1 and 2.7 microlitres a day during the period from 12 months to 36 months. In comparison, over the same period, the synthetic stoppers let in much more oxygen while screwcaps let in noticeably less.
Research by the Australian Wine Research Institute has suggested that reduced or 'rubbery' aromas can develop in wine sealed under screwcap as a result of the closure's low OTR. The same researchers found that wines under synthetic stoppers tend to lose fruit attributes and develop oxidised or 'wet wool' aromas.
Cork, it seems, appears to allow some, but not too much, oxygen into the wine, preserving fruit intensity and minimising the tendency for reduced characters.
Wine experts and consumers already have a layman's appreciation of how a wine develops under cork over a period of years. But the scientific basis for this process is not well described. For example, it is not clear how oxygen actually enters a cork-sealed bottle.
Amorim is funding research to gain a better understanding of oxygen entry — for example, to determine whether oxygen diffuses through the cork from the atmosphere or comes from within the cork itself.
As with micro-oxygenation in the winery, researchers have only just begun to explore these and other issues. Amorim's research and development director, Dr Miguel Cabral, says it is imperative the cork industry supports such research.
“Amorim believes further research on oxygen permeability will lead to the development of improved cork closures that will give the winemaker more precise control over post-bottling wine development,” he said.
“Cork is a complex material and it would be erroneous to suggest that we understand all of its unique properties as well as we should. We have learned from its long history that cork is wonderfully suited as a wine seal but we do not know all the reasons why.”
For more information: P Lopes, C Saucier, P-L Teissedre and Y Glories. 'Impact of storage position on oxygen ingress through different closures into wine bottles,' Journal of Agricultural and Food Chemistry, 54(18), 6741-6746, 2006.
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