First Results of the Australian Wine Research Institute's Wine Bottle Closure Trial
The Australian Wine Research Institute (AWRI) announced the first publication of results arising from the AWRI trial of the technical performance of various wine bottle closures. The findings were published in the June 2001 issue of the Australian Journal of Grape and Wine Research (AJGWR) and are considered to be of major importance to the wine industry as a whole and to closure manufacturers and suppliers.
A Semillon wine was bottled under controlled conditions on 26th May 1999, using 14 different closures: a screw-cap type, two grades of conventional cork, two types of 'technical' cork (natural cork with a synthetic component), and nine closures manufactured from synthetic polymer material. During the 20 months following bottling, the physical aspects of the closures' performance, and the chemical and sensory changes in wine sealed with each of the closures has been assessed periodically. The study is ongoing, with sufficient wine bottled for up to 10 years of testing.
It is apparent from the results to date that many of the closures trialled are suited to short term (approximately 6 to 12 months) storage of wines, but for longer time periods there is doubt over particular closures' abilities to act as an adequate seal, with consequent effects on sulfur dioxide (SO2) levels in the wine, browning, and on oxidized aroma. No one closure tested in this study could be considered entirely suitable by all criteria assessed, for the long-term storage of wine.
Wine sealed with the screw cap retained the greatest concentration of SO2 and ascorbic acid, and had the slowest rate of browning. For other closures, the trend of SO2 loss relative to wine sealed with the screw cap was apparent from an early stage, and was most evident in the group of synthetic closures, intermediate in the conventional corks, and least evident with the technical corks. The dwindling concentration of SO2 was a strong predictor of future browning in the wine.
After 12 months of storage, wine in bottles sealed with the closures showing the lowest SO2 concentration were rated as sufficiently high in oxidized aroma to consider them as markedly lower in quality. For closures with intermediate SO2 concentration at this time, there were diminished ratings in the fruit sensory attributes, and increased developed aroma. Whilst TCA taint was a noticeable problem for some cork and technical corks, any plastic-type taint appeared not to be a problem with most synthetic closures, with one notable exception. After 18 months of storage a rubber-like aroma had developed in wine sealed with the screw cap closure. This has been attributed to the placement of a rubbized insert inside the screwcap to aid in sealing. Critics pointed out that there are plenty of other insert materials that could be used in screwcap manufacture that would solve this problem.
The closures differed widely in regard to physical characteristics, and in general synthetic corks appeared least 'consumer-friendly' in terms of the forces and energy required to extract them from the bottles and from a corkscrew, and in the ease of manual reinsertion into the bottleneck.
For a range of performance criteria, the technical corks were found to exhibit less variability than synthetic closures which in turn tended to be less variable than the natural cork closures.