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Cork: Unsung Hero or Villain?
In some senses, cork is both. Its use was at least partially instrumental in the discovery of wine’s aging potential, but recently became the source of a major wine fault.
Cork’s role in unmasking wine’s aging potential, a sine qua non of wine quality, is little recognized. For wine to avoid turning to vinegar, wine must be stored in the absence of oxygen. Otherwise, acetic acid bacteria metabolize the wine’s alcohol and residual sugars to acetic acid. Protection from oxygen requires both a container and a closure that are gas and liquid impermeable, as well as inert. Cork not only provides an impermeable and inert closure, but it also compresses without lateral distortion, springs back to shape almost instantaneously, and retains its elastic adherence to the opening it closes for decades.
Since at least Etruscan times (circa 600 B.C.), cork has been used to seal wine amphoras. Stoppers were cut out of slabs of cork oak bark, their edges usually being coated with pitch to augment adherence in the neck. Pine pitch was also applied to the amphora’s inner surfaces to make them waterproof. Impervious amphoras became available only during the Classical Roman period, when a technique was developed to generate a vitreous inner lining. By obviating the need for a pitch lining, expression of the wine’s inherent fragrance became possible, devoid of an overwhelming resinous flavor. Coincidentally, this provided conditions whereby an essentially hitherto unsuspected attribute of wine could shine unmasked – its potential to improve with age. When this property was first realized is unknown, but the fame of old wine was clearly recognized by Pliny’s time (23–79 AD).
With the decline of the Roman Empire, wine storage switched to wooden barrels. Although comparatively non-porous, enough oxygen often entered barrels for the wine to turn vinegary by summer, especially when transported over rough medieval roads. Cork bungs, if available, would have had little effect in limiting oxygen entrance.
When the full value of cork as a wine closure again became realized is unknown, but depended on technological developments in glass production (early to mid-1600s). The use of coal provided the additional heat and sulfur to produce strong glass. Thus, wine could again be stored in an inert, impervious container, and sealed with an elastic, gas-impermeable closure. The conditions were ripe for the rediscovery of wine’s aging potential. The first reports of wine being purposely aged occur shortly after the introduction of strong glass bottles (notably port shipped to England, and botrytized Tokaji wines exported to Poland and Russia). In the case of port, this is associated with a shift in bottle shape, from the initial onion-shape, to mallet-like, and finally the cylindrical-form typical today. The latter had the advantages of standing upright, could be laid on their side (maintaining the cork’s moisture content and elasticity), and conveniently stored. Slowly, additional benefits of bottled wine for transport and sale came to be realized, resulting in barrels being relegated to use as fermentation and/or maturation vessels.
The villainous side of cork began to be realized only in the 1970s. In all fairness, it was not the cork’s fault. It was a consequence of beginning to use a commercial pesticide (TCP, 2,4,6-trichlorophenol) in cork forests. Unknown at the time, several harmless, cork-dwelling fungi could detoxify TCP to TCA (2,4,6-trichloroanisole). TCA in contaminated stoppers could subsequently diffuse into bottled wine. Although present in only trace amounts (ppb), TCA can generate a pronounced, distinctive, moldy odor. Even more insidiously, TCA can suppress the activity of odor receptors, and at concentrations below those that generate its corky/moldy odor. Thus, TCA is doubly nefarious, potentially reducing even wines not noticeably ‘corked’ to flavorlessness.
Even when the origin of the off-odor was discovered (1980), the reluctance of the cork industry to accept its involvement meant that implementation of corrective action was slow. Even with termination of TCP usage, and other changes in cork processing, it can take years for trees to produce TCA-free cork – bark is usually removed on a 9-year cycle (but can be up to 15 years, depending on the growth rate of the tree). In addition, there are other sources of TCA, and another pesticide–flame retardant (tribromophenol) can be microbially modified to tribromoanisole, a compound with a corky odor similar to TCA.
In the interval before effective action was taken by the cork industry, alternative closures began to establish themselves in the wine industry, notably plastic stoppers and screw caps. The first plastic corks were too oxygen permeable to be used to close bottles housing wine benefitting from long aging. Newer plastic corks are better, but still not as good as screw caps. By contrast, screw caps are as good at limiting gas exchange as the best slow-grown cork (those with many, narrow, growth rings). In addition, the rate of gas diffusion using screw caps can be adjusted to winery specifications – by selecting among several cap liners. Finally, screw caps have considerable consumer appeal, being easy to remove and reapply (no corkscrew or pulling required). In contrast, cork, being a natural product, is inherently variable in gas permeability. Although visual inspection can detect surface faults, X-ray tomography is needed to discover internal faults. Consistent minimal gas exchange with cork stoppers is currently assured only with agglomerate cork (produced by gluing small pieces of cork under heat and pressure). In addition, TCA can be more effectively removed from the small chips of cork used to produce agglomerate cork than from whole cork.
Although cork stoppers are still preferred by most premium wine producers (for tradition, presumed superiority, or consumer expectation), there is growing acceptance of screw caps by leading producers. This has been aided by studies indicting no detriment during prolonged wine aging, and possibly the benefit of enhanced flavor retention. For the botanist in me, and my desire to encourage the preservation of natural cork-oak forests, I find a weakening in the demand for top quality cork regrettable. However, the practical side of me cannot deny the appeal of screw caps. Thus, I’m in the unenviable position of being “betwixt the devil and the deep blue sea.”
About the Author
Ronald S. Jackson received his bachelor’s and master’s degrees from Queen’s University and the doctorate from the University of Toronto. His time in Vineland, Ontario, and subsequently at Cornell University, redirected his interest in plant disease toward viticulture and enology. As part of his regular teaching duties at Brandon University, he developed the first wine technology course in Canada. For many years, Dr. Jackson was a technical advisor to the Manitoba Liquor Control Commission, developed sensory tests to assess the tasting skills of members of its Sensory Panel, and was a member of its External Tasting Panel.
Dr. Jackson has left his position as a professor and the chair of the Botany Department at Brandon University to concentrate on writing. He is allied with the Cool Climate Oenology and Viticulture Institute, Brock University.
He is also the author of Elsevier books Wine Science: Principles and Applications, Fourth Edition, which won the prestigious OIV Award in 2015, and the forthcoming Wine Tasting: A Professional Handbook, Third Edition.
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Additionally, please click one the links below to listen to Dr. Jackson’s lectures and learn more.
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