Superior Wine Aging
Virtually every "great" wine of the past two centuries has been finished with natural cork. The reasons for this tradition are largely due to the physical properties that protect bottled wine under a variety of storage conditions. Natural cork also provides an important role in a wine's successful maturation process.
Wines finished with natural cork undergo a predictable process of aging that results in softened tannins, added complexity and the creation of bottle bouquet.
Because most "great" wines are evaluated with the understanding that a predictable aging process will be realized, natural cork remains an important component of modern wine.
History of Cork Usage
Natural cork has been associated with the storage of valuable foods and beverages for thousands of years. Ancient Egyptians, Greeks and Romans referenced cork as a preferred material for stoppers used with wine and olive oil.
The most significant development occurred in the 1600s, when Dom Pérignon, developed his methode champenoise. The wooden stoppers used to store still wines had considerable disadvantages when applied to sparkling wine. Dom Pérignon successfully adopted cork stoppers and cork soon became essential for wine bottling.
Over the next two centuries the spread of mass-produced glass bottles and standardized neck dimensions greatly advanced the use of cork, not just for wine, but a wide range of liquids and foodstuffs.
Cork Physical Attributes
Natural cork provides unique physical properties that are perfectly suited for preservation and development of fine wine. Cork's intricate cell structure, joined together like minute "honeycombs", creates a material that is compressible, resilient and impervious. These physical properties of cork have not been duplicated by man-made materials.
Natural Cork has the ability to adapt to the vagaries of bottle shape and size. It is equally capable at withstanding environmental changes in heat, cold and moisture. This benefits the wine by protecting against leakage and early oxidation and allows the wine to gain complexity and bouquet throughout its storage life.
Controlled Oxygen Ingress
Natural Cork – Diffusion
Oxygen ingress with natural corks is primarily a result of diffusion. A typical 44mm cork contains an estimated at 3.5ml of oxygen. When the cork is compressed the internal air pressure increases to between 6 and 9 atmospheres. This establishes a pressure imbalance that is solved by the gradual equalization of gasses between cork and headspace.
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The exchange of gasses explains why studies of oxygen ingress show that bottles with natural cork “pick up” a small amount of oxygen over the first 6-9 months of aging. After that, oxygen ingress is no longer significant (the referenced study ran for 36 months).
Variations in oxygen diffusion between corks appear in the first six months of storage and likely reflect differences in cellular structures. After the initial diffusion period, additional variation was not observed.
Alternative Closures – Permeation
Artificial closures provide oxygen ingress primarily through permeation. Oxygen passes directly through the closure from the outside air. This can happen at a controlled rate, but unlike diffusion, the permeation does not stop. Oxygen continues to enter the bottle at whatever rate is determined by the closure.
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The majority of synthetic closures will exhibit significant oxygen permeation within 18 months. Some manufacturers now advertise optional products with reduced permeation rates. Recent developments with screwcap manufacturers show an interest in fitting screwcaps with a permeable seal, so that more oxygen can be introduced into the wine.
In both instances, the mechanism for oxygen ingress is by permeation of outside air. This will occur for the duration of wine storage.
Lopes, P.; Saucier, C.; Teissedre, P.L.; Glories, Y. Impact of storage position on oxygen ingress through different closures into wine bottles. J. Agric. Food Chem. 2006. 54.6741-6746
The Best Solution
A permeable closure is fine if you know how much oxygen you want to introduce and you know how long it will take for that to occur. In the absence of that control, a natural cork is the preferable solution. Natural cork introduces a small amount of oxygen over the first 6 to 9 months. After that, additional oxygen ingress is virtually nonexistent, and if stored properly the wine will develop without reductive or oxidized flavors.