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.
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.
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.
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.
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