Wine Must Breathe, But Not Too Much
Over the past twenty years the wine industry has witnessed the introduction of many new bottle closures. The stated intention with the new closures was / is to provide an alternative to traditional, natural wine corks with the promise of improving an assortment of factors. Some allowed easier removal. Some present cost savings to the winery. Some promise elimination of the inconsistencies measured in natural corks. All these alternatives needed to fulfill the primary role of the closure – protecting the wine inside. In addition, a good closure must provide the proper environment that regulates the beneficial effects of oxygen.
With a few exceptions, alternative closures have been adequate at keeping wine in the bottle. The track record becomes less successful at managing oxygen from getting inside. Is not interesting how, for hundreds of years, there has been praise and appreciation for corks ability to “just control the right amounts of oxygen to reach the bottled wine”; by the act of Mother Nature it has become the gold standard of closures. The alternative closure market is still trying to find a way to control oxygen in the bottle to just the right amount then stop the process automatically.
Most wines benefit from small amount of oxygen. Too much, and the wine will “oxidize” and exhibit a range of flavors from a “loss of fruit tastes” to the development of “vinegar-like” characteristics. Too little oxygen can develop “reduced” characteristics. These often include sulfide aromas, which can run from the aroma of “struck matches” to the memorable essence of an angry skunk. In some cases the reduced aromas can be improved by aerating the wine before drinking. This is also called letting the wine “breathe”.
Before the introduction of alternative closures, most research on wine and oxygen was focused on the wine making processes in the cellar and barrel room. With the advent of alternative closures, the study of oxygen and wine now includes packaging options and bottling conditions.
In the March 31, 2011 issue of “The Telegraph”, Victoria Moore writes, “My view is that a large number of aromatic whites with screwcaps are slightly reduced and that it’s a phenomenon that’s largely brushed aside. There’s also a middle ground of which the effect of reduction is to dumb down the nose and give it a very particular taste. ”
Why this interest coincides with the introduction of alternative closures is because their advent demonstrated that the oxygen behavior of a natural cork is impossible to duplicate. Natural cork consists of millions of tiny “cells” – 40 million cells fit in one cubic centimeter. Each cell is filled with air, and that air provides a small dose of oxygen for each bottle.
Synthetic closures appeared to allow too much oxygen ingress, and are not recommended for wine that will spend more than 18 months in the bottle. Some screwcap liners are virtually airtight. Reduced aromas were noticed in some of the wines using this alternative. Manufacturers have tried to compensate for these shortcomings by adjusting the oxygen permeability of their products. In the case of synthetics, there are now options promising reductions in the rate of permeation. For screwcaps, there are options that replace the airtight tin liner with a plastic disk that allows for oxygen permeation.
Both solutions are problematic because the correction involves oxygen permeation. Permeation is like filling a bucket with a garden hose that can not be controlled or even turned off. You can adjust the rate, but once established, the rate will not change. Natural cork does not involve permeation. Cork introduces a finite amount of oxygen, and when complete, the oxygen entry ceases. The compression of the cork in the bottle and saturation of the cork seals to container. In reality, cork is a wood of the oak tree species that also offers tannin values.
Using the visual image of the garden hose illustration, it is important for wine that is to be stored for several months not to be constantly being inundated with oxygen; kind of the magic of cork.
Natural Cork – Diffusion
Oxygen ingress with natural corks is primarily a result of diffusion. A typical 44mm cork contains an estimated 3.5ml of oxygen. When the cork is compressed the internal air pressure increases to between 6 and 9 atmospheres. (1 atmosphere = 14.6 psi- thus 6-9 atmospheres equals 88-132psi.) Cork compression is from 24mm to 15.5mm, then expands to 18mm to fit inside bottle neck.
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.
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.
Ms. Jancis Robinson, MW has written a review of all closure options and does state a preference for natural cork on ascetic reasons and does go into specifics on what some oxygen permeation is needed to make for a good wine. “We still do not know enough about exactly how different fine wines will age under screwcaps to be able to give very precise advice … It is thought that red wines destined for long ageing may require a greater degree of interaction with oxygen than is currently permitted by screwcap, though further research is needed in order to give definitive advice, “says Robinson.
In a study done by Lopes, P .; Saucier, C .; Teissedre, PL Glories, Y. which appeared in J. Agri. Food Chem, 2006 their study of various closure on Oxygen Ingress by Closure Type noted that a premium natural cork allowed small amounts of oxygen to come into contact with the wine for approximately 12 months then remained sealed to oxygen ingress for the test period of 3 years . In the case of screw caps oxygen interaction was for a period of approximately 2 months and then virtually no ingress appeared. Synthetic corks allowed oxygen ingress at ever increasing levels with no measureable sign of abating.
For hundreds of years, cork has been a preferred closure without any understanding of why, except it was easy to install and it did keep wine sealed naturally. With the advent of synthetic closure options, science started taking a look at wines interaction with these closures and “lo and behold” some oxygen became recognized as a benefit to wine’s aging qualities; but not to much and / or to little. Whether oak barrels or cork, oxygen plays an important role in the ways these natural materials impact fine wine making.