It’s years since I did a scientific experiment but there is one I would like to see done with corks and screwcaps. It’s quite amazing that even with a more than a decade’s experience of bottling wines under screwcaps, the long term effects of the type of closure remain controversial. One issue that I think should be finally resolved is just what effect exposure to oxygen has on long term maturation.

When the same wine is bottled both under cork and under screwcap, it’s evident within a few months that they develop differently. Most of the comparative tastings that I have done have been with white wines, where the usual difference is that wine under screwcap retains brighter fruits with more evident freshness. Preferences are usually split at such tastings between the bottlings: some people prefer the fresh, young style of screwcaps, while others find more complexity in the greater development of the wine under cork.

When I was out in New Zealand and Australia earlier this year, I had several opportunities to compare older red wines that had been bottled under both types of closure. The results were completely consistent.  The wine under screwcap always seemed younger – in a blind tasting you might have said by a couple of years – with more primary fruits, whereas the wine under cork showed some development towards more savory, sometimes even tertiary, aromas and flavors. All of these wines were Pinot Noir (mostly from the first few years of this millennium) but I assume the results would be generally true for all red wines. (The tastings are described in more detail in my recent book, In Search of Pinot Noir). This may not be a completely fair comparison, because the reason for the switch to screwcaps was often the terrible condition of the corks available down under. In fact, when there was the opportunity to taste multiple bottles under cork, they often tasted as different from one another as they did from screwcap, an immediate validation of the decision to switch to screwcaps.

So wines under screwcap clearly develop more slowly: the question in my mind is whether they develop in the same way at a slower pace or whether the overall pattern of  development is different, reflecting a different relative timing of the loss of primary fruits and the appearance of tertiary flavors. Among the wines I tasted, when the wines under cork were in perfect condition, I generally preferred them: but that may be because my taste generally runs to older wines. I would be really interested to repeat the comparison in a few years when the screwcap wines have developed further to see which I prefer then.

Anyway, back to the experiment. The difference between screwcaps and corks is the rate with which oxygen gets into the bottle. It can be close to zero for screwcaps: in fact, there have been problems involving reduction for wines bottled under screwcap, just as damaging in their way as problems with oxidation for wines with faulty corks. Sulfur levels need to be reduced when bottling under screwcap and it may take a while to establish the most appropriate levels for wines intended for any aging. The very best corks (defined operationally as the tightest) have very low oxygenation levels close to those of screwcaps. But the unanswered question is whether you do actually need some level of oxygen exposure; for corks this comes from the supposed “breathing” of the cork; for screwcaps it could come in the future from liners with defined rates for passage of oxygen.

So the experiment I want to do is to determine definitively whether the difference between corks and screwcaps is solely due to oxygen exposure. It is very simple in principle. Bottle a (red) wine under both cork and screwcap. Take bottles with each type of closure and keep one set in normal cellar conditions (cool, dark, humid). Put the other set under identical conditions but in an atmosphere of nitrogen. Then see whether the wines with cork and screwcap closures develop in exactly the same way under nitrogen (which is what you would expect if oxygen is the sole relevant factor). And of course see what differences emerge with and between the wines in the normal cellar. All that’s required is a cellar filled with nitrogen (and I suppose a means of retrieving bottles for periodic testing). Then finally we would know the answer instead of speculating and arguing about it.