Wine closures: do we need a new screwcap liner?
In short, yes.
Screwcaps are great, but in all the discussion on the topic (which has generated a lot of heat), one thing people keep forgetting is that the screwcap is not the closure. It's merely a way of keeping a liner (the actual closure) in apposition to the bottle rim. A crown cap does the same.
Now this sounds geeky and pedantic, but it's actually incredibly important. The gas transmission properties of the liner are crucial, because this determines how much oxygen gets into the wine after bottling.
There are two liners used for wine. The most commonly encountered is one with a metal (tin) layer in its construction, as well as a layer of saranex, which allows very little oxygen transmission. This is the one used in Australia and New Zealand almost exclusively.
The other is saranex-only. This allows more oxygen transmission, by a factor of 10. It's commonly used in Europe, especially for more commercial wines.
How does cork compare? Corks vary in their oxygen transmission (OTR) properties, but typically they fall somewhere between the tin/saran liner and the saranex-only. For the technical minded, Jim Peck of G3 in California has published the following guideline figures based on his research with a technique known as MOCON.
Typical OTRs in air (cc Oxygen/closure per day)
Screwcap, tin/saran liner 0.0001
Screwcap, saranex liner 0.001
Natural cork 0.0005
Synthetic cork 0.005
What are the implications of these figures? Do we want any closure OTR? The answer seems to be yes, just a little, to avoid problems with sulfur compounds (reduction). Othwerwise, as little as possible seems to give the best results for most wines.
The tin/saran liner is risky. It doesn't really allow enough OTR to reduce the risk of reduction to an acceptable level. Winemakers often have to resort to copper fining when they use this closure, which isn't ideal. Otherwise, it's great for shelf life and keeping wines fresh for a long time.
The saranex liner is great for wines that are going to be drunk in the first three to five years after bottling. But it's not really suitable for wines that will be cellared for longer.
So we have a need for a liner with properties somewhere between the two, and this is just what Jim Peck and his colleagues at G3 have been working on. Their solutions? Microperforations in the tin layer of the liner.
They have developed a mathematical formula that allows them to predict the level of OTR depending on how many perforations are made and where they are located. This could be great news for winemakers who are currently unprepared to switch to screwcaps because of the risk of reduction, or for winemakers already using screwcaps who are unhappy making the foot fit the slipper in terms of preparing their wines differently to suit the closure they are bottling with.