Tainted choice
Published: 1st May 2007
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The debate on closures must seem perplexing to an outsider. Surely putting a stop in a bottle is a simple process? Jamie Goode lifts the lid on this fast-changing issue
In this age of iPods and Black Berrys, devising novel ways of closing a wine bottle that avoid the risks associated with natural cork should be a trivial matter. But the subject of closures is a complex one. To do it justice requires some fairly heavy wine chemistry, plus a soupçon of physics.
Cork has been the dominant closure type since wines were first bottled at the end of the 17th century. It has been the one-size fits-all-closure for most of the past 300 years and, considering this is an old technology, it has done a good job.
But cork has one flaw: it can become infected and impart a musty taint to a proportion of bottles that it seals. This is caused principally by two, four, six-trichloroanisole - more commonly known as TCA - and it is the curse of the cork industry. Humans are extremely sensitive to this compound, produced by fungi living in the gas exchange pores of the cork oak bark, and most people can detect it at concentrations as low as five parts per trillion (ppt, the same unit as nanograms per litre). This makes it hard to eradicate. To give a better idea of this figure, it's equivalent to one second in 64 centuries.
It's likely that cork taint has always been an issue but it has increasingly been perceived as a serious problem over the past few decades. Perhaps this is because the rate has been increasing - enhanced demand for cork could have led to standards slipping and the use of poorer quality material - or perhaps it is only with increased consumer quality expectations and better winemaking that the real taint rate has become apparent.
Either way, the percentage of tainted bottles documented in large surveys, such as the International Wine Challenge, has, at around a little over 4 per cent, been far too high .
This has been the driving force for seeking alternatives, with an extra push coming from the variable nature of cork which can result in sporadic cases of oxidised wine and bottle-to-bottle variation.
Early experiments
Alternatives to cork aren't new. In the 1970s the Australian wine industry had a brief flirtation with screwcaps : with favourable outcomes for a couple of trials , a number of screwcapped wines were released commercially. Ultimately, though, this experiment failed because consumers weren't ready. They continued to associate screwcaps with cheap wine.
The fact that many of these 1970s screwcapped wines have survived in good shape indicates that there was nothing wrong with the technology. As an aside, Switzerland adopted screwcaps back in the early 1970s: these were a commercial success and have been widely used ever since for many Swiss wines (predominantly for easy-drinking whites from the Chasselas grape).
It wasn't until the mid-Nineties that the next development with alternative closures occurred: the synthetic cork. Th is was hailed as a major breakthrough in the fight against cork taint and, while many questioned the aesthetics of sticking a bit of plastic in the neck of wine bottles, it w as widely adopted , even for some high end wines. Before long, though, it became clear that wines sealed with synthetics weren't lasting as long as expected, showing signs of premature oxidation. There were also some problems with extraction forces: for some of the formats, it took a heroic physical effort to pull them out.
While plastic corks were creating a good seal in terms of keeping the wine in, plastic allows quite high levels of diffusion of oxygen, hence the oxidation problems. In fairness, a lot of work has since been done by synthetic cork manufacturers, to the extent that many of the products on the market now have much better aesthetics, are easier to extract and reinsert, and have improved oxygen transmission properties. But not all plastic corks are the same: in Italy, for example, there are dozens of synthetic brands , many of which are made by plastics companies which previously made shoe heels and have lost this business to the Far East.
By the late Nineties, consumer preferences had shifted enough to allow a return of screwcaps, at least in certain markets. In 1999 Australian wine company Orlando released one of its top Rieslings in both screwcap and cork-sealed versions, deliberately choosing a high-end wine to address consumer perceptions of screwcaps. The former sold out first. In 2000, 14 Clare Valley producers banded together and bottled their Riesling s with screwcaps. The following year, screwcaps really caught on in New Zealand; now they are the predominant closure type in New Zealand and Australia.
Screwcap revolution gathers pace
Screwcaps have, in some markets at least, finally shed their image of being for cheap wines only. Even resistant markets such as France have seen a number of wines switch to screwcap , led by Michel Laroche in Chablis and subsequently a scattering of producers in Alsace and Burgundy.
For a while it looked as if screwcaps would sweep the board. The advantages of this closure are immediately apparent : inexpensive, easy to use , easy to reseal, consistent and allows very little oxygen transmission , thus keeping wines fresh for longer. But there have been complications.
The first is the "reduction" issue. The oxygen transmission properties of screwcaps are determined by the composition of the liner in the cap, which is held tightly in place over the rim of the bottle. This is where it gets a bit technical. The majority of screwcaps have a metal layer in the liner which is then covered by a thin PVDC film. These are the ones with a silver appearance and they allow very little oxygen transmission. The others have a liner made of a material known as saranex, which is white and allows a bit more oxygen transmission. When the metal liner is used the bottled wine develops what is called a low redox potential (because hardly any oxygen is getting into the bottle) and, under these conditions, some jiggling around with sulphur compound chemistry can occur such that below-threshold levels of one smelly sulphur compound can change to above-threshold levels of another, more reduced, even more smelly sulphur compound.
This is known as reduction. At its worst, it can impart an odour of eggs; more commonly it will result in a burnt match, rubbery or struck flint characteristic on the nose. Reduction isn't unique to screwcap-sealed wines; it's just that, in large studies looking at closure performance, some low-level reduction has been a consistent result in wines sealed with screwcaps with a metal layer.
How much of a problem is screwcap reduction in the real world? It's hard to be sure. Reports likening it to cork taint are way off the mark: for a start, cork taint is always a fault, whereas a little bit of low-level reduction won't be spotted by almost all consumers and in some contexts can add complexity. To an extent, it can be avoided by careful winemaking and adjusting the procedure at bottling. Still, it's clear that working with a low redox closure such as a metal-lined screwcap requires a degree of caution.
The second issue is the change that has occurred in opinions of whether any oxygen transmission through the closure is needed for successful ageing of wines. It is now clear that a closure that seals hermetically, with no oxygen transmission whatsoever, is undesirable - this would be asking for trouble from reduction, as just described.
On the other hand, closure that allows too much oxygen transmission results in prematurely developed and then oxidised wine. And, as the results of the extended closure trial by the Australian Wine Research Institute have shown, wines sealed with closures that have different oxygen transmission levels will develop quite differently in bottle, all other things being equal.
So here's a new world view: closure choice is a winemaking decision, and winemaking continues after bottling. Winemakers need to carefully select the right closure : designer closure use, if you like. The problem is that our understanding of just how much oxygen is required by a wine post-bottling is still in its infancy: this is something winemakers need to experiment with.
The third issue is that there now exists a range of alternative closures which offer consistent, taint-free performance . Notable newcomers include: D IAM, from Sabaté - a closure made of granulated cork, cleaned of any taint by supercritical carbon dioxide then recombined with synthetic microspheres to make a taint-free cork (check www.oeneoclosuresusa.com); ProCork - a five-layer membrane applied to either end of a natural cork to reduce variability and eliminate taint (www.procork.com.au); Vino-Lok - a glass closure with a plastic O-ring as the seal, held in place with a metal cap (www.vino-lok.de); and Zork - a space-age plastic closure consisting of a cap that holds a liner in place over the bottle rim
(www.zork.com.au).
The final issue is that cork producers themselves are finally making a huge effort to clean up their act. They are working hard on quality control through the production process to prevent the development of TCA in batches of cork, as well as devising curative methods to remove any TCA that may be present in this cork.
There are reports that taint rates are lower than they used to be; if this is the case , the drive towards alternatives may lose some momentum.
Customised approach
As a consequence of all these issues, it looks as though the future for closures may well be one in which a range of closure types are in use, with winemakers selecting the best one for each particular wine. It may well be that cork will retain some of its market share and exist alongside a range of alternatives. How much business cork loses will depend on how successful attempts to eradicate taint problems are.
Screwcaps are certainly here to stay, although more work needs to be done on their liner properties. Plastic corks may well retain some market share, even though they've had some bad press: let's face it, they are cheap and most wines aren't going to be kept for more than a few months after bottling, anyway.
It's too soon to say how the various novel closure types will fare, but there's no reason why at least some of them shouldn't take off. It's an interesting time for closures, that's for sure. ?
n A member of the Association of British Science Writers, Jamie Goode has a PhD in plant biology and publishes a website, www.wineanorak.com. He has written two books: Wine Science (Mitchell Beazley, November 2005) and Wine Bottle Closures (Flavour Press, May 2006)