The last decade has seen a revolution in the dull-sounding but vitally important field of wine bottle closures. Ten years ago, cork was still pretty much the universal closure, although the murmurs of dissatisfaction about its poor performance, with unacceptably high taint rates, were getting louder. The most progressive winemakers were beginning to try out alternatives, and back in 1997, most people were backing plastic corks as the taint-free alternative to succeed cork.

Screwcaps weren't yet on the radar: they'd been trialed in Australia in the 1970s and had been abandoned because of poor consumer acceptance. It wasn't until 1999 that Orlando's Phil Laffer reintroduced them for a high-end Riesling in Australia, offering consumers a choice between the same wine in screwcap and cork: The former sold out long before the latter. The following year, a group of 14 vignerons from the Clare Valley banded together and released their Rieslings under screwcap. This prompted a mass migration to this closure in both Australia and, even more strongly, in New Zealand. It became clear that in some markets, notably Australia, New Zealand and the UK, there was little consumer opposition to this novel way of sealing wine bottles.

Now cork's major opponent was no longer plastic corks, but screwcaps. The battle lines were drawn between those who still championed cork, and those who insisted that all wines should be sealed with screwcap. Screwcaps had established themselves as the key alternative to cork philosophically, if not yet in volume terms--plastic corks currently still sell significantly more worldwide than screwcap, but they lack the same sort of advocacy that screwcaps enjoy.

A Matter of Balance

But with the widespread use of screwcap, some technical issues have emerged, surrounding post-bottling sulfur chemistry, known more commonly as "reduction" in the trade. It's hard to discuss these dispassionately, because such is the volume of the war of words between advocates of screwcap and cork that these discussions rapidly get fanned into flames. Add to this that the subject matter itself is horridly technical, and the fact that we don't have all the data we'd like, and there's a need for calm, balanced treatment of these issues, which is what I'm attempting with this article.

The issue under discussion, screwcap reduction, first came to light in the closures study by the Australian Wine Research Institute (AWRI), which commenced in 1999. "It can be argued that closing the bottle remains one of the greatest technical issues facing the wine industry," suggested lead authors Peter Godden and Leigh Francis in the report's introduction. "The winemaker can control many aspects of wine production to create a wine suitable for the marketplace, and yet there can be an unpredictable incidence of problems once the wine is bottled, due in large part to the properties of the closures used."

The same wine, a respectable Clare Valley Sémillon, was bottled using 14 different closures, and followed with regular chemical and sensory analysis. The results after 21 months in bottle were published in 2001, and showed that while the screwcapped wine kept fruit freshness and retained free sulfur dioxide the best of all the closures, it also suffered from a sensory defect described by the expert tasters as "rubber/struck flint." This was a surprising finding, and caused a great deal of head scratching and anxiety within the trade.

Shortly after publication, Godden had this to say about it: "We are very confident that the 'rubber-like' character is not a taint, but is an unwelcome modification due to chemically reduced sulfur, as a result of lack of oxygen. However, it is certainly an important character in screwcap-closed wine, and we have highlighted its existence to avoid mass-bottling of wine under extremely anaerobic conditions which might then develop a similar character somewhere in the future." This reduction was still evident five years post-bottling with the Sémillon used in the trial.

What Causes Reduction?

The AWRI results raised a number of questions. What is the explanation for this reduction? Was this problem specific to this wine? How much of an issue is it with real-world wines being drunk by consumers? And what does it suggest about the role of the closure in wine development?

The prevailing view at the time was that the ideal closure would be one that seals hermetically, allowing no oxygen transmission at all. But a further trial from AWRI seemed to indicate that an anaerobic closure is entirely unsuitable for wine. This trial involved comparing a Chardonnay wine sealed three different ways: with cork, with screwcap and then hermetically sealed in a glass ampule. The Chardonnay wine underwent some development in all three cases, but with the screwcap there was a bit of reduction and with the ampule a lot. Another study, this time looking at a Penfolds Bin 389 red wine sealed with synthetic cork, natural cork and screwcap, also encountered some reduction in the screwcapped bottle.

We need to pause here for a technical aside. The screwcap itself is just a means for holding a liner in apposition to the rim of the bottle. It is the properties of this liner that determine the oxygen transmission levels of the closure. Currently, two different liners are available for wine. The first, and most widely used, is called "tin/Saran." This consists of a Saran backing, with a metal layer (usually tin, but it can be aluminium) that is covered by a thin layer of PVDC, which is in contact with the wine. This metal liner allows very little oxygen transmission at all. It is the liner used almost universally in Australia and New Zealand. In the U.S. and Europe, it is still the most widely used liner, but you are also likely to encounter the alternative liner, known as "Saranex only," and consists solely of Saranex. This allows more oxygen transmission, and is not implicated in the screwcap reduction story.

Back to the issue of reduction. What is happening here? It all has to do with the issue of sulfur-compound chemistry. During fermentation, yeasts can produce sulfides. This occurs when they are stressed: for example, when there is low must nitrogen they may turn to the sulfur-containing amino acid cysteine as a nitrogen source, resulting in sulfide production. They can also produce sulfides when they are forced to operate anaerobically or are temperature-stressed, which is why good winemaking practice is to monitor juice nitrogen status, give plenty of aeration to fermenting musts and avoid dramatic shifts in temperature. In the worst case scenarios, reduced wines smell of hydrogen sulfide, which gives them an eggy, dirty-drains aroma. Hydrogen sulfide can be oxidized to mercaptans (also known as thiols, and smelling of burned-match, rubber, earth or cooked cabbage), which can be oxidized to dislufides (described variously as having aromas that are vegetal, cabbagey, garlic-like, rubbery or onion-like).

Mercaptans can also arise by other means. The presence of these sulfur compounds in wine is known as "reduction," but this term is actually a bit of a misnomer, and can lead winemakers into trouble when they respond to their presence by giving the wine oxygen: This is because it is possible for part-oxidized wines to have these aromas, and giving even more oxygen to them clearly wouldn't be a good idea.

There's another complication here. Some of the sulfur-containing compounds we're talking about can be bad in some contexts and good in others. The best example of this is in Sauvignon Blanc, where a range of thiols have been implicated as being positive contributors to the aroma of wines made from this variety. In the right context, a little bit of thiol can be positive--a complexing factor.

Redox Potential

So why is reduction a problem that's associated with screwcap use? It's to do with a concept known as redox potential. This is a measure of how oxidative or reductive a system, such as a wine in barrel or bottle is, and it is measured in millivolts (mV)--the higher the reading, the less reductive. Typically, an aerated red wine will have a redox potential of 400--450 mV, whereas storage in the absence of air for some time will reduce this to 200--250 mV. If levels get as low as 150 mV, then there is a danger that reduction problems can occur.

Exposure to oxygen through winemaking practices such as racking, topping up barrels and filtering, increases the level of dissolved oxygen in the wine and increases the redox potential, which will then return to 200--300 mV.

In white wines, this redox level will change much more rapidly than in red wines, because red wines have a higher concentration of phenolic compounds such as tannins that are able to interact with oxygen, and act as buffers. Another variable here is the level of free sulfur dioxide in the wine, which will act protectively by reacting with the products of oxidation.

Yeast lees also scavenge oxygen and protect the wine in a similar fashion, helping to lower the redox potential and create a more reductive environment. In modern winemaking, reductive conditions are encouraged: The protection of wines from oxygen by the use of stainless steel tanks and inert gases helps to preserve fresh fruit characters. These reductive conditions--those in which oxygen is more or less excluded--can also favor the development of smelly forms of sulfur compounds.

Post-bottling, the redox state of the wine will be influenced by a number of factors, including the state of the wine at bottling, the free sulfur dioxide levels, the oxygen pick-up during the bottling procedure, headspace extent and composition (air or inert gas), and the oxygen transmission by the closure. As we've seen, reduction seems to be a problem in these sorts of analytical studies involving metal-lined screwcaps, and the obvious explanation is that the low redox environment of the screwcap-sealed wine is causing some unwanted sulfur chemistry to occur, with sulfur compounds shifting from a less smelly (and thus unnoticed) form to a more smelly (and thus noticeable), more reduced form. This is assuming the wine is bottled clean, of course.

A Minor Technical Problem?

What are we to make of screwcap reduction? Is it a real world problem on a par with cork taint, or is it just a minor technical problem--a teething issue that just needs a bit of tweaking? The latter position has been the one consistently adopted by proponents of screwcaps.

Since the publication of the first AWRI report in 2001, there has been just a trickle of data on the subject of screwcap reduction. But little by little, a clearer picture has emerged, and the current weight of evidence suggests that the issue of mercaptans in screwcapped wines is problematic enough that some caution should be exercised in their use. Winemaker (Stonecroft, Hawkes Bay) and Ph.D. chemist Alan Limmer has been a bit of a thorn in the side of the screwcap lobby. He has written widely on the subject, bringing his knowledge of wine chemistry to bear. In particular, Limmer has pointed out that screwcap reduction is not a problem that can be completely eliminated by better winemaking, as many have claimed.

"In essence we are talking about thiol accumulation, post-bottling, from complex sulfides that do not respond to pre-bottling copper treatment," Limmer claims, in response to the assertion that fining with copper removes reduction defects. "This reaction occurs to all wines containing the appropriate precursors, irrespective of closure type. But the varying levels of oxygen ingress between closures leads to significantly different outcomes from a sensory point of view."

Limmer's explanation for screwcap reduction is that sulfides present in the wine at bottling necessitate a very small level of oxygen ingress through the closure, otherwise they can become reduced to thiols. Because sulfides are less smelly, it is possible for a wine that is clean at bottling to taste reduced after bottling if the closure doesn't permit enough oxygen ingress. So the use of a closure, such as cork, which does allow a little oxygen ingress (but not too much) is a necessary concession to the vagaries of sulfur chemistry.

Of course, we'd rather not have the sulfides in the wine at all, which would then avoid problems with reduction to mercaptans at a later stage. But, as Limmer points out: "Controlling ferments to not produce the complex sulfides is beyond our means currently. This sulfide behavior of the ferment is more controlled by the yeast genetics than the winemaker," he explains.

"It is not the winemaker's fault these compounds exist in the wine at bottling. We can minimize it to some extent by providing optimum nutrient conditions for the ferment, and employing some specific winemaking regimes. But, the research tells us this only has a slight impact on the complex sulfide pattern produced by the yeast." Limmer reinforces his point: "The patterns are quite specific to each yeast type, almost irrespective of nutrient conditions. Every wine contains these complex sulfides."

Keep it Clean

But others think that care taken during winemaking, and particular at bottling, can reduce the risk of any reduction significantly. Wise counsel would be that winemakers intending to use tin-lined screwcaps should ensure that they are in control of bottling parameters. It is important to get the wine clean before bottling, without any trace of mercaptan or disulfide. A healthy ferment should be the first priority.

When it comes to bottling, free sulfur dioxide levels that are a little lower than those used with other closures are advisable, as is a knowledge of the variation in oxygen pick-up on the bottling line used. It should be emphasized that allowing more oxygen pick-up or leaving a larger headspace of air does not counter post-bottling reduction, but rather results in oxidation.

Copper fining can help in some circumstances: This certainly gets rid of mercaptans, but it doesn't eliminate disulfides which, as we have seen, can revert in a low redox environment to mercaptans. Besides, copper fining will also remove the desirable sulfides which are important for varietal character in Sauvignon Blanc and other grapes.

Do Consumers Notice?

The extent of screwcap reduction is currently unclear. The only large survey to look for it, the faults clinic of the UK-based International Wine Challenge (IWC), reported that 2.2% of screwcapped wines suffered from mercaptan problems in 2006, and in 2007 provisional results were slightly up, at 2.7%. This sounds alarming, but it should be borne in mind that cork taint irredeemably ruins bottles it affects, while very few consumers will have a problem with low-level mercaptans in their wines.

I doubt that most of the wine trade would spot this as a problem in all but the most extreme cases, so it is unfair to equate it with the very well recognized problem of cork taint. Having said this, though, screwcap-sealed wines affected by mercaptans should be a major concern for winemakers, because the closure is modifying the flavor of the wine, which is emphatically not reaching the consumer "the way the winemaker intended."

It would be dangerously complacent for the industry to take the view that if the consumer doesn't notice it, then it doesn't matter. How do you spot low-level mercaptans? "They impact from an organoleptic perspective towards the end of the palate," Limmer says, "imparting a 'mineral' or bitter/hard/astringent aspect. This has the appearance of shortening or closing up the palate, so the wine does not display a fine, fresh, long finish, but ends abruptly, and somewhat harshly."

The picture emerging is a complex one, but a simplified "screwcaps taint wine" message would be dangerous if it caused producers to back away from adopting alternative closure solutions, which would then have the secondary effect of removing any incentive from the cork industry to put its house in order and do all it can to reduce taint levels. However, complications like this reduction issue should put pressure on winemakers to be more curious about the closures they are using, and pay more attention to the potential loss of quality that can occur during the bottling process.

(London-based writer Jamie Goode is the publisher of wineanorak.com and specializes in wine science issues. His first book was published in 2005 by Mitchell Beazley. To comment on this article, e-mail edit@winesandvines.com.)

quelle: http://winebusiness.com