Reductive reasoning: getting to the bottom of 'reduction' problems in screwcap wines

Wines & Vines, August, 2007 by Jamie Goode

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 Semillon, 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 Semillon 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.