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

Wines & Vines, August, 2007 by Jamie Goode

Exposure to oxygen through wine-making 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."