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

Wines & Vines, August, 2007 by Jamie Goode

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, cab-bagey, 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.