Additionally, since there is no “one size fits all” answer on the use of sulfur dioxide in wine, it is important to have an understanding of the chemistry of sulfur dioxide and how it reacts in a given wine before it can be used properly. The subject of chemistry can be daunting for those who have not studied it since high school and the chemistry of sulfur dioxide in wine is no exception. Because of this, some professional winemakers have only a basic knowledge of how sulfur dioxide reacts in wine and the different forms that it takes.

Oxidation is the term used by chemists to describe when an element or compound, such as sulfur, loses electrons. While oxidation reactions do not necessarily have to have the presence of oxygen to occur, they often do because when oxygen reacts with an element or compound it readily accepts electrons.

The burning of sulfur in the air oxidizes it and produces SO2 in the chemical reaction: S + O2 = SO2.

Sulfur dioxide gas has a sharp pungent aroma that smells like a burnt match, this is hardly surprising because match heads contain sulfur and when they ignite, they release SO2.

In a chemical reaction where sulfur gains electrons it is said to be reduced. Compounds that are made up of reduced sulfur are called sulfides. Sulfide compounds are characterized by a strong unpleasant odor.

Hydrogen disulfide (H2S), is a wine spoilage compound that has an aroma that smells like rotten eggs. It can have several causes, but it is most frequently a result of residual sulfur dust present on grapes when they are harvested, being reduced by yeast to H2S during fermentation.

Hydrogen disulfide can also be caused bya shortageofyeastnutrientsduringfermentation. H2S can undergo further chemical reaction to form compounds called mercaptans. Mercaptans also have strong unpleasant aromas that are reminiscent of cabbage, garlic, and skunk. Although sulfurdioxide andsulfides both contain the element sulfur, there is nodanger of added SO2 forming H2S.

ulfur dioxide was first used in winemaking when the Romans discovered that if you burn candles made of sulfur inside empty wine vessels it would keep them fresh and prevent them gaining a vinegar smell. 2000 years later sulfur dioxide, or SO2, remains without a doubt the most important additive that is used in winemaking.

Used as both an antimicrobial agent and antioxidant, winemakers find it indispensable to preserve wine quality and freshness. However, if used improperly, the effect can be just as adverse as they can be beneficial.

All wines benefit from tender care, whether one is crushing, racking, or bottling, the gentlest method of accomplishing a task is often the best. Just as it is with any of the other tools and techniques a vintner may use, the addition of sulfur dioxide works best when enough is added at the proper time to accomplish the desired task without adding too much and adversely affecting wine quality.

This text has been edited from first publication by WINEMAKERMAG.COM, August/September 2008, with Author's cooperation.

The amount and timing of sulfur dioxide additions depends on the style of wine that is being made and the composition of the wine to which it is being added. While it is possible to make wine without adding sulfur dioxide, you cannot make wine that contains no sulfur dioxide at all. This is because yeast produce a small amount, about 10 parts per million, during fermentation.

When adding sulfur dioxide to must or wine it is important to consider the stage of winemaking that it is in, such as fermentation, ageing or prebottling. You should also consider what the status of the malolactic fermentation is and whether you want to encourage or discourage it, and how turbid or clean the wine is, and how long it will be before the wine is consumed.

Because of these variables and the variation in winemaking styles, every winemaker or winemaking text is likely to have a different answer to the question: “how much sulfur dioxide should be added?” This confusion can be frustrating to novice winemakers but it illustrates one important point; there are few absolute rules of wine making and depending on the situation and the wine style that is being made there are many options.