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w i inneeGM A K IN GG w R O WIN SULFUR DIOXIDE The science behind this anti-microbial, anti-oxidant wine additive BY Pat Henderson, Senior Winemaker, Kenwood Vineyards, Kenwood, Calif., Author of About Wine S 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. 2,000 years later sulfur dioxide, or SO 2 , still 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 effects 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 usually the most successful. 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 result without adding too much and adversely affecting wine quality. 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. This text edited from first publication: Jan./Feb. 2009. 54 p racti c al w i ne ry & v i n e yard JANUARY 20 14 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 vintners but it illustrates one important point; there are few absolute rules of winemaking and depending on the situation and the type of wine that is being made there are many options. Every winemaker or winemaking text is likely to have a different answer to the question: "How much sulfur dioxide should be added?" 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 winemakers have only a basic knowledge of how sulfur dioxide reacts in wine and the different forms that it takes. Chemistry of sulfur dioxide Sulfur is an element found on the periodic table. In its pure form, it can be dusted or sprayed on grapevines during the growing season to prevent rot and mildew from developing. If sulfur is oxidized, it forms sulfur dioxide or SO2. 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 SO 2 in the chemical reac- tion: S + O2 = SO 2 . 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 SO 2 . 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 (H2 S), 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 H2 S during fermentation. Hydrogen disulfide can also be caused by a shortage of yeast nutrients during fermentation. H2 S 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 sulfur dioxide and sulfides both contain the element sulfur, there is no danger of added SO2 forming H2 S. Sulfur dioxide itself is a gas that readily dissolves in water, once dissolved it reacts with the water to form new compounds called sulfites as shown in the following reaction: H2O + SO2 ↔ H+ + HSO3- ↔ 2H+ + SO3= In solution with H2 O (water), SO2 is called molecular SO 2 , HSO 3- is called bisulfite and SO3= is called sulfite. The negative signs (- and =) denote the negative charge of the bisulfite and sulfite ions (molecules with a charge are called ions). The double arrows (↔) of the equation denote that the reaction is at equilibrium. At equilibrium, the rate at which bisulfite ions become sulfite is the same as the rate sulfite ions become bisulfite. The reaction between the different types of sulfite is going both ways at a steady state so the concentration of the sulfite compounds remains constant. While the concentration of the different forms of sulfites may be steady, it does not mean there are equal amounts of the compounds in solution; the acidity or pH of the water has a huge affect on their concentration. The more acidic or the lower the pH of the water, the more heavily the reaction is weighted to the molecular SO 2 side. The more basic or higher the pH is, the more sulfite that is present as illustrated in Figure I. How does it differ when SO2 dissolves in wine? Since wine is more than 80%