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w i inneeGM A K IN GG w R O WIN 100 % of Sulfite form in solution 90 80 70 60 50 40 30 20 10 0 0 1 2 3 4 5 6 7 pH % SO2 %HSO3– %SO3= Figure I. The percentage of the different forms of free sulfite over the pH range (0 to 7). Wines _ usually range from pH 3 to 4, so bisulfite (HSO3 ) is the dominant form of free sulfite in wine. Total sulfur dioxide Free sulfur dioxide Bound sulfur dioxide Molecular Bisulfite Sulfite SO2HSO3-SO3= Sulfites attached to sugars, acetaldehyde and phenolic compounds Figure II. Different forms of SO2. water, the reaction, or disassociation, of molecular SO2 into sulfites happens in much the same manner. Since wine is acidic (between 3 and 4 pH), molecular SO2 and bisulfite make up the vast majority (99.99% at pH 3.4) of the sulfite compounds present in wine. Sulfites will also react with other chemical constituents found in wine such as sugars, acetaldehyde, and phenolic compounds. When a sulfite reacts with another molecule and becomes part of its structure it no longer takes part in the equilibrium reaction and it is called bound. Sulfites that still are part of the equilibrium reaction are called free. The combined amounts of free and bound sulfites are called Total SO2 . The more compounds that are available in a wine for sulfites to bind to, the higher the ratio of bound to total sulfites there will be. Therefore, wines that are sweet or have high solids because they have not been settled or filtered will have a lower ratio of free to total SO2 . When sulfite binds to anthocyanins (the phenolic molecule that gives red wines their color), the anthocyanins go from a colored to a colorless form. Red wines that receive an SO2 addition will have a slightly less intense color. This effect is particularly noticeable in light colored reds or rosé wines. Knowing both the amount of free and total sulfites is very important because only the free forms of sulfites are available for providing a preservative role in wine. This is often expressed as ppm free SO 2 /ppm total SO 2 to denote which number is free and which is total; these numbers can readily be determined by chemical analysis. Sulfur dioxide concentration is measured in parts per million (ppm) or as milligrams per liter (mg/L). A wine with 19 ppm free SO2 and 51 ppm total SO2 would be noted with 19/51. The amount of bound sulfites would be equal to the total SO2 minus the free SO2 , (51 – 19 = 32). For dry table wines the level of free sulfur is usually somewhere around 35% to 75% of the level of total SO2 . Anti-oxidation role of sulfur dioxide Oxidative reactions can occur in both red and white wines but are particularly noticeable in the latter. Dissolved oxygen in wine can react with phenolic compounds giving the wine a brownish hue. Another product of this oxidation is the compound acetaldehyde, which has a nutty, sherry-like aroma. Sulfur dioxide is used by winemakers to prevent this oxi- 510-732-7010 Custom made - Handcrafted Barrels Include Code 14SE on your first order for an extra 10% off! wineryseals.com A Division of Bay Seal Company Providing Sealing Solutions for over 50 Years pr actica l win ery & vin eya r d JANUARY 20 14 55