Adding Sulfur Dioxide (SO2)

Abstract

The judicious use of sulfur dioxide (SO2) will inhibit the growth of microorganisms (e.g., bacteria) present on the grape skins as the berries come from the vineyard. Its early use presumes the vintner has decided that the adventitious wild yeasts which might be destroyed or inhibited by sulfur dioxide will not contribute to the vintage. It appears that Saccharomyces cerevisiae might be less susceptible to the action of sulfur dioxide than other yeasts that may be present. So, if the particular strain of S. cerevisiae used can cope, it may be able to function unimpeded. Regardless, sulfur dioxide might still be used because, in addition to suppression of deleterious microorganisms, it appears to reduce oxidation of particularly fragile white wine components. In industrial settings, both gaseous sulfur dioxide and sulfur dioxide as a liquefied gas (boiling point – 10 °C [14 °F]) are used. In either form it is a dangerous tool. It is dangerous first because it is toxic and second because an excess of it will ruin the wine. In many cases, because its value is recognized as beneficial, sulfur dioxide is replaced by addition of either sodium metabisulfite (Na2S2O5) or potassium metabisulfite (K2S2O5) with the latter generally preferred. Indeed, while it is best to look at the MSDS. (Manufacturer’s Safety Data Sheet) before use, the solubility of the two salts is the same and given as 450 grams/ liter (g/ L) at 68 °F (20 °C) and the pH on dissolution as between 3.5 and 4.5. The potassium (K) salt appears, at this writing, to be more readily available in food quality (as opposed to chemical quality) grade. So, with regard to sulfur dioxide (SO2), and as shown in Figure 17.1, its structure is much more similar to water and to ozone than it is to carbon dioxide (CO2); sulfur lies beneath oxygen (O2) in the periodic table (silicon, Si, lies beneath carbon). Nonetheless, sulfur dioxide (SO2) reacts with water much the same way that carbon dioxide (CO2) does.