Chapter 18

Sulfur Dioxide Use in Winemaking

Free Sulfur Dioxide Requirements in Wine

The amount of sulfur dioxide in the free form, and the effectiveness of the free sulfur dioxide, depends on the pH of the wine. Therefore, all other things being equal, at a higher pH not only will more sulfur dioxide need to be added to achieve the desired level of free sulfur dioxide, but the concentration of free sulfur dioxide required to have the desired antioxidant and antimicrobial properties will in itself also be greater.

Molecular Sulfur Dioxide Concentration

Most commercial wineries maintain at least 0.8 mg/L of molecular sulfur dioxide for white and blush table wines but only maintain 0.5 mg/L of molecular sulfur dioxide for red table wines. White wines are on average bottled with higher sulfur dioxide either because they are more likely to contain residual sugar or because oxidation has more deleterious effects on the aroma and color of white wines than on red wines. Furthermore, acetaldehyde, an important binder of sulfur dioxide, is on average at higher concentrations in white wines than in red wines.

Free Sulfur Dioxide Conversion

As previously mentioned, not all the added sulfur dioxide is converted to free sulfur dioxide. A certain percentage of the added sulfur dioxide will bind with wine constituents (such as acetaldehyde, sugars, phenolic compounds, anthocyanins, yeast, and others). Since only a portion of the sulfur dioxide that is added will be free sulfur dioxide, it is necessary to estimate what percentage will become free sulfur dioxide. In general, the ratio free to total sulfur dioxide will range from 35 to 65 percent (depending on pH and other factors; therefore, a greater addition is made to compensate for the binding phenomenon.

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