Chapter 18

Sulfur Dioxide Use in Winemaking

Chemistry of Sulfur Dioxide

When sulfur dioxide is added to wine, part of the added sulfur dioxide combines with other wine materials and becomes bound, and part of the sulfur dioxide added to wine remains in the free form. The combined sulfur dioxide is chemically bound to these materials and no longer exists as free sulfur dioxide. The remainder of the added sulfur dioxide remains in a free state. The sum of the concentrations of both the bound and free portions of sulfur dioxide are referred to as the total sulfur dioxide.

Free Sulfur Dioxide

Free sulfur dioxide in wine exists in three states: molecular sulfur dioxide (SO2), bisulfite (HSO3-), and sulfite (SO32-) and are in dynamic equilibrium (Figure 18.1). The quantities of these different forms of free sulfur dioxide depend on the pH. Each form has different properties. The molecular form, usually present in relatively small amounts, is the primary form responsible for the antimicrobial activity of sulfur dioxide. The bisulfite form has an important role in the antioxidant properties of sulfur dioxide, while the sulfite form is present only in small quantities and has no enological interest.

Molecular Sulfur Dioxide

To ensure adequate protection of the wine, the winemaker will have target a specific free sulfur dioxide level to ensure there is enough molecular sulfur dioxide. To complicate matters, as mentioned, the amount of molecular sulfur dioxide will depend on the pH of the must or wine.

Bisulfite

Of the three forms, the bisulfite ion dominates in wine due to the pH (wine pH is usually between 3 and 4). In addition, the bisulfite ion can exist in a free or bound form due to its ability to bind with various wine components such as acetaldehyde, pyruvic acid, anthocyanins, and sugars. When bound to acetaldehyde, the once unpleasantly fragrant aldehyde becomes odorless and no longer presents itself as a problem.

Bound Sulfur Dioxide

When sulfur dioxide is added to wine, part of the added sulfur dioxide combines with other wine materials, and part of the sulfur dioxide added to wine remains in the free form. The combined sulfur dioxide is chemically bound to these materials and no longer exists as free sulfur dioxide. Wine materials that bind with sulfur dioxide fall into two general classes.

The Debate Over Sulfites

The use of sulfur dioxide, or sulfites, in wine is a hot topic these days, and a topic with widespread misinformation. The confusion is partly due to misleading warning labels required for wines at far lower levels mandated for foods containing sulfites, giving the impression that sulfites are more dangerous than they really are. Almost all vintners, including most organic practitioners, add a small amount of sulfur dioxide to grapes before fermentation to improve winemaking hygiene. Wines without sulfur dioxide do not exist.

Click on the following topics for more information on sulfur dioxide use in winemaking.