Sulfur Dioxide (SO2)

Since the 1487 Royal Decree of Germany allowed winemakers the use of burned and charred sulfured wood chips when barrel aging wine, using sulfur dioxide has been a common practice in winemaking. The use of sulfur dioxide allowed vintners to control and stabilize the wine during winemaking and bottling, especially in less than ideal winemaking conditions.

When sulfur dioxide is added to wine, it interacts with the wine and becomes a sulfite compound, some of these compounds, remain free, while others combine with the phenolic compounds, sugars and acetaldehyde in the wine and become bound to those compounds. The ratio of free to bound sulfites in the wine is important, as only free sulfites act as a preservative; the bound sulfites change the wine. For instance, bound sulfites prevent oxidation in red wine by bonding to the phenolic molecule anthocyanins. The wine will have less vibrancy and intensity in color, but not the brown hue typically associated with oxidation. The addition of sulfur dioxide is considered controversial as modern winemaking employs sanitation, temperature controls and testing that should eliminate the need for the addition of sulfur dioxide, but it should be noted that naturally-occurring sulfites are found in wines that have no sulfur dioxide added to them, although in much lower concentrations.