The Use of Sulphur Dioxide (SO2) in Winemaking

By Charles Plant

Overview

This technical article explains how winemakers use sulfur dioxide to protect wine quality. The piece covers sourcing SO2, calculating proper dosages, testing methods, and application techniques throughout the winemaking process.

Key Sourcing Information

The most practical SO2 source for home winemakers is potassium metabisulfite, which “is 57% SO2.” Commercial Campden tablets contain this compound but deteriorate over time, making bulk potassium metabisulfite a more reliable and economical choice.

Dosing Strategies

Proper SO2 levels depend on grape condition, pH, and fermentation plans. For sound red grapes at pH 3.4, approximately 30 ppm works well. White wines destined for champagne-method sparkling production may require as little as 10 ppm if grapes are perfect. Moldy fruit demands 50–60 ppm or higher.

A practical approach involves creating a 10% potassium metabisulfite solution in water for precise measurements and consistent application.

The pH Connection

The article emphasizes that “what is really protecting your wine is molecular SO2,” which varies based on pH levels. At pH 3.2, achieving 0.8 ppm molecular SO2 requires 22 ppm free SO2, while pH 3.5 demands approximately 43 ppm—essentially double the amount.

Testing Methods

The article provides detailed procedures for testing free and total SO2 using iodine solutions, including specific chemical preparations and measurement techniques suited for white and rosé wines, though testing reliability decreases significantly with red wines.