Sulphur dioxide is a colourless gas with a strong and unpleasant smell. Although it is toxic, many processed foods, and wines in particular, contain sulphur dioxide as a preservative.
A small amount is used, yet some people are highly allergic to sulphur dioxide. The WHO (World health Organization) set .7 mg per kilogram of weight per day as safe. Hence, a 60 – 80 Kg. Weighing person can ingest 42 – 56 mg. per day.
Some wines contain 10 – 200 mg. per litre. According to US laws, the label must state that the wine contains sulphite (the generic term used for sulphur dioxide) but not quantity, simply because sulphur dioxide dissipates over time.
Winemakers can and do use sulphur dioxide at four stages of wine production – harvesting, crushing, fermentation and bottling.
Harvesting – sulphur dioxide is applied to inhibit an uncontrolled fermentation caused by wild yeasts.
Crushing – sulphur dioxide may be added to prevent wild yeasts from interfering with fermentation. Winemakers prefer cultured yeasts, which are sulphur dioxide resistant.
Fermentation – sulphur dioxide may be added at any time to stop or prevent malolactic fermentation. Fermenting yeasts naturally produce 10 and up to 30 mg/L, but the latter is rare.
Bottling – sulphur dioxide is added during bottling to prevent oxidation and an accidental fermentation in the bottle. Both are highly undesirable. Sweet wines are particularly prone, and some winemakers use large amounts to avoid both. In Germany winemakers like “sterile” bottling, which largely eliminates fermentation in the bottle.
Sulphur dioxide manifests itself in two forms – dissolved in wine (fixed) and volatile (free). The latter smells, the former affects flavour and can be tasted.
Generally, white and sweet wines are treated with sulphur dioxide. Sweet wines contain more sulphur dioxide. Red wines do not require sulphur dioxide as they contain naturally higher levels of natural sulphur dioxide and antioxidants.
Excessive oxidation ruins wine, controlled oxidation adds complexity, particularly to sherries, vin jaune and Madeiras.