The role of sulphites in winemaking is to prevent oxidation (the conversion of alcohol to acetaldehyde which removes fruit aromas and flavours) and to prevent microbiological spoilage of the wine by killing undesirable yeasts and bacteria.
Sulphite (SO2) exists in two main forms in wine – free SO2 and bound SO2. The addition of free and bound is referred to as total SO2. Free SO2 refers to molecular SO2 and lightly bound bisulphite and sulphite anions. Bound SO2 refers to the sulphite anions which are bound strongly to other compounds such as acetaldehyde, colour pigments and tannins in the wine.
The presence of sulphites in foods and beverages, even in very small amounts, has been shown to cause allergic reactions, in particular asthmatics. Reactions can include wheezing, flushing, low blood pressure, migraines and skin rashes. It is estimated that around 2% of the population may be sulphite sensitive and this does not include asthmatics who can also be affected.
Preservative free red wines will age normally due to the presence of naturally occurring tannin and acid and alcohol which act to preserve the wine and allow it to age slowly. Preservative free white wine has alcohol and acid but not tannin and so whites made without sulphite are subject to aging more rapidly than white wines made with SO2.