Potassium Metabisulphite is a stable source of sulfur dioxide in winemaking. The use of sulfur compounds is not a recent innovation. The great Dutch shipping empire popularized the use of sulfur in the 16th century by refusing to ship any wines not treated. They insisted on sulphites because sulphite treated wines were the only ones that survived a long sea voyage without turning into vinegar.

Sulphites work by releasing free sulfur dioxide, which inhibits yeast, mold and bacteria. It does this in two ways: one, it kills some of the organisms outright, and two, it blocks the surviving organisms ability to reproduce.

Sulphites are also added directly to wine after fermentation, to help prevent oxidation. Oxidation in wine follows the same pattern that you see in the cut edge of an apple—the wine turns brown and takes on a flat 'cardboard' taste. Sulfur binds with the oxygen in the wine and prevents this damage.

Many people worry that they may be allergic to sulphites. True sulphite allergies are very rare. It's more likely that they have a histamine reaction to red wine, or that they have been over exposed to sulphites in the past. In the 1970's restaurants would douse their salad bars with 2000 PPM (parts per million) sulphite solutions in order to keep the produce fresh. Mixing with food acids, such as dressings or vinegar, would cause the salad to release clouds of sulphite gas, provoking unpleasant reactions.

Some facts that might clear up any misunderstanding about sulphites:

This is not to say that sulphites are totally benign. People with asthma or emphysema should avoid inhaling sulphite powder or the gas that comes off the prepared solution. It can act as a bronchial constrictor, aggravating any breathing problems.