Sulphur dioxide (SO2) is the most widely used and controversial additive in winemaking. Its main functions are to inhibit or kill unwanted yeasts and bacteria, and to protect wine from oxidation.
SO2 is added at several points in the process of conventional vinification and is present in the finished/ bottled wine in the form of sulphites.
Sulphites occur naturally in all living things and are present in small quantities even in unsulphured wines. They can cause allergic reactions with a very small number of people.
Sulphur dioxide should not be confused with the powdered sulphur which is sometimes dusted onto vines to protect them from powdery mildew, even used in organic viticulture.

 

The first definite mention of its use in winemaking is a German royal decree of 1487. This permitted winemakers to burn sulphured woodchips in barrels used for storing wine. This is an effective way of disinfection and is still practiced (using pure powdered sulphur rather than woodchips) although steam-cleaning is now an alternative. The systematic use of sulphur to control fermentation and to stabilise the wine at bottling was perfected by the French in North Africa early in the 20th century. It was a way for them to make wine in conditions that were very hot. This technique caught on in other warm climates, as a way of making wine without having to worry about it.
In the winery it may be used as a disinfectant in between vintages, and may be added to 'must' and finished wines as an antibacterial agent to prevent spoilage. It may also be used in winemaking as a method of stopping fermentation. Excessive use may result in an unpleasant mothball or burnt match aroma in the wine.
The amount of wine sulphur (potassium metabisulphite) to add to a wine is not a simple matter. An accurate set of scales are needed to weigh 1 or 2 grams. White wines, and sparkling wines, are treated completely differently from red wines (NB: sometimes on the wine bottle it may stated as preservative 220 added). Different varieties consume SO2 at different rates, so it is necessary to measure the SO2 regularly with a test such as the Sulfacor test.