Sulphite is used in winemaking for several reasons:

Sulphite is an important preservative that also prevents the acidification of wine. Young and sweet wines are especially susceptible to this problem. The sugar in these wines can be transformed into acid by acetic acid- or lactic acid-forming bacteria; this conversion alters the taste. Sulphite also enhances the colour of red wine. Without it the intensity of the colour would decrease with the passing of time and other colours (purple/blue) could emerge. Sulphite thus prevents the loss of colour and enhances the aesthetic value of red wine. For white wine, this function is, obviously, less important.

Last but not least, sulphite improves the taste of wine by removal of acetaldehyde, which is formed by yeast during the winemaking process and imparts an unpleasant taste.

In wine, 200 – 400 ppm (ppm stands for parts per million, for instance, mg/kg or mg/l) is allowed as free sulphur dioxide. As this is a gas, sodium sulphite or sodium bisulphite is usually used.