Sulphur dioxide (SO₂) dioxide is a colourless but highly toxic gas that has been used almost universally in wine since the 17th century. It is even produced in small quantities (ca, 10mg/l) by yeast during fermentation, so that its total absence in wine is rare.

In wineries, it is found in 4 forms:

• Generated from burning sulphur candles or pastilles
• Gas compressed into liquid form
• Dissolved in solution (normally 5%)
• Potassium (or sodium) metabisulphite powder that, when dissolved in an acid solution, releases sulphur dioxide.

Sulphur dioxide has the following properties in winemaking:

• Antiseptic: It inhibits the development of microorganisms, e.g. acetobacter and wild yeasts. Selected winemaking yeasts are generally more tolerant.
• Antioxidant: It combines with products of wine oxidation, such as acetaldehyde, quinones and peroxide which limits the damage to must or wine. It also neutralises oxidative enzymes (Tyrosinase), slowing the down the rate of oxidation

When it is added to must or wine, some of it combines with wine components, such as acetaldehyde, and this renders it inactive (known as bound SO₂). The level of activity of the uncombined sulphur dioxide (Free SO₂) is affected by the pH of the must/wine: the more acid the must/wine the more active it is.

Due to health concerns (especially for asthmatics) and so as to encourage quality winemaking, the use of SO₂ is strictly controlled, with total additions being restricted by law to 150mg/l for reds, 185mg/l for sparkling wines and 200 mg/l for dry whites and Rose. Sweet wines are higher while organic wines are required to have even lower levels.

Sulphur dioxide is often added to juice pre-fermentation in order to control microbial activity and oxidation.
General recommendations are:

• For white musts 60 –100 mg/l
• For red musts 10 – 60 mg/l

Levels are increased according to:

• Style of wine required (it preserves primary aromas)
• Sanitary state of grapes, especially the level of grey rot
• pH: high pH low acid wines need more SO₂.

When adding, it is important to make sure that it is well mixed in.

Ascorbic acid (vitamin C) can effectively supplement the antioxidant role of SO₂, but this cannot replace SO₂ completely, as it has no antiseptic effect.