Wine Arcana

Concept

Sulfur dioxide

Also known as Sulphur dioxide, Sulfites, Sulphites

Sulfur dioxide (SO₂) is a preservative used in winemaking to limit unwanted microbial growth and protect wine against oxidative change. Its effectiveness depends on the chemical forms it takes after entering the wine. The amount added, the amount measured, and the amount available to protect the wine can differ substantially.

Yeast can also produce sulfites during alcoholic fermentation. A wine made without added sulfur dioxide can therefore contain sulfites from fermentation.1. Andrew L. Waterhouse, University of California, Davis, “Sulfites,” discussing sulfite production by yeast.

Free, bound, and total

Winemakers distinguish free SO₂ from SO₂ bound to other wine components. The two fractions together constitute total SO₂. The free fraction includes dissolved molecular SO₂ and the related bisulfite and sulfite ions. At wine pH, bisulfite is the predominant form and sulfite is scarce. These forms interconvert as the chemical environment changes.

Bisulfite binds to compounds including acetaldehyde, a product of fermentation and oxidation. Binding reduces the amount of free SO₂ and can also suppress acetaldehyde's aroma. Different binding partners retain SO₂ with different strengths. A wine rich in compounds that bind sulfite can absorb an addition while yielding relatively little free SO₂.

Consequently, total SO₂ alone gives an incomplete picture of protection. Winemakers measure the free fraction as well, while considering the wine's composition and history. Even that measurement needs interpretation in young red wines, where analytical methods can include SO₂ loosely bound to anthocyanin pigments.2. Geoff Cowey and Adrian Coulter, “How much sulfur dioxide (SO₂) is needed at bottling?”, Australian & New Zealand Grapegrower & Winemaker 687 (April 2021), pp. 76–77.

Acidity and protection

Molecular SO₂ is especially important for antimicrobial action. At a lower pH, a larger proportion of free SO₂ is molecular. Two wines with the same measured free SO₂ can therefore have different protection against yeasts and bacteria. Temperature, alcohol, sugar, and the organisms present also affect the result.

The antioxidant action works within a chain of reactions. Oxygen entering wine can lead to hydrogen peroxide, which participates in reactions that damage aroma and oxidise ethanol. SO₂ intercepts peroxide and helps limit those consequences. It can also inhibit oxidising enzymes and bind certain compounds with unwanted odours. These are among the purposes identified in the International Organisation of Vine and Wine's account of sulphiting.3. International Organisation of Vine and Wine, “Sulphiting,” International Code of Oenological Practices, II.3.4.4.

Protection diminishes as SO₂ reacts. Transfers, bottling, and subsequent oxygen entry through packaging can all affect how much remains. A concentration measured before bottling consequently needs to be considered alongside oxygen pickup and the intended storage period.

Decisions in the cellar

Sulfur dioxide management follows the intended microbial sequence. Limiting bacteria too early can interfere with a desired malolactic conversion; leaving insufficient protection after fermentation can allow spoilage. Hygiene, sound fruit, appropriate storage, and filtration help determine how much protection the wine requires and how effectively an addition works.

Excess molecular SO₂ has a pungent, irritating smell. The rotten-egg aroma associated with hydrogen sulfide belongs to a different sulfur compound, discussed under reduction. Identifying the compound matters because their formation and management differ.

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