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Chemistry
SO₂ & Sulphites
Sulphur dioxide (SO₂) is the most important preservative in winemaking. It provides antimicrobial and antioxidant protection at multiple stages.
Forms of SO₂
- Total SO₂ — all SO₂ in the wine, bound and free. This is what appears on EU labels when >10 mg/L.
- Free SO₂ — SO₂ not bound to other compounds. Only the free form provides protection.
- Molecular SO₂ — the truly active antimicrobial form. Calculated from free SO₂ and pH.
Addition Forms
- Campden tablets (potassium metabisulphite) — 1 tablet per 4.5 L (one demijohn, or an imperial gallon) ≈ 50–65 mg/L SO₂. Simple for home use.
- Potassium metabisulphite (KMS) powder — 1g per 10L ≈ 50 mg/L. More precise.
- Sodium metabisulphite — similar to KMS, used for equipment sterilisation.
Target Free SO₂ by pH
| pH | Target Free SO₂ (mg/L) |
|---|---|
| 3.0 | 10 |
| 3.2 | 13 |
| 3.4 | 18 |
| 3.6 | 24 |
| 3.8 | 31 |
⚠️ Commission Delegated Regulation (EU) 2019/934 (oenological practices, Annex I Part B) sets maximum total SO₂ at 150 mg/L for dry reds, 200 mg/L for dry whites/rosés, with higher ceilings for sweeter wines (up to 400 mg/L for certain botrytis-affected wines). Always log additions for compliance records.
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