Understanding SO₂ (Sulphur Dioxide): Protector and Preserver

Sulphur dioxide (SO₂) is the single most important preservative and protective agent in winemaking. Used correctly, it allows wine to survive bottling and ageing without oxidation or microbial spoilage. Used incorrectly — too much or too little — it either damages the wine or fails to protect it. Understanding SO₂ at a chemical level is one of the clearest differentiators between experienced and novice winemakers.

The Three Forms of SO₂

SO₂ in wine exists simultaneously in three forms, in dynamic equilibrium depending on pH and temperature:

1. Total SO₂

The sum of all SO₂ forms in the wine — bound, free, and molecular. This is the number that appears on wine labels ("Contains sulphites") and is regulated by legal limits. It is measured by the Ripper titration or aeration-oxidation method.

2. Free SO₂

SO₂ not bound to other compounds in the wine. This includes both the molecular form and the bisulphite form. Free SO₂ provides the actual protection — it is available to react with oxygen and inhibit microorganisms. Bound SO₂ (linked to aldehydes, sugars, and other compounds) is essentially inactive.

When SO₂ is added to wine, some is immediately bound by:

• Acetaldehyde (the primary binding compound — if significant, the wine will consume most of the SO₂ you add)
• Pyruvate
• α-ketoglutarate
• Glucose and other sugars

The remaining SO₂ stays free. This is why a wine with high acetaldehyde (e.g. from oxidation) requires much more SO₂ to achieve target free SO₂ levels.

3. Molecular SO₂

The truly active antimicrobial form — actual SO₂ gas dissolved in wine. Only molecular SO₂ kills bacteria and inhibits wild yeast. Its concentration is determined by:

Molecular SO₂ (mg/L) = Free SO₂ / (1 + 10^(pH − 1.81))

This is why pH is critical. At lower pH (more acidic wine), more of the free SO₂ exists in the molecular form:

Practical implication: A wine at pH 3.8 needs 6× more free SO₂ than a wine at pH 3.0 to achieve the same molecular SO₂ protection. High-pH wines are inherently harder to protect.

Target Free SO₂ by pH

The conventional target for molecular SO₂ is 0.8 mg/L for whites/rosés and 0.5 mg/L for reds (lower because tannins provide additional antioxidant protection).

Working backwards to free SO₂ targets:

Table values for 0.8 mg/L molecular target (whites) and 0.5 mg/L (reds)

Note that at pH 3.8, you need 84 mg/L free SO₂ in a white wine — this approaches the EU maximum (200 mg/L total). High-pH wines are genuinely difficult to protect without violating regulatory limits.

Addition Forms

Potassium metabisulphite (KMS) powder

The most common form for home and professional winemakers. Contains approximately 57% active SO₂ by weight (some variance by purity).

Grams KMS to add = target mg/L SO₂ × volume (L) / 570

Example: Adding 30 mg/L free SO₂ to 20 litres:
30 × 20 ÷ 570 = 1.05g KMS

Campden tablets (potassium metabisulphite compressed)

One tablet per 4.5L provides approximately 50–65 mg/L SO₂ (varies by brand). Convenient for home use but less precise than weighing powder.

Sodium metabisulphite

Same efficacy as KMS but adds sodium ions — used for equipment sanitisation only, not wine additions.

Liquid SO₂ (gaseous SO₂ under pressure)

Used in commercial wineries via a wand directly into wine. Not practical for home winemakers.

Testing Free SO₂

You cannot manage SO₂ without measuring it. The options:

Ripper titration

A standard acidimetric titration: a measured wine sample is acidified, then titrated against iodine (standard 0.02 N). Each mL of iodine solution used ≈ 6.4 mg/L SO₂. Simple, cheap, slightly inaccurate (iodine also oxidises other reducing compounds). Good enough for most home use.

Available as ready-made test kits from home brew suppliers.

Aeration-oxidation (AO) method

More accurate but more equipment-intensive. SO₂ is stripped from the wine with an air stream, collected in hydrogen peroxide, and titrated. Gold standard for commercial winemaking.

Commercial colorimetric kits

Enzymatic or colorimetric test strips/kits. Less accurate but very simple for cellar monitoring.

When to Add SO₂

Legal SO₂ Limits

*USA requires declaration "Contains Sulfites" if total SO₂ exceeds 10 mg/L.

"Contains Sulfites" must appear on EU and US labels when total SO₂ exceeds 10 mg/L. Since virtually all wine contains at least this level, the declaration is essentially universal.

SO₂ in "Natural" / Low-Intervention Wine

The natural wine movement aims to reduce or eliminate SO₂ additions. The practical challenges:

• Without SO₂, wines require meticulous winemaking to avoid oxidation and bacterial spoilage
• Very low-SO₂ wines often show increased volatile acidity, brett character, or oxidative notes
• Some natural winemakers add a small SO₂ dose at bottling only (20–30 mg/L total)

For home winemakers without professional-grade temperature control and oxygen-free bottling equipment, eliminating SO₂ dramatically increases risk of spoilage.