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). With a 100 mL sample, each mL of iodine solution used ≈ 6.4 mg/L SO₂ — halve the sample and that factor doubles, so always use the sample size and factor from your kit's own instructions. 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

*The USA applies a single limit of 350 mg/L total SO₂ to all wine (27 CFR 4.22(b)(1)), plus the "Contains Sulfites" declaration above 10 mg/L. The UK retained the EU limits on leaving the EU and has not adopted the EU's post-2023 amendments — the practical numbers are the EU ones.

"Contains sulphites" ("Contains Sulfites" in the US) 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.