Sulfur Dioxide Management in Winemaking

Problem Definition

Sulfur dioxide (SO₂) is the most widely used and essential additive in winemaking, serving dual roles as antioxidant and antimicrobial agent. Effective SO₂ management requires understanding the equilibrium between free and bound forms, calculating molecular SO₂ concentrations for antimicrobial activity, and adjusting dosing strategies based on wine pH, temperature, and style objectives. Both insufficient and excessive SO₂ present problems: underprotection leads to oxidation and microbial spoilage; overuse causes sensory defects (sulfurous odors) and regulatory non-compliance.

Technical Context

SO₂ Equilibrium Chemistry

Free SO₂ exists in three forms in aqueous solution:

Molecular SO₂ (SO₂·H₂O): Antimicrobial active form
Bisulfite (HSO₃⁻): Dominant at wine pH; antioxidant
Sulfite (SO₃²⁻): Minimal at wine pH

The equilibrium is pH-dependent:

Wine pH	Molecular SO₂ (% of free)
3.0	6.1%
3.2	4.0%
3.4	2.5%
3.6	1.6%
3.8	1.0%

Key Implication: Higher pH wines require higher free SO₂ to achieve equivalent molecular SO₂.

Molecular SO₂ Calculation

Formula:

Molecular SO₂ (mg/L) = Free SO₂ (mg/L) × (% molecular at pH)

Target: 0.5-0.8 mg/L molecular SO₂ for microbial control

Example (pH 3.4 wine, targeting 0.6 mg/L molecular):

Required Free SO₂ = 0.6 / 0.025 = 24 mg/L

Bound vs. Free SO₂

Binding agents in wine:

Acetaldehyde (strongest binder)
Glucose/fructose
Pyruvic acid
α-ketoglutaric acid
Anthocyanins (red wines)

Total SO₂ = Free SO₂ + Bound SO₂

Bound SO₂ is largely inactive; only free SO₂ provides protection.

Options and Interventions

Addition Timing

At Crush/Must:

Immediate antioxidant protection
Inhibits wild yeast/bacteria
Typical dose: 25-50 mg/L total
Risk: Can inhibit desired wild fermentation

Post-Fermentation:

After MLF completion (if conducted)
Target free SO₂ for style
White wines: 25-40 mg/L free
Red wines: 20-30 mg/L free

At Bottling:

Final adjustment for bottle aging
Account for oxygen pickup during bottling
Verify molecular SO₂ target

Dosing Calculations

1 g/L potassium metabisulfite (K₂S₂O₅) = ~0.57 g/L SO₂

Common Forms:

Potassium metabisulfite (57% SO₂)
Sodium metabisulfite (65% SO₂)
Liquid SO₂ (100%)
Sulfur wicks (for barrel treatment)

Style-Specific Strategies

Fresh White Wines (Sauvignon Blanc, Riesling):

Higher free SO₂ (30-45 mg/L)
MLF blocked
Reductive handling
Molecular target: 0.6-0.8 mg/L

Barrel-Fermented Chardonnay:

Moderate free SO₂ (25-35 mg/L)
MLF typically completed
Oxidative handling tolerance higher
Molecular target: 0.5-0.6 mg/L

Red Wines:

Lower free SO₂ acceptable (20-30 mg/L)
Higher bound SO₂ (anthocyanin binding)
Molecular target: 0.5-0.6 mg/L

Sparkling Base Wines (Champagne):

Very low SO₂ at base wine stage
Avoid inhibiting secondary fermentation
Final dosage adjustment at disgorgement

Trade-offs and Risks

Insufficient SO₂

Risks:

Oxidation: Browning, premature aging
Microbial spoilage: Brettanomyces, acetic acid bacteria
Volatile acidity development
Acetaldehyde accumulation

High-risk scenarios:

High pH wines (>3.6)
Warm cellar temperatures
Residual sugar present
Extended lees contact

Excessive SO₂

Risks:

Sulfurous odor (burnt match)
Palate harshness
Regulatory limits exceeded
Consumer sensitivity reactions

Legal Limits (vary by region):

EU: 150-400 mg/L total (varies by style/sugar)
USA: 350 mg/L total
Organic wines: Lower limits apply

SO₂ and Reduction

SO₂ additions can mask reduction issues
H₂S and mercaptans may recombine with SO₂
Release of sulfides possible if free SO₂ drops
Aeration before SO₂ addition recommended if reduction present

Practical Implications

High pH Wine Considerations

Wines above pH 3.6 require:

Higher free SO₂ levels (35-50 mg/L)
More frequent monitoring
Consider acid adjustment for stability
Barossa Valley, Napa Valley reds often high pH

Low SO₂ / Natural Wine Approaches

If minimizing SO₂:

Rigorous hygiene essential
Lower pH wines more stable
Cold storage mandatory
Shorter production cycles
Accept increased variability risk

Monitoring Protocol

Recommended frequency:

Weekly during fermentation
Monthly during aging
Before/after transfers
Pre-bottling verification

Methods:

Aspiration-oxidation (Ripper method)
Iodometric titration
Aeration-oxidation (accurate)
Analyzer instruments

References

Ribéreau-Gayon, P., Glories, Y., Maujean, A., & Dubourdieu, D. (2006). “Handbook of Enology, Volume 2: The Chemistry of Wine Stabilization and Treatments.” 2nd Edition. Wiley. Publisher Link
Waterhouse, A.L., Sacks, G.L., & Jeffery, D.W. (2016). “Understanding Wine Chemistry.” Wiley. Publisher Link
OIV (2023). “International Code of Oenological Practices.” https://www.oiv.int
Jackson, R.S. (2014). “Wine Science: Principles and Applications.” 4th Edition. Academic Press. Publisher Link

Last Updated: January 6, 2026