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Reduction and Sulfide Management: From Fermentation to Bottle

Technical guide to hydrogen sulfide and mercaptan formation, prevention strategies, and remediation protocols for reductive wine faults.

6 min read
Eno Codex Editorial

Reduction and Sulfide Management: From Fermentation to Bottle

Problem Definition

Reductive faults in wine arise from volatile sulfur compounds (VSCs), primarily hydrogen sulfide (H₂S) and mercaptans (thiols), that produce aromas ranging from struck match and rubber to rotten eggs and sewage. These compounds form during fermentation under yeast stress and can persist or evolve during aging, particularly under reductive (low-oxygen) conditions.

The problem is amplified in:

  • Wines fermented with nutritional deficiencies
  • Varieties with genetic predisposition to sulfur compound production (Syrah, Mourvèdre)
  • Reductive winemaking protocols (aromatic whites under screwcap)
  • Wines with extended lees contact without stirring

Technical Context

Chemistry of Volatile Sulfur Compounds

Hydrogen sulfide (H₂S):

  • Produced by yeast during fermentation
  • Primary metabolic pathway: reduction of elemental sulfur or sulfate for amino acid synthesis
  • Sensory threshold: 1-10 μg/L (extremely low)
  • Aroma: Rotten eggs

Mercaptans (thiols):

  • Form from H₂S reaction with alcohols, aldehydes, or other wine components
  • Examples:
    • Methanethiol (MeSH): Cooked cabbage, rubber
    • Ethanethiol (EtSH): Onion, rubber
  • Sensory threshold: 1-10 μg/L
  • Formed post-fermentation; more difficult to remediate than H₂S

Disulfides:

  • Form from oxidation of mercaptans
  • Dimethyl disulfide (DMDS): Cooked cabbage
  • Can revert to mercaptans under reductive conditions (cyclical problem)

Formation Mechanisms

During fermentation:

  1. Sulfate reduction pathway: Yeast reduce sulfate (SO₄²⁻) to sulfide for amino acid synthesis. If nitrogen is limiting, excess sulfide is released as H₂S.

  2. Elemental sulfur residues: Sulfur dust from vineyard applications is reduced to H₂S by yeast.

  3. Cysteine desulfuration: Yeast enzymatically release H₂S from sulfur-containing amino acids when nitrogen is limited.

YAN and H₂S relationship:

  • YAN <140 mg/L: Elevated H₂S risk
  • Higher methionine/cysteine: Elevated H₂S from desulfuration
  • Timing: H₂S production peaks at 2/3 through fermentation when nitrogen is depleted

Post-fermentation:

  • Extended reductive conditions (lees, screwcap, stainless steel) preserve or increase VSCs
  • Mercaptan formation from H₂S + acetaldehyde or alcohols
  • Disulfide cycling under variable oxygen conditions

Varietal Predisposition

Research indicates variety-specific differences in VSC production:

  • Syrah/Shiraz: Genetically predisposed to higher VSC production regardless of nitrogen status. Sulfidic character may be considered varietal at low levels.

  • Mourvèdre: High sulfide production documented; often described as “meaty” or “gamey”—partly attributable to VSCs at low concentrations.

  • Chardonnay: Moderate risk; sur lie aging requires vigilant management.

  • Sauvignon Blanc: Paradoxically contains positive varietal thiols (3MH, 3MHA) alongside potential negative sulfides; winemaking must distinguish.

Options and Interventions

Prevention

Nitrogen management:

  • Measure YAN at crush
  • Supplement to 250-350 mg/L for normal fermentations
  • Higher YAN for known high-sulfide varieties or stressed vintages
  • Split additions: 50% at inoculation, 50% at 1/3 sugar depletion

Elemental sulfur control:

  • Observe pre-harvest interval (PHI) for sulfur applications
  • Minimum 3-4 weeks between last sulfur application and harvest
  • Micronized sulfur is more completely metabolized than eleite

Yeast selection:

  • Some strains produce less H₂S than others
  • Strain selection should consider sulfide tendency for high-risk varieties

Oxygen management during fermentation:

  • Micro-oxygenation during fermentation (5-10 mg/L O₂)
  • Rack-and-return (délestage) promotes oxidation of H₂S
  • Avoid completely anaerobic conditions

Detection

Sensory:

  • Swirl wine in glass covered with hand, then smell
  • VSCs volatilize readily; detectable at very low concentrations
  • Distinguish between transient post-fermentation “reduction” and persistent fault

Analytical:

  • Cadmium sulfide method: Detects H₂S
  • GC-MS: Quantifies specific VSCs
  • Copper addition test: If 0.5 mg/L CuSO₄ eliminates aroma, VSCs confirmed

Remediation

Hydrogen sulfide (immediate post-fermentation):

  1. Aerate by racking with splashing
  2. Add copper sulfate: 0.25-0.5 mg/L copper (max 0.5 mg/L residual per EU regulations)
  3. Rack off lees (H₂S source)

Mercaptans (more persistent):

  1. Copper fining: 0.5-1.0 mg/L copper; may require higher doses
  2. CuSO₄ reacts: 2RSH + Cu²⁺ → RSSR + Cu⁰ + 2H⁺
  3. May require repeat treatment
  4. Residual copper must be verified (<1.0 mg/L most jurisdictions)

Disulfides:

  • Copper alone ineffective
  • Ascorbic acid addition (50-100 mg/L) can reduce disulfides to mercaptans for subsequent copper treatment
  • Multiple treatments often necessary

Aeration:

  • Simple aeration (racking, splash decanting) volatilizes H₂S
  • Less effective for mercaptans and disulfides
  • May oxidize other wine components; use judiciously

Trade-offs and Risks

Copper fining:

  • Effective for H₂S and mercaptans
  • Legal limits on residual copper (0.5-1.0 mg/L)
  • Excess copper promotes oxidation and instability
  • May strip positive thiols from aromatic whites

Aeration:

  • Volatilizes H₂S effectively
  • Can oxidize delicate aromatics
  • Not suitable for fresh, reductive white wine styles

Ascorbic acid addition:

  • Reduces disulfides but creates new chemistry
  • In presence of oxygen, can accelerate browning
  • Not reversible

Screwcap/closure considerations:

  • Screwcaps with low ROTR (rate of oxygen transmission) preserve VSCs
  • Synthetic closures or Saranex screwcaps may reduce post-bottling sulfide issues
  • Closure strategy should match wine style

Practical Implications

Variety-specific considerations:

  • Syrah: Côte-Rôtie AOC producers traditionally accept some reductive character as varietal; excess is fault. YAN supplementation and racking protocols critical.

  • Mourvèdre: Bandol AOC requires 18-month oak aging; extended lees contact must be managed. “Gamey” character at low concentration considered acceptable.

  • Sauvignon Blanc: Marlborough GI style emphasizes reductive handling for thiol preservation. Distinguishing positive varietal thiols from negative sulfides requires careful management.

Appellation-specific implications:

  • Côte-Rôtie AOC: Traditional Syrah production acknowledges reductive tendencies. Modern practice includes careful nitrogen management and controlled oxidative handling.

  • Bandol AOC: Mourvèdre’s sulfide tendency combined with mandatory 18-month oak aging requires vigilant monitoring. Foudre aging (larger volume, less surface oxidation) may exacerbate issues vs. barrique.

References

  • Siebert, T.E., Solomon, M.R., Pollnitz, A.P., & Jeffery, D.W. (2010). “Selective Determination of Volatile Sulfur Compounds in Wine by Gas Chromatography with Sulfur Chemiluminescence Detection.” American Journal of Enology and Viticulture, 61(4), 474-484. DOI: 10.5344/ajev.2010.61.4.474

  • Ugliano, M., & Henschke, P.A. (2009). “Yeasts and Wine Flavour.” Food Chemistry, 121, 1-8. DOI: 10.1016/j.foodchem.2008.09.076

  • Jiranek, V., Langridge, P., & Henschke, P.A. (1995). “Regulation of Hydrogen Sulfide Liberation in Wine-Producing Saccharomyces cerevisiae Strains by Assimilable Nitrogen.” American Journal of Enology and Viticulture, 46(1), 75-83. AJEV Link

  • Rauhut, D. (1993). “Production of Sulfur Compounds.” In Wine Microbiology and Biotechnology (ed. Fleet, G.H.), pp. 183-223. Harwood Academic Publishers. DOI: 10.1201/9780203749883