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Malolactic Fermentation: Initiation, Blocking, and Troubleshooting

Evidence-based protocols for managing malolactic fermentation including timing decisions, inoculation strategies, and prevention methods for different wine styles.

6 min read
Eno Codex Editorial

Malolactic Fermentation: Initiation, Blocking, and Troubleshooting

Problem Definition

Malolactic fermentation (MLF) is the bacterial conversion of L-malic acid to L-lactic acid and CO₂. This secondary fermentation reduces total acidity, increases pH, and modifies sensory character (butter notes from diacetyl, reduced green/tart character). The decision to allow, encourage, block, or control MLF timing is among the most consequential choices in winemaking.

MLF management failures include:

  • Uncontrolled MLF in wines intended to remain malic (high-acid whites)
  • Incomplete MLF leaving residual malic acid and microbial instability
  • Stuck MLF due to environmental stress
  • Elevated diacetyl from improper management

Technical Context

Biochemistry

Oenococcus oeni (formerly Leuconostoc oenos) is the primary MLF bacterium in wine. The reaction:

COOH-CHOH-CH₂-COOH → COOH-CHOH-CH₃ + CO₂

L-malic acid (dicarboxylic) → L-lactic acid (monocarboxylic) + CO₂

Chemical impact:

  • TA reduction: ~0.1-0.3 g/L per g/L malic acid converted
  • pH increase: 0.1-0.3 units typically
  • Sensory: Reduced tartness, potential butter/cream notes

Factors Affecting MLF

FactorOptimal for MLFInhibitory
pH3.3-3.5<3.0 or >4.0
Temperature18-22°C<15°C or >30°C
Alcohol<13% v/v>14% v/v (strain-dependent)
Free SO₂<10 mg/L>30 mg/L
Residual sugar<5 g/LHigh residual (osmotic stress)

Diacetyl Dynamics

Diacetyl (2,3-butanedione) is a byproduct of MLF metabolism. Its concentration depends on:

  • Citric acid availability (metabolized by O. oeni producing diacetyl)
  • Yeast activity post-MLF (yeast reductases convert diacetyl to less aromatic acetoin)

Sensory threshold: ~0.2-2.0 mg/L (varies by wine matrix and individual sensitivity)

Management: Maintain wine on yeast lees during and after MLF for 2-4 weeks; lees reduce diacetyl to below threshold.

Options and Interventions

Encouraging MLF

Co-inoculation (simultaneous with AF):

  • Add O. oeni culture 24-48 hours after yeast inoculation
  • Advantages: Faster completion; reduced VA risk; nutrients available
  • Risks: Bacterial population crash if SO₂ at crush is high

Sequential inoculation (after AF):

  • Traditional timing: inoculate after alcoholic fermentation complete
  • Advantages: Controlled timing; can assess wine first
  • Risks: Inhibitory conditions post-AF (high alcohol, low nutrients, SO₂)

Spontaneous MLF:

  • Rely on indigenous LAB population
  • Advantages: Potential complexity from diverse strains
  • Risks: Slow, unreliable; potential spoilage organism activity; stuck MLF

Protocol for reliable MLF:

  1. Complete AF to dryness
  2. Rack off gross lees (retains fine lees)
  3. Ensure SO₂ <10 mg/L free
  4. Warm wine to 18-22°C
  5. Inoculate with O. oeni culture (direct addition or starter)
  6. Monitor malic acid weekly (enzymatic or paper chromatography)
  7. Completion: <0.1 g/L malic acid

Blocking MLF

Wines where MLF is typically blocked:

Blocking methods:

  1. Sulfur dioxide: Maintain free SO₂ >30 mg/L; molecular SO₂ >0.8 mg/L

    • Most effective at lower pH
    • May require frequent additions post-AF

  2. Lysozyme: Enzyme that lyses Gram-positive bacteria cell walls

    • Dosage: 250-500 mg/L
    • Does not affect yeast or Gram-negative bacteria
    • Some binding to tannins in red wine (reduced efficacy)

  3. Sterile filtration: 0.45 μm membrane removes bacteria

    • Apply before bottling
    • Does not prevent re-inoculation if contaminated

  4. Temperature: Maintain <12°C to inhibit bacterial growth

    • Effective during storage
    • Not sufficient alone if high bacterial population present

  5. pH adjustment: Very low pH (<3.0) inhibits O. oeni

    • Rarely practical as primary strategy
    • Can be combined with other methods

Partial MLF

Some producers allow partial malic acid conversion for stylistic balance:

  • Monitor malic acid; intervene when target reduction achieved
  • Intervention: add SO₂ (30-50 mg/L), chill, filter
  • Difficult to achieve consistently; timing critical

Trade-offs and Risks

Complete MLF:

  • Reduces total acidity; may result in flabby wines if initial TA low
  • pH increase elevates VA and oxidation risk
  • Microbial stability achieved if completed fully

Blocked MLF:

  • Preserves freshness and varietal character
  • Risk of uncontrolled MLF in bottle if bacteria survive
  • Requires higher SO₂ management

Stuck MLF:

  • Residual malic acid creates instability
  • Neither fresh nor MLF-softened; worst of both
  • Must restart or fully block

Diacetyl management:

  • Over-reliance on MLF for “buttery” character considered dated in many markets
  • Sur lie aging reduces diacetyl but extends cellar time
  • Consumer preferences vary by market

Practical Implications

Variety-specific considerations:

  • Chardonnay: Traditional Burgundy style includes full MLF plus sur lie aging for integrated butter notes. Chablis producers often block or limit MLF to preserve minerality.

  • Pinot Noir: MLF nearly universal; completes fermentation profile. Retention on fine lees post-MLF standard for complexity.

  • Sangiovese: High natural acidity benefits from MLF. Complete conversion standard for Chianti Classico and Brunello.

  • Riesling: MLF typically blocked to preserve characteristic acidity. German and Alsatian traditions emphasize malic acid freshness.

Appellation-specific implications:

  • Champagne AOC: Base wines may undergo MLF before assemblage and second fermentation. Decision varies by house style. Non-MLF base wines retain freshness but risk in-bottle MLF.

  • Bourgogne AOC: Both white and red Burgundy traditionally undergo complete MLF. White wines are typically aged sur lie for diacetyl reduction.

  • Chablis AOC: Divided tradition—some producers complete MLF, others block partially or completely for freshness. No regulatory requirement either way.

  • Chardonnay — full MLF for Burgundy style; blocked for Chablis freshness
  • Pinot Noir — MLF nearly universal
  • Sangiovese — benefits from MLF due to high natural acidity
  • Riesling — MLF typically blocked to preserve acidity
  • Sauvignon Blanc — usually blocked for aromatic preservation

References

  • Lonvaud-Funel, A. (1999). “Lactic Acid Bacteria in the Quality Improvement and Depreciation of Wine.” FEMS Microbiology Reviews, 23, 1-20. DOI: 10.1111/j.1574-6976.1999.tb00371.x

  • Bartowsky, E.J. (2005). “Oenococcus oeni and Malolactic Fermentation—Moving into the Molecular Arena.” Australian Journal of Grape and Wine Research, 11(2), 174-187. DOI: 10.1111/j.1755-0238.2005.tb00285.x

  • Bartowsky, E.J., & Henschke, P.A. (2004). “The ‘Buttery’ Attribute of Wine—Diacetyl—Desirability, Spoilage and Beyond.” International Journal of Food Microbiology, 96(3), 235-252. DOI: 10.1016/j.ijfoodmicro.2004.03.013

  • Ribéreau-Gayon, P., Dubourdieu, D., Donèche, B., & Lonvaud, A. (2006). Handbook of Enology, Volume 1: The Microbiology of Wine and Vinifications (2nd ed.). Wiley. ISBN: 978-0-470-01034-1. Publisher Link

  • AWRI (2024). “Malolactic Fermentation Fact Sheet.” Australian Wine Research Institute. https://www.awri.com.au