Malolactic Fermentation

The enzymatic heart of MLF is the malate decarboxylase (malic enzyme) present in lactic acid bacteria, which catalyzes a coupled decarboxylation–oxidation reaction: L-malate + NAD⁺ → pyruvate + CO₂ + NADH. Oenococcus oeni then reduces pyruvate to L-lactate via lactate dehydrogenase, regenerating NAD⁺. The net stoichiometry is L-malate → L-lactate + CO₂, generating a modest ATP yield that supports bacterial growth under the harsh conditions of wine (pH 3.1–3.5, ethanol 10–14 %, SO₂ 20–40 ppm). The drop in titratable acidity is typically 1–3 g/L, and pH rises 0.1–0.3 units. Beyond deacidification, MLF bacteria produce diacetyl (buttery aroma, threshold 0.2 mg/L), acetoin, and 2,3-butanediol — compounds that contribute butteriness and complexity. Sulfur dioxide management is critical: free SO₂ above 30–50 ppm inhibits MLF; winemakers time additions accordingly. Temperature must remain above 15 °C for O. oeni to remain active.

• MLF is the primary tool for softening high-acid wines from cool climates (Burgundy, Champagne, Willamette Valley) where malic acid levels are naturally elevated — without it, many Pinot Noirs would taste harsh and unbalanced.
• In Chardonnay and other barrel-fermented whites, intentional MLF plus lees contact creates the hallmark 'creamy, buttery' style associated with white Burgundy and premium California Chardonnay.
• Blocking MLF (by maintaining high SO₂ or cold-stabilizing at 10 °C) preserves primary fruit and crisp acidity — the winemaker's choice to retain freshness in Riesling, Sauvignon Blanc, and sparkling wine bases.
• Diacetyl production during MLF is a double-edged variable: at low concentrations it adds complexity; at high concentrations it overwhelms fruit and makes a wine taste like movie-theater popcorn butter.
• In ripened cheeses, residual lactic fermentation including malolactic activity at the surface contributes to rind development and paste softening in bloomy-rind styles.

1. 1To encourage MLF naturally, complete primary alcoholic fermentation, rack to barrel or tank, raise cellar temperature to 18–22 °C, and avoid SO₂ additions until MLF is confirmed complete via paper chromatography or enzymatic malic acid assay.
2. 2Inoculate with a commercial O. oeni culture (e.g., Lalvin 31, Chr. Hansen Viniflora Oenos) 24–48 hours after the end of primary fermentation if natural MLF is unreliable or if speed is critical.
3. 3Monitor malic acid disappearance with paper chromatography: spot wine on chromatography paper with malic acid and lactic acid standards, develop in solvent, and compare. A single yellow-green spot (lactic) replacing two spots (malic + lactic) confirms completion.
4. 4To block MLF in white wines intended to retain high acidity, add 25–40 mg/L free SO₂ immediately after primary fermentation and hold the wine below 13 °C.
5. 5In butter and crème fraîche production, cultured-cream MLF activity contributes lactic acid and diacetyl: adding a mesophilic culture (Leuconostoc cremoris, Lactococcus lactis subsp. cremoris) and incubating at 20–22 °C for 16–18 hours converts cream into cultured butter base with characteristic tang and butterscotch depth.

• Bubbling or light effervescence in a tank or barrel 2–4 weeks after primary fermentation ends signals active MLF — the CO₂ released from malic acid decarboxylation.
• Aroma shifts from green apple and tart citrus toward soft stone fruit, cream, and light buttery notes as MLF progresses.
• A wine tasted mid-MLF often shows an oddly flat, 'furry' mouthfeel — the acidity is in transition and feels unresolved until MLF completes.
• Completion: the wine's acidity feels rounded and integrated, without the sharp acidic cut of unresolved malic acid; the finish is longer and creamier.

• Adding SO₂ too early after primary fermentation, inadvertently blocking MLF in a wine that would have benefited from it — particularly a common error with red wines in cool vintages.
• Allowing MLF to proceed at temperatures below 15 °C, where the process stalls partway through and leaves residual malic acid alongside lactic acid — an unstable, microbiologically risky wine.
• Over-inoculating with O. oeni in conjunction with high lees contact, which can lead to excessive diacetyl and biogenic amine (histamine, tyramine) production.
• Failing to monitor MLF completion before bottling — wine with residual malic and viable bacteria will re-ferment in bottle, creating CO₂ pressure, cloudiness, and off-flavors.
• Assuming MLF is always desirable: for crisp styles (Vinho Verde, Muscadet, German Riesling), MLF destroys the essential aromatic freshness and fruit definition.

• Lactic Acid Fermentation

  MLF is a specialized form of lactic fermentation; understanding the general pathway clarifies why lactic acid bacteria dominate the process.

• Microbial Succession in Fermentation

  MLF follows alcoholic fermentation in a classic succession; yeast first convert sugar to alcohol, then bacteria convert malic to lactic acid.

• Acid-Base Balance in Cooking

  The pH shift from MLF is the biological analog of adding a buffer to a sauce — both raise pH and reduce perceived sharpness.

• Acetification (Acetic Acid Fermentation)

  If MLF bacteria are stressed or wine is exposed to oxygen, acetic acid rather than lactic acid accumulates — the failure mode that links these two processes.

1. 1.Winkler, A.J. et al. — General Viticulture (1974, University of California Press)
2. 2.Ribéreau-Gayon, P. et al. — Handbook of Enology, Vol. 1: The Microbiology of Wine and Vinifications (2006, Wiley)
3. 3.Fugelsang, K.C. & Edwards, C.G. — Wine Microbiology: Practical Applications and Procedures (2007, Springer)
4. 4.Davis, C.R. et al. — Practical implications of malolactic fermentation, American Journal of Enology and Viticulture 36 (1985)

Confidence: high