Malolactic Fermentation (MLF): Mechanism and Effect

Malolactic fermentation is a process in winemaking in which tart-tasting malic acid, naturally present in grape must, is converted to softer-tasting lactic acid by lactic acid bacteria (LAB). Although called a fermentation, it is technically a decarboxylation: the malolactic enzyme (MLE) removes a carboxyl group from L-malic acid, yielding L-lactic acid and CO₂. The principal organism is Oenococcus oeni, formerly known as Leuconostoc oenos until reclassified in 1995, a gram-positive, obligately heterofermentative bacterium with ellipsoidal to spherical cocci morphology that grows in pairs or chains. O. oeni is uniquely suited to wine's harsh environment, tolerating conditions of low pH, high ethanol, and sulfur dioxide that inhibit most other microorganisms.

• Other LAB genera, including Lactobacillus, Pediococcus, and Leuconostoc, can also carry out MLF but are generally less desirable; some species increase volatile acidity or produce off-flavors.
• O. oeni is heterofermentative, producing lactic acid, acetic acid, ethanol, and CO₂ from sugar metabolism, as well as characteristic flavor compounds such as diacetyl.
• MLF most often follows alcoholic fermentation but can run concurrently with it; spontaneous MLF can originate from bacteria resident in winery equipment such as barrels, tanks, and presses.

The MLF reaction is a single enzymatic decarboxylation: L-malic acid (a dicarboxylic acid) is converted to L-lactic acid (a monocarboxylic acid) and CO₂ by the malolactic enzyme. Because diprotic malic acid is replaced by the less acidic monoprotic lactic acid, titratable acidity falls by 1 to 3 g/L and pH rises by approximately 0.3 units. The magnitude of this shift depends directly on the starting concentration of malic acid in the wine; cool-climate grapes, which retain higher malic acid at harvest, experience the most dramatic change. The process also consumes citric acid as a secondary substrate, generating diacetyl as an intermediary. Diacetyl concentration peaks when malic acid is exhausted and is strongly influenced by oxygen exposure and SO₂ levels.

• Each gram per litre of malic acid that is fully converted causes titratable acidity (expressed as tartaric acid) to drop by approximately 0.56 g/L.
• Diacetyl is an intermediary of citric acid metabolism; its final concentration is strongly dependent on oxygen availability and redox potential, with more oxidative conditions (such as a partially filled barrel) promoting higher levels.
• The decarboxylation reaction generates a proton gradient across the bacterial cell membrane, providing energy to drive ATP synthesis, which is why O. oeni can survive and grow in the nutrient-poor environment of finished wine.

MLF's most immediate effect is a reduction in perceived sourness, as the titratable acidity decreases and the sharper character of malic acid is replaced by the softer sensation of lactic acid. Mouthfeel also changes: the increase in pH, along with the production of polyols such as erythritol and glycerol, and the presence of ethyl lactate at concentrations up to 110 mg/L, contribute to a rounder, creamier texture. In Chardonnay, wines that complete MLF are often described as having hazelnut, dried fruit, and freshly baked bread aromas, while in red wines some O. oeni strains metabolize methionine to produce roasted and chocolate notes. Critically, MLF improves microbiological stability: by consuming malic acid, it removes a key nutrient source for spoilage bacteria, reducing the risk of refermentation in bottle.

• Diacetyl, the key buttery aromatic compound, derives from O. oeni's metabolism of citric acid; levels above 5 mg/L produce an intense butterscotch character perceived as a flaw, and its sensory threshold is approximately 15 times lower in Chardonnay than in Cabernet Sauvignon.
• Red wines completing MLF in barrel may gain enhanced spice and smoke aromas, though some studies show a loss of primary fruit notes such as raspberry and strawberry in Pinot Noir.
• Post-MLF color in red wines can diminish slightly due to pH-driven shifts in anthocyanin equilibrium; winemakers may add tartaric acid after MLF to restore acidity and stabilize color.

The decision to encourage, block, or partially complete MLF is one of the most consequential stylistic choices a winemaker makes. MLF is standard practice for virtually all red wines and for most white wines from Burgundy, including Chablis, where naturally high acidity makes softening desirable. By contrast, aromatic white varieties such as Riesling, Sauvignon Blanc, Gewürztraminer, and Pinot Grigio typically have MLF blocked to preserve their varietal freshness and crisp structure. Partial MLF, where only a proportion of lots are fermented and then blended, is another tool for achieving balance. Winemakers in warm climates, where grapes arrive with lower malic acid and higher pH, often block MLF to avoid producing flat, flabby wines lacking freshness.

• White Burgundy Chardonnay and Chablis undergo complete MLF as standard practice; the high natural acidity of these cool-climate wines means MLF softens rather than strips freshness.
• Virtually all red wines, including Bordeaux, Burgundy, Rhône, and New World Cabernet and Pinot Noir, complete MLF to soften acidity, integrate tannins, and achieve microbiological stability before bottling.
• MLF is generally suppressed in aromatic whites such as Riesling, Sauvignon Blanc, and Gewürztraminer, where bright acidity and primary varietal aromatics are the hallmarks of style; sulfur dioxide additions and refrigeration are the primary blocking tools.

Successful MLF management hinges on controlling temperature, SO₂, and microbial population. Optimal conditions for O. oeni activity require temperatures between 20 and 37°C, with the process significantly inhibited below 15°C; many traditional cellars relied on spring warming to trigger spontaneous MLF. Free SO₂ must be below approximately 8 mg/L before inoculation, as molecular SO₂ above roughly 0.8 molecular SO₂ (pH-dependent but equivalent to around 35 to 50 ppm free SO₂) will inhibit LAB. After MLF is confirmed complete, a prompt SO₂ addition arrests residual LAB activity and protects the wine from oxidation. Inoculation with certified commercial O. oeni cultures is strongly preferred over spontaneous MLF, which is less predictable, slower to complete, and carries a higher risk of spoilage from undesirable LAB strains.

• Monitoring: winemakers confirm MLF completion using paper chromatography or enzymatic HPLC assay; residual malic acid above 30 mg/L indicates incomplete conversion and future instability risk.
• SO₂ strategy: do not add SO₂ post-alcoholic fermentation until MLF is confirmed complete; re-addition immediately after completion arrests LAB activity and stabilizes the wine.
• Spontaneous MLF carries higher risk: uninoculated ferments can take months, can be dominated by undesirable LAB strains, and may produce elevated biogenic amines or off-flavors such as excess diacetyl.
• Tools to block MLF include early SO₂ addition (50 to 100 mg/L total), refrigeration to below 14°C, sterile filtration, and biological inhibitors such as lysozyme.

Regional winemaking traditions illustrate how differently MLF can be deployed. White Burgundy, including wines from Chablis through to Meursault and Montrachet, undergoes complete MLF as standard, a practice that delivers texture and complexity without eliminating the high acidity that distinguishes these wines. In contrast, most New Zealand Sauvignon Blancs deliberately block MLF via SO₂ and cool fermentation temperatures to preserve the herbaceous, citrus, and tropical aromatics that define the style. Red wines across virtually all serious producing regions, from Bordeaux to Barossa, complete MLF as a baseline step for stability and textural integration. California's exuberant, full-MLF Chardonnay style, where barrel fermentation and complete malolactic conversion combine to produce rich buttery complexity, stands in deliberate contrast to the leaner, more restrained approach favored in cooler European appellations.

• White Burgundy and Chablis: complete MLF is standard, softening naturally high malic acidity without compromising the wines' characteristic mineral-driven structure.
• New Zealand Sauvignon Blanc: MLF is typically blocked with SO₂ and cold temperatures to preserve primary varietal aromatics; the resulting wines retain bright acidity suited to early drinking.
• Bordeaux reds: complete MLF is standard, typically conducted in barrel or tank post-harvest; Bordeaux white wines occasionally use partial MLF in high-acid vintages to improve balance.