Malolactic Fermentation

Malolactic fermentation is a secondary biological process in winemaking in which lactic acid bacteria convert L-malic acid, a sharp, green-apple-tasting dicarboxylic acid, into the softer, monoprotic L-lactic acid and carbon dioxide. Technically a decarboxylation rather than a true fermentation, MLF does not produce ethanol; instead, it deacidifies wine, improves microbial stability, and modifies aroma and texture. It is standard practice for virtually all red wine production and is selectively employed for certain white wine styles such as Chardonnay and sparkling wine base wines.

• L-malic acid (dicarboxylic) is converted to L-lactic acid (monocarboxylic) and CO₂ via the malolactic enzyme
• Most often occurs after alcoholic fermentation, but can run concurrently (co-inoculation)
• Raises pH by 0.2–0.5 units; reduces total acidity by roughly 0.6 g/L per gram of malic acid converted
• Improves microbial stability by removing malic acid, a substrate for potential spoilage organisms

The lactic acid bacteria responsible for MLF belong primarily to the genera Oenococcus, Lactobacillus, and Pediococcus. Of these, Oenococcus oeni (formerly known as Leuconostoc oenos) is the species most prized by winemakers due to its exceptional tolerance of the harsh wine environment: low pH, high ethanol, elevated SO₂, and limited nutrients. It decarboxylates malic acid via an intracellular malolactic enzyme, generating energy through a proton gradient across its cell membrane. Modern winemakers routinely inoculate with commercial freeze-dried O. oeni cultures to ensure predictable, timely MLF and to guard against spoilage organisms that could otherwise thrive during the lag phase of spontaneous MLF.

• O. oeni was reclassified from Leuconostoc oenos to its current genus in 1995; it grows optimally at pH 4.5–6.5 but can function at wine pH of 3.0–3.8
• Commercial inoculation at more than 2 × 10⁶ cells/mL is recommended to ensure reliable, complete conversion
• Co-inoculation (adding bacteria 24–48 hours after yeast) can shorten production time and produce fruitier, lower-diacetyl wine styles
• Yeast can inhibit LAB through medium-chain fatty acids (octanoic and decanoic acids); yeast hull additions help mitigate this effect

MLF fundamentally reshapes a wine's sensory profile. Malic acid's sharp, green-apple character softens into lactic acid's creamier, rounder quality, producing greater palate weight and a smoother mouthfeel. This change is partly attributable to the rise in pH, but also to the production of polyols such as glycerol and ethyl lactate. Diacetyl, an intermediate of citric acid metabolism during MLF, introduces buttery, nutty, or toasty aromas when present at moderate concentrations. Because diacetyl thresholds differ significantly by wine type, winemakers must manage its levels carefully through strain selection, inoculation timing, and oxygen exposure.

• Diacetyl at 1–4 mg/L contributes desirable buttery and butterscotch notes; above 5–7 mg/L it is considered undesirable by consumers
• In Chardonnay, MLF wines are often described as having hazelnut, dried fruit, and freshly baked bread notes alongside a creamy texture
• In red wines, some O. oeni strains metabolise methionine to produce roasted aroma and chocolate notes
• MLF can reduce primary fruit intensity in some red wines, including raspberry and strawberry notes in Pinot Noir, due to pH-driven changes in aroma compound equilibria

Red winemakers universally promote MLF, which softens tannin perception and integrates oak character. Many also prefer MLF to occur during barrel aging for greater integration of wood and wine. For white wines, the decision is stylistic: producers of white Burgundy (Chardonnay) routinely encourage MLF for complexity and roundness, while makers of aromatic varieties such as Riesling and Gewürztraminer actively block it to preserve freshness and fruit definition. Preventing MLF requires maintaining adequate free SO₂ (at least 25 ppm), keeping cellar temperatures between 10–14°C (50–57°F), and sterile filtration at bottling to remove residual bacteria.

• Promote MLF: virtually all reds, cool-climate Chardonnay, sparkling base wines, premium oak-aged whites
• Block MLF: Riesling, Gewürztraminer, Albariño, fruit-forward Sauvignon Blanc, low-acid warm-climate whites
• Rosé producers often use partial MLF (on a portion of the blend) to balance freshness and textural softness
• MLF in the bottle is a wine fault, producing CO₂, cloudiness, and off-flavours; sterile filtration or adequate SO₂ prevents this

In Burgundy, white wine producers such as Domaine Leflaive ferment exclusively with indigenous yeasts and practice bâtonnage between alcoholic and malolactic fermentation, allowing MLF to proceed naturally in barrel as part of a traditional, minimally interventionist style. Cool-climate regions including Burgundy and Champagne, where high malic acid concentrations are common, depend on MLF for balance. In contrast, New Zealand Sauvignon Blanc producers typically block MLF to preserve the variety's hallmark herbaceous and tropical character. California Chardonnay producers vary widely, with many opting for full MLF in barrel to achieve a rich, creamy style, while others use partial MLF or no MLF to retain more vibrant acidity.

• Domaine Leflaive (Puligny-Montrachet): indigenous yeast fermentation in oak barrel; bâtonnage between alcoholic fermentation and MLF; bottling in the spring of the second year
• Champagne: MLF is standard for non-vintage blends for textural integration; some prestige cuvées block it to preserve acidity and freshness
• Chablis: producers are split, with some using partial or no MLF to preserve the appellation's characteristic steely, mineral acidity
• Barossa Valley Shiraz: MLF is standard; winemakers select O. oeni strains to retain fruit-forward character while gaining tannin softness

Stuck MLF, where bacteria fail to complete the conversion of malic acid, is among the most disruptive problems in the cellar. The most common causes are excessive SO₂, temperatures below 15°C, pH below 3.0–3.2, high ethanol, nutrient deficiency, or yeast-bacteria incompatibility. Prevention relies on avoiding post-fermentation SO₂ additions until MLF is complete, maintaining cellar temperatures above 18°C (65°F) during the process, and supplementing with bacterial nutrients where required. If MLF stalls, the wine should be assessed for residual malic acid and, if above an acceptable threshold, re-inoculated with a fresh, acclimatised O. oeni culture after addressing the root cause.

• Excessive free SO₂ (above 10 mg/L) drastically inhibits LAB; total SO₂ above 50 ppm is generally incompatible with active MLF
• Temperatures below 15°C significantly inhibit MLF; below 10°C the process essentially stops
• Nutrient deficiency post-alcoholic fermentation is a common cause of stuck MLF; yeast autolysis releases amino acids that can support bacterial growth
• A prolonged lag phase before MLF begins increases the risk of Brettanomyces contamination; prompt inoculation reduces this window of vulnerability