Malolactic Fermentation (MLF)
The bacterial conversion of sharp malic acid to softer lactic acid, reshaping a wine's texture, flavor, and microbial stability from the inside out.
Malolactic fermentation (MLF) is a secondary process in which lactic acid bacteria, primarily Oenococcus oeni, convert tart malic acid into softer lactic acid and CO2. Standard practice for virtually all red wines, it reduces acidity, improves microbial stability, and generates flavor compounds including diacetyl, which contributes buttery or nutty character. MLF is also selectively used in white wines such as Chardonnay to add richness and roundness.
- Oenococcus oeni (formerly Leuconostoc oenos) is the dominant LAB species preferred by winemakers for MLF, valued for its tolerance to low pH, high alcohol, and SO2 compared to other LAB species
- Chemically, MLF is a decarboxylation: L-malic acid (dicarboxylic) converts to L-lactic acid (monocarboxylic) plus CO2, with each gram per liter of malic acid reducing titratable acidity (expressed as tartaric acid) by approximately 0.56 g/L
- Diacetyl, the compound behind buttery and nutty aromas in MLF-treated wines, has a sensory detection threshold of 0.2 mg/L in white wines and 2.8 mg/L in red wines; concentrations above 5 to 7 mg/L are generally considered undesirable
- Optimal MLF temperatures are between 20 and 37°C; the process is significantly inhibited at temperatures below 15°C, which is why cool-cellar barrel MLF in Burgundy can extend across the winter months
- Free SO2 above roughly 25 to 50 ppm (depending on pH) inhibits LAB; total SO2 above 50 ppm generally limits LAB growth, making SO2 management critical when MLF is desired
- Commercially prepared LAB starter cultures became available to winemakers in the 1990s, enabling far greater control and predictability over spontaneous MLF
- The process is standard for most red wine production and common for white varieties such as Chardonnay; winemakers may block MLF in aromatic whites, high-acid styles, or warm-climate whites where further acidity reduction is unwanted
Definition and Scientific Background
Malolactic fermentation is the bacterial-driven decarboxylation of L-malic acid to L-lactic acid and carbon dioxide, carried out by lactic acid bacteria (LAB) of the genera Oenococcus, Lactobacillus, and Pediococcus. Unlike alcoholic fermentation driven by Saccharomyces cerevisiae yeast, MLF is a bioconversion that reduces total acidity, raises pH, and generates a range of flavor and texture compounds. The process is sometimes called a 'second fermentation,' a term noted as early as 1837 by the German enologist Freiherr von Babo, though the term 'malolactic fermentation' itself came into scientific use in the early 20th century as oenologists recognized its bacterial origin. Malic acid is a dicarboxylic acid with a harsher taste; lactic acid, a monocarboxylic acid, is perceived as softer, making the transformation palpable on the palate.
- Enzymatic pathway: L-malic acid (dicarboxylic) is decarboxylated to L-lactic acid (monocarboxylic) plus CO2
- Driven by LAB, primarily Oenococcus oeni, the species most adapted to wine's low pH and high ethanol environment
- Described as a 'second fermentation' in wine literature since the 19th century; scientifically characterized in the early 20th century
Why MLF Matters for Wine Quality
MLF brings about three major outcomes in wine: deacidification, microbial stabilization, and sensory modification. Because malic acid is dicarboxylic it contributes twice as many protons per molecule as lactic acid, so its removal causes a measurable drop in titratable acidity and a rise in pH. Each gram per liter of malic acid converted reduces TA (expressed as tartaric acid) by approximately 0.56 g/L. For red wines, this softening of acidity is integral to structure and age-worthiness. MLF also removes malic acid as a potential carbon source for spoilage organisms, improving the long-term stability of the wine. The process generates compounds including diacetyl, acetoin, ethyl lactate, and various esters that contribute textural richness and aromatic complexity. For white wines, the decision to use MLF is stylistic: it adds body and creaminess to Chardonnay and similar varieties, while winemakers may deliberately block it in aromatic or high-acid styles to preserve freshness.
- Each gram per liter of malic acid converted reduces titratable acidity by approximately 0.56 g/L expressed as tartaric acid
- Removes malic acid as a nutrient source for spoilage organisms, enhancing microbiological stability of the finished wine
- Generates diacetyl, acetoin, ethyl lactate, and esters that contribute creamy texture and secondary aromatic complexity
Sensory Identification in the Glass
MLF-treated wines display distinctive sensory signatures rooted in the compounds produced during bacterial metabolism. Diacetyl is the most discussed, imparting buttery or nutty aromas at low to moderate levels; its odor threshold is just 0.2 mg/L in white wines and 2.8 mg/L in reds, and at levels above 5 to 7 mg/L it becomes overwhelming. At medium concentrations of roughly 1 to 4 mg/L, diacetyl contributes desirable butterscotch notes. Acetoin and 2,3-butanediol, downstream products of diacetyl metabolism, contribute more subtle creaminess. Ethyl lactate, which can reach as high as 110 mg/L after MLF, contributes to the round, oily mid-palate mouthfeel characteristic of MLF wines. In red wines, the higher concentration of polyphenols raises the sensory threshold for diacetyl, making it less detectable than in whites; MLF in reds is also associated with reduced green or vegetal character and enhanced red fruit aromas.
- Diacetyl threshold: 0.2 mg/L in white wines, 2.8 mg/L in reds; pleasant at 1 to 4 mg/L, undesirable above 5 to 7 mg/L
- Ethyl lactate, which can reach up to 110 mg/L post-MLF, contributes to the oily, rounded mid-palate mouthfeel
- In reds, MLF can reduce vegetal green notes and enhance red fruit character; buttery diacetyl is less perceptible due to polyphenol masking
Regional Applications and Stylistic Choices
MLF is standard practice for virtually all red wines globally, from Burgundy Pinot Noir and Bordeaux Cabernet blends to Barolo, Rioja, and New World Cabernet Sauvignon. In each case, conversion of harsh malic acid to softer lactic acid is essential to textural balance and long-term stability. For white wines, the decision is stylistic. Chardonnay, particularly in California and Burgundy, commonly undergoes full or partial MLF; the resulting wines show creamier texture, reduced sharp acidity, and, in high-diacetyl expressions, pronounced butterscotch character. Many Chablis producers, contrary to common myth, do allow MLF, though their wines remain mineral and taut due to the appellation's cool climate and high natural acidity. Winemakers in warmer climates or those working with aromatic varieties such as Sauvignon Blanc, Riesling, and Pinot Gris typically block MLF to preserve fresh fruit character and primary aromatics.
- MLF is standard for virtually all red wines, including Pinot Noir, Cabernet Sauvignon, Nebbiolo, Tempranillo, and Merlot
- Chardonnay is the most common white wine variety to undergo MLF, both in Burgundy and in New World regions such as California and Australia
- Winemakers block MLF in aromatic whites and warm-climate styles to preserve freshness, primary fruit, and higher natural acidity
Winemaking Control and Modern Practices
The four key parameters governing MLF success are pH, temperature, ethanol, and sulfur dioxide, and they interact in a compounding way. The optimal temperature range is 20 to 37°C; the process is significantly inhibited below 15°C and temperatures above 25°C can be lethal for LAB. Free SO2 above roughly 25 to 50 ppm inhibits bacteria, with total SO2 above 50 ppm generally limiting LAB growth. Winemakers who want MLF are advised not to add SO2 after alcoholic fermentation until the process is complete. The introduction of commercially prepared freeze-dried LAB starter cultures in the 1990s gave winemakers far greater control, enabling reliable completion and consistent flavor outcomes. Spontaneous MLF, relying on indigenous bacteria, introduces risk including sluggish conversion, volatile acidity production, and biogenic amine formation. Co-inoculation, where LAB are added concurrent with or shortly after yeast inoculation, can significantly shorten the total fermentation timeline and reduce the window during which the wine is unprotected by SO2.
- Optimal MLF temperature: 20 to 37°C; significantly inhibited below 15°C; temperatures above 25°C can be lethal for LAB
- Total SO2 above 50 ppm generally limits LAB growth; free SO2 management is critical when MLF is desired
- Commercial LAB starter cultures, available since the 1990s, improved reliability and reduced risks of spoilage compared to spontaneous MLF
Chemistry, Byproducts, and Related Concepts
Beyond the core malic-to-lactic conversion, LAB also metabolize citric acid during MLF, producing diacetyl as an intermediary compound. The maximum diacetyl concentration during MLF coincides with the exhaustion of malic acid in the wine, and is strongly influenced by oxygen concentration, redox potential, and SO2 levels. Diacetyl binds reversibly with SO2, meaning early sulfiting after MLF can suppress buttery aroma temporarily, only for it to re-emerge as SO2 dissipates during aging. Acetoin and 2,3-butanediol are further reduction products of diacetyl, contributing subtler creamy and nutty notes. Ethyl lactate, synthesized from ethanol and the lactic acid produced during MLF, adds fruity, buttery, and creamy notes and contributes to mouthfeel roundness. MLF also intersects with wine stability concepts including volatile acidity (uncontrolled spoilage bacteria can generate excess acetic acid), biogenic amine formation, and color stability in red wines.
- Diacetyl production peaks at malic acid exhaustion and is amplified by oxidative conditions, citric acid content, and delayed SO2 addition
- Diacetyl binds reversibly with SO2; if SO2 decreases during storage, free diacetyl can increase again, affecting the wine's sensory profile
- Ethyl lactate, produced during MLF, contributes to fruity and creamy aromas and the rounded mouthfeel associated with MLF-treated wines
MLF-treated wines show a shift from sharp, angular acidity toward a softer, rounder palate driven by the substitution of malic acid with lactic acid. Diacetyl contributes buttery and nutty aromas, perceptible at low levels in whites and at higher concentrations in richer, barrel-fermented Chardonnay. Acetoin and 2,3-butanediol add subtle cream and hazelnut notes, while ethyl lactate delivers a fruity, waxy, oily richness to the mid-palate. In red wines, MLF can reduce green or vegetal characters and enhance red fruit expression. The overall effect is wines with increased textural weight, a creamier finish, and reduced sharp or herbaceous acidity, without necessarily exhibiting pronounced butter aromas, which depend heavily on diacetyl levels and winemaking choices.