Wine Arcana

Concept

Malolactic fermentation

Also known as Malolactic conversion, MLF

Malolactic fermentation is a bacterial conversion in which malic acid becomes lactic acid and carbon dioxide. It commonly follows alcoholic fermentation, although the two processes can overlap. By changing one of wine's principal acids, it can reduce acidity, alter aroma and texture, and remove a substance that might otherwise support microbial activity later.

The conversion is usually carried out by Oenococcus oeni. Producers also call it MLF or malolactic conversion. Its effects depend on the wine, bacterial strain, timing, and subsequent handling.

How the conversion works

Malic acid has two carboxyl groups, while lactic acid has one. Lactic acid bacteria remove one group from each molecule of malic acid as carbon dioxide, leaving lactic acid. This reduces titratable acidity and generally raises pH, although the size of the change depends on the starting malic-acid content and the wine's buffering capacity. This conversion leaves tartaric acid unchanged.1. International Organisation of Vine and Wine, “Microbiological de-acidification by lactic acid bacteria,” International Code of Oenological Practices, II.3.1.2.3; Eveline J. Bartowsky, “Oenococcus oeni and malolactic fermentation—moving into the molecular arena,” Australian Journal of Grape and Wine Research 11 (2005), pp. 174–187.

Oenococcus oeni is especially important because it can function despite wine's low pH, alcohol, limited nutrients, and sulfur dioxide. Other lactic acid bacteria can also carry out the conversion, with effects that vary by strain. A population already present on grapes or in the winery may begin spontaneously, but onset and completion can be unpredictable. Inoculating with a selected culture gives the producer more control over strain and timing.

The bacteria may be introduced during alcoholic fermentation, a practice called co-inoculation, or after yeast has consumed the grape sugars. Early inoculation can shorten the unprotected interval before both processes finish and exposes the bacteria to less alcohol. Sequential inoculation separates the two stages and reduces the chance that bacteria will metabolise sugar, but the finished alcoholic wine may be a harsher environment. Yeast strain, pH, temperature, alcohol, nutrients, and sulfur dioxide can promote or inhibit the bacteria in combination and guide the timing.

Acidity, aroma, and texture

The most direct sensory effect is a different impression of acidity. Converting malic acid to lactic acid usually makes a wine seem less sharp and can change how its tannin, alcohol, and fruit are perceived. This is often described as a rounder texture. Other bacterial metabolism, yeast-lees contact, oak, and the wine's original composition may contribute separate textural effects.

The buttery-smelling compound diacetyl can accumulate while lactic acid bacteria metabolise citric acid. Its concentration and perception vary with bacterial strain, oxygen and redox conditions, timing, sulfur dioxide, contact with active yeast or bacteria, and the surrounding wine. Yeast and bacteria can reduce it to less aromatic compounds, and continued bacterial activity after malic acid is exhausted can change it further.2. Jan Clair Nielsen & Marianne Richelieu, “Control of Flavor Development in Wine during and after Malolactic Fermentation by Oenococcus oeni,” Applied and Environmental Microbiology 65 (1999), pp. 740–745, https://doi.org/10.1128/AEM.65.2.740-745.1999. Malolactic fermentation can therefore be conspicuously buttery, subtly modify aroma, or show little obvious diacetyl character.

Bacterial enzymes and metabolites can also change esters, grape-derived aroma precursors, acetaldehyde, and other compounds. These changes may preserve, redirect, or mute fruit expression depending on strain and wine matrix.

Stability and risk

Completing the conversion before bottling consumes malic acid that surviving bacteria might otherwise use later. An unplanned conversion in bottle can produce carbon dioxide, haze or sediment, lower acidity, and change aroma. This is why completion is often a stability goal even when a producer does not seek an obvious sensory signature. It is especially common in red wine and is also used for some white wines and traditional-method sparkling wine base wines.

A slow or uncontrolled bacterial population may form excessive volatile acidity, biogenic amines, or other unwanted compounds, and the accompanying rise in pH can make some wines more permissive to later microbial growth. Selected cultures, monitoring, and hygiene help manage these risks. Once the intended conversion is complete, producers generally clarify, cool, add sulfur dioxide, filter, or combine measures appropriate to the wine before packaging.3. Carla Virdis et al., “Lactic Acid Bacteria in Wine: Technological Advances and Evaluation of Their Functional Role,” Frontiers in Microbiology 11 (2021), article 612118, https://doi.org/10.3389/fmicb.2020.612118; University of California, Davis, “Diagnostic Key: Prevention of Post-Fermentation Off-Character Production.”

Why producers encourage, limit, or prevent it

A producer may encourage full conversion when the fruit retains substantial malic acid, when a softer acid profile suits the intended structure, or when removing malic acid before maturation and bottling is the safest route. This is the usual logic in many red wines. In a cool season or an acidic sparkling base, deacidification may be useful; in a warmer or riper wine, the same conversion may remove acidity the producer wants to keep.

Limiting the effect can mean putting only some lots through malolactic fermentation and blending them back, then stabilising the result. This allows a producer to retain part of the original acid profile without treating partial conversion as an uncontrolled stopping point. Delaying inoculation until after alcoholic fermentation can preserve the option to assess each lot, separate the microbial stages, or manage red-wine colour and aroma differently. Conversely, co-inoculation may be chosen for speed, bacterial survival, or a different sensory outcome.

Preventing malolactic fermentation preserves malic acid and avoids bacterial aroma changes. That can suit wines built around firm acidity or delicate grape-derived aroma, including many examples of Riesling, but it creates a different stability task: bacteria must be suppressed or removed so that conversion does not begin later. Sulfur dioxide, low temperature, clean handling, and microbial filtration are among the controls; their effectiveness depends on pH, wine composition, timing, and local rules. The choice is therefore a balance among acid, aroma, texture, microbial risk, and intended development.

Sources