Malolactic fermentation is a process in winemaking where bacteria convert sharp-tasting malic acid (the kind found in green apples) into softer lactic acid (the kind found in milk and yogurt). Despite its name, it’s not technically a fermentation at all. It’s an enzymatic reaction that reduces a wine’s acidity, rounds out its texture, and can add flavors like butter and cream. Nearly every red wine you’ve ever had has gone through it, and it plays a defining role in the taste of many white wines too, especially Chardonnay.
How the Conversion Works
During the primary fermentation of wine, yeast converts grape sugar into alcohol. Malolactic fermentation is a separate, secondary process carried out by bacteria rather than yeast. The key player is a species called Oenococcus oeni, a lactic acid bacterium prized for its ability to survive in the harsh conditions inside wine: low pH, high alcohol, and the presence of sulfur dioxide, which is used as a preservative.
The bacteria produce an enzyme that strips one of the two acid groups from malic acid, releasing carbon dioxide and leaving behind lactic acid. Malic acid is a dicarboxylic acid (it has two acid groups), while lactic acid is a monocarboxylic acid (just one). That simple subtraction is why the wine tastes less tart afterward. The reaction depends on specific cofactors to proceed, and the bacteria also need a transport protein to shuttle malic acid into the cell and lactic acid back out. But from a winemaker’s perspective, the chemistry is straightforward: a stronger acid becomes a weaker one, and CO₂ bubbles off.
Why It Changes How Wine Tastes
The most obvious effect is a drop in acidity. Malolactic fermentation typically raises a wine’s pH by up to 0.2 units and noticeably lowers its titratable acidity. In practical terms, the wine shifts from tasting sharp and angular to feeling rounder and smoother on the palate.
But deacidification is only part of the story. While breaking down malic acid, the bacteria also metabolize small amounts of citric acid in the wine. A byproduct of that citric acid metabolism is diacetyl, the compound responsible for buttery and butterscotch flavors. In white wines, the sensory threshold for diacetyl is quite low (around 0.2 mg/L), meaning even small amounts register on your palate. In red wines, where tannins and fruit flavors are more dominant, the threshold is higher (roughly 0.9 to 2.8 mg/L), so the buttery note is less prominent. This is why a Chardonnay that has gone through malolactic fermentation can taste distinctly creamy and buttery, while a Cabernet Sauvignon that went through the same process may not taste buttery at all.
Red Wine vs. White Wine
Virtually all red wines undergo malolactic fermentation. Red grapes tend to have higher malic acid levels, and the softening effect complements the wine’s tannins and fruit character. Completing the process also provides biological stability, since the bacteria consume the nutrients that could otherwise feed spoilage organisms later in the bottle. For red wine producers, skipping malolactic fermentation would be unusual.
White wines are a different calculation. Winemakers choose whether to allow, encourage, or prevent the process depending on the style they want. A rich, full-bodied Chardonnay often goes through complete malolactic fermentation, sometimes in oak barrels, to maximize that creamy, buttery profile. A crisp Sauvignon Blanc or Riesling, on the other hand, relies on bright acidity for its character, so winemakers typically block the process entirely. Some producers split the difference, putting part of a batch through malolactic fermentation and blending it back with the untreated portion to balance richness and freshness.
How Winemakers Control the Process
Winemakers have several tools for encouraging or preventing malolactic fermentation. The most important are sulfur dioxide (SO₂), temperature, and timing.
- Sulfur dioxide: Lactic acid bacteria are far more sensitive to SO₂ than yeast. Free SO₂ levels as low as 10 mg/L can be lethal to O. oeni, and bound SO₂ above 30 mg/L delays bacterial growth significantly. Above 50 mg/L of bound SO₂, malolactic fermentation is strongly inhibited. When winemakers want the process to happen, they keep SO₂ levels minimal until it’s complete. Red wines destined for malolactic fermentation are typically held below 20 mg/L of free SO₂ through both primary and secondary fermentation.
- Temperature: The bacteria work best in a moderate range, roughly 18 to 22°C (64 to 72°F). Cooling a wine down after primary fermentation can stall or prevent bacterial activity, which is one reason why some white wines stored cold never undergo the process spontaneously.
- Inoculation timing: Winemakers can add commercial bacterial cultures either after primary fermentation is finished (sequential inoculation) or at the same time as yeast (co-inoculation). Co-inoculation shortens the overall winemaking timeline without causing excessive volatile acidity. It also tends to produce wines with lower buttery character, because the yeast present during fermentation can break down diacetyl into compounds with much higher sensory thresholds, effectively neutralizing the butterscotch flavor. Sequential inoculation, by contrast, allows more diacetyl to persist and gives the winemaker more control over the final flavor profile.
Biological Stability
Beyond flavor, malolactic fermentation serves a practical purpose: it makes wine more stable. If malic acid remains in a finished, bottled wine, stray lactic acid bacteria can eventually start converting it on their own. This uncontrolled secondary fermentation in the bottle produces off-flavors, haziness, and unwanted carbonation. By completing the process deliberately before bottling, winemakers remove the bacteria’s food source and eliminate the risk of this kind of spoilage.
Biogenic Amines: A Downside to Watch
One less-discussed consequence of malolactic fermentation is the production of biogenic amines, including histamine, putrescine, and tyramine. These compounds form when bacteria break down amino acids in the wine, and they’re the reason some people experience headaches, flushing, or digestive discomfort after drinking red wine. Red wines consistently show higher levels of biogenic amines than whites, with median histamine concentrations around 2.45 mg/L and putrescine around 7.30 mg/L in one study of commercial wines. The difference maps directly onto the fact that red wines routinely undergo malolactic fermentation while most whites do not.
The bacterial strain matters. Some strains of O. oeni and Lactobacillus produce significant amounts of these amines, while others produce very little or can even degrade them. Researchers have identified specific Lactobacillus plantarum strains that break down putrescine and tyramine without producing new amines, making them promising candidates for winemakers who want the benefits of malolactic fermentation with fewer downsides. Poor storage conditions after fermentation can also drive biogenic amine levels higher, as bacteria with strong amino acid-degrading activity continue to work.
How to Spot It in Your Glass
You can often tell whether a wine has gone through malolactic fermentation just by tasting it. A Chardonnay with prominent butter, cream, or butterscotch notes almost certainly completed the process. A Chardonnay that tastes lean, mineral, and apple-crisp likely did not. In red wines the clues are subtler, since the process is nearly universal, but comparing a young, acidic red made without malolactic fermentation (some Beaujolais, for example) against a typical Merlot or Pinot Noir reveals how much smoother and rounder the converted wine feels on the palate. Wine labels rarely mention malolactic fermentation directly, but tasting notes that reference “creamy mouthfeel” or “buttery character” are usually pointing to it.