What Is Malolactic Fermentation?

Quick answer

Malolactic fermentation is a stage of winemaking in which lactic-acid bacteria convert malic acid into lactic acid and carbon dioxide.

Malic acid is a naturally occurring grape acid. It can contribute to a sharper, more angular impression in wine. Lactic acid is generally perceived as softer, so MLF usually reduces the sense of sharpness and changes the wine’s acid balance. It does not remove all acidity, and it does not guarantee that a wine will taste soft or creamy.

Despite its name, MLF is not the same as the alcoholic fermentation that produces wine’s alcohol. Alcoholic fermentation is mainly performed by yeast, which converts grape sugars into alcohol and carbon dioxide. MLF is a bacterial conversion of acid.

Winemakers may allow MLF to happen naturally, use selected bacteria to manage it, prevent it, or allow it in only part of a wine. The decision depends on the grape, vintage, acidity, intended style and the wine’s need for stability.

How does malolactic fermentation work?

During MLF, lactic-acid bacteria convert malic acid into lactic acid and carbon dioxide. One important wine-associated bacterium is Oenococcus oeni, although other lactic-acid bacteria can also be involved.

The two main biological processes look like this:

MLF often occurs after alcoholic fermentation, although winemakers can manage the timing in different ways. The important point for wine drinkers is that MLF is not simply “a second alcoholic fermentation”. It is more precisely a bacterial conversion, even though the traditional name remains malolactic fermentation.

What does MLF do to wine?

It can make acidity seem less sharp

MLF generally lowers a wine’s titratable acidity and raises its pH. In practical tasting terms, the wine may seem less sharp, angular or pointed.

That does not mean MLF makes a wine flat. A wine can still have plenty of refreshing acidity after MLF. Its final impression also depends on grape ripeness, alcohol, tannin, residual sugar, pH and the balance of other components.

This is why “softer” is best understood as a tasting description, not a guarantee. A soft-tasting wine has not necessarily undergone MLF.

It may change texture

Some wines that have undergone MLF seem rounder or smoother on the palate. The altered acid balance can make the wine feel less tense, while other winemaking choices may contribute to a fuller or creamier impression.

MLF is only one possible influence on texture. Lees contact, oak, alcohol, blending and grape composition can also affect how a wine feels. These processes may be used alongside MLF, but they are not the same thing.

It can sometimes add buttery or creamy aromas

MLF can contribute to the formation of diacetyl, a compound associated with buttery, creamy, nutty or dairy-like aromas. This is one reason people often connect MLF with buttery Chardonnay.

But MLF does not automatically make a wine taste like butter. The amount of diacetyl formed and retained varies with the bacteria, the wine and how it is handled. Some wines complete MLF with little obvious buttery aroma.

A creamy or buttery impression may also come from oak, lees contact or a combination of influences. Aroma alone is therefore not a reliable test for whether MLF occurred.

Why do winemakers use MLF?

MLF is a choice, not a compulsory stage in every wine.

A winemaker may encourage it to:

• reduce a wine’s sharper acid impression;
• shape the wine’s texture;
• develop a particular style;
• contribute certain aroma characteristics; or
• reduce the chance of an unwanted bacterial conversion happening later in the bottle.

A winemaker may prevent or limit it to:

• preserve bright, energetic freshness;
• retain more of the original malic-acid character;
• protect a delicate aromatic style;
• avoid excessive softening; or
• avoid prominent buttery or dairy-like aromas.

Sometimes only part of a wine undergoes MLF. This can help a winemaker balance freshness with texture before blending.

Completing MLF may also support biological stability by reducing the amount of malic acid available for a later, unwanted bacterial conversion. However, it is not a complete guarantee of bottle stability; many aspects of wine chemistry and packaging contribute to that.

Does every wine undergo malolactic fermentation?

No. MLF is common in many red wines, but it is not universal. It is also used in some white wines, while others undergo only partial MLF or none at all.

For Australian wine drinkers, MLF is especially relevant because it is encountered frequently in Australian red wine and selectively in Australian white wine. A historical Australian Wine Research Institute survey estimated that MLF was used in approximately 99% of Australian red table wine and 11% of Australian white table wine in its surveyed dataset. Those figures are historical survey results, not current universal statistics.

Australian Chardonnay is a useful example of winemaking choice. It may be made with full, partial or no MLF. A wine with MLF may seem rounder or creamier, while one that avoids it may retain a sharper, more linear acid profile. Oak, lees contact, ripeness and other decisions also influence the result.

Australian practice is not uniform. Choices vary by grape variety, vintage, winery, fruit acidity and intended style.

MLF, oak and lees: what is the difference?

These terms are easy to mix up:

• MLF is a bacterial conversion of malic acid into lactic acid.
• Oak maturation means wine has contact with oak, which can contribute characters such as vanilla, spice or toast.
• Lees contact means wine is kept with the spent yeast cells left after fermentation, which can influence texture and savoury or creamy impressions.

A winemaker may use two or all three techniques in the same wine. A buttery or creamy wine does not therefore point to one process by itself.

For broader context, see how wine is made, what Chardonnay is and what oak does to wine.

Is malolactic fermentation good or bad?

Neither. MLF is a winemaking tool, and whether it is useful depends on the wine being made.

In a high-acid wine, reducing some sharpness may help create a more balanced style. In another wine, preserving that sharpness may be the goal. A winemaker may want buttery complexity in one Chardonnay but a bright, clean and linear style in another.

The question is not whether MLF is inherently good. It is whether the conversion suits the wine’s intended balance, texture, aroma and stability.

Can you taste whether a wine has undergone MLF?

Not reliably from one sensation alone.

A wine that has undergone MLF may seem less sharply acidic, rounder or more textured, and it may show buttery or creamy aromas. But those impressions can also result from alcohol, oak, lees, fruit ripeness, blending and other production choices.

The most useful approach is to treat MLF as one possible explanation rather than a tasting diagnosis. If a wine tastes soft, that does not prove MLF occurred. If it tastes buttery, that does not prove oak was used—or that MLF was the only cause.

Frequently asked questions

Is malolactic fermentation the same as fermentation?

It is commonly called a fermentation, but technically it is a bacterial conversion or decarboxylation. It is different from yeast-driven alcoholic fermentation.

Does MLF make wine less acidic?

It generally reduces titratable acidity and raises pH, so acidity may seem less sharp. It does not remove all acidity or guarantee a flat wine.

Is buttery Chardonnay always made with MLF?

No. MLF can contribute to buttery aromas through diacetyl, but oak, lees contact and other factors may also contribute. MLF does not always produce a buttery character.

Does MLF happen in red wine or white wine?

It is common in many red wines and is selective in white wines. Australian Chardonnay may undergo full, partial or no MLF.

Can MLF happen naturally?

Yes. It can occur through bacteria already present in the winery or wine, or it can be managed using selected bacterial cultures.