Malolactic Fermentation

What Is Malolactic Fermentation?

Malolactic fermentation (also called biological acid reduction, lactic acid fermentation, or simply MLF or malo) is a biochemical process in which the sharp malic acid (malate) in wine is converted by lactic acid bacteria into the softer lactic acid (lactate). This process takes place after alcoholic fermentation and considerably changes the character of the wine.

Although it is called a "fermentation," it is technically not a true fermentation but rather a bacterial conversion. The term has, however, become established in wine terminology.

Chemical Process

During malolactic fermentation, the following occurs:

Malic acid (C₄H₆O₅) → Lactic acid (C₃H₆O₃) + CO₂

Lactic acid bacteria (primarily Oenococcus oeni) convert the diprotic, sharp-tasting malic acid into the monoprotic, softer lactic acid. Carbon dioxide is produced as a by-product and can rise as fine bubbles through the wine during MLF.

The result: total acidity decreases, and the remaining acid tastes softer and rounder.

Effects on the Wine

Acid Reduction

The most important effect is the reduction of acidity by approximately 1–3 g/L. The sharp, green malic acid is replaced by the gentler, creamier lactic acid. The wine therefore appears:

• Softer and rounder on the palate
• Less angular and aggressive
• Fuller and creamier in texture

This softening is particularly desirable with acid-rich grape varieties such as Chardonnay, Pinot Noir, or wines from cool climates.

Aromatic Changes

Malolactic fermentation adds characteristic secondary aromas to the wine:

• Butter, butterscotch: Through the formation of diacetyl
• Cream, milk: Creamy, milky notes
• Yogurt, buttermilk: Lightly sour-creamy
• Hazelnut, almond: Nutty components

These aromas are particularly pronounced in Chardonnay from California or Burgundy, where MLF is deliberately used to create the typical buttery character.

Stabilization

An important practical aspect: if malolactic fermentation is carried out in a controlled way in the cellar, it can no longer occur spontaneously in the bottle. This prevents unwanted changes after bottling and biologically stabilizes the wine.

When Is Malolactic Fermentation Used?

Not every wine undergoes malolactic fermentation. The decision depends on the desired style:

Almost Always MLF:

Red wines: With virtually all red wines, malolactic fermentation is standard. Without MLF, red wines would taste too acidic and astringent. MLF softens the tannins and integrates them better.

Barrel-fermented Chardonnay: Premium Chardonnays aged in barrique almost always undergo MLF to produce the characteristic creaminess and butteriness — typical of Burgundy, California, and many New World Chardonnays.

Often MLF:

Pinot Noir, Pinot Blanc, Weißburgunder: These varieties benefit from MLF, as it adds more fullness and creaminess.

Wines from cool climates: In regions with high natural acidity (e.g., Germany, Austria, northern France), MLF can help bring the acidity down to a drinkable level.

Usually No MLF:

Fresh, fruity white wines: Wines such as Sauvignon Blanc, Riesling, Grüner Veltliner, or simple Pinot Grigio deliberately skip MLF to preserve their crisp freshness, liveliness, and primary fruit aromas.

Aromatic white wines: Grape varieties with intense primary aromas (Gewürztraminer, Muscat, Albariño) would lose expressiveness through MLF.

Sparkling wines (partially): Champagne and high-quality sparkling wines usually undergo MLF, but Prosecco and many other fresh sparkling wines do not.

Controlling Malolactic Fermentation

Winemakers have various options for managing malolactic fermentation:

Encouraging MLF

• Raise temperature: Lactic acid bacteria work optimally at 18–22°C
• Keep sulfur dioxide (SO₂) low: Too much sulfur inhibits the bacteria
• Add bacterial cultures: Targeted inoculation with Oenococcus oeni
• Provide nutrients: Bacteria need nitrogen and other nutrients
• Optimize pH: Bacteria prefer higher pH values (above 3.3)

Preventing MLF

• Lower temperature: Below 15°C, MLF stops
• Add sulfur dioxide: Kills or inhibits bacteria
• Early filtration: Mechanically removes bacteria
• Sorbic acid: Chemical inhibition (for white wines)

Timing of MLF

Malolactic fermentation can take place at various points:

During alcoholic fermentation (co-inoculation):

• Modern technique, both fermentations run in parallel
• Faster, more efficient, lower risk
• Slightly less aromatic complexity

After alcoholic fermentation (traditional):

• Classic approach, especially in Burgundy
• Slower (several weeks to months)
• More complexity, but higher risk of off-flavors
• Often spontaneous, using natural bacteria

Risks and Problems

If not carried out correctly, malolactic fermentation can cause problems:

Off-flavors:

• Too much diacetyl: Excessive buttery notes (rancid)
• Acetic acid: Volatile acidity with wrong bacteria
• Mousiness: Unpleasant, musty notes

Spontaneous MLF in the bottle:

• Cloudiness and carbonation in the bottle
• Unpleasant change in aromas
• Avoidable through stabilization before bottling

Loss of freshness:

• Excessive acid reduction in wines already low in acidity
• Wine seems flat and dull

Partial Malolactic Fermentation

Some winemakers deliberately stop MLF only partway to achieve a balance between freshness and creaminess. One portion of the wine is vinified with complete MLF, while the other retains its malic acid. The resulting blend has more complexity than a fully blocked MLF, but more freshness than a fully completed one.

This technique is particularly popular with modern Chardonnays that should be neither too buttery nor too acidic.

Practical Tip

If you prefer buttery, creamy white wines, choose Chardonnays that have undergone malolactic fermentation — often indicated by terms such as "barrel-fermented" or "buttery" on the label. Californian Chardonnays typically display strongly pronounced MLF aromas.

If you prefer fresh, crisp white wines, go for wines without MLF: Riesling, Sauvignon Blanc, Grüner Veltliner, or Chardonnays labeled "unoaked" or "stainless steel."

For red wines, MLF is standard and a sign of proper vinification — you do not need to worry about it here.

Wines that have undergone malolactic fermentation are generally somewhat more age-worthy and develop more harmoniously in the bottle, since biological stability is ensured.