Malolactic Fermentation

Malolactic fermentation (MLF), also known internationally as malolactic conversion (malolactic fermentation) or apple-to-lactic acid fermentation, is an important biochemical process in winemaking. In it, sharp malic acid (malate) is converted by lactic acid bacteria into the softer lactic acid (lactate) and carbon dioxide.

What happens during MLF?

After alcoholic fermentation, wine contains two primary acids: tartaric acid and malic acid. Malic acid tastes sharper, greener, and more angular than the softer tartaric acid. During malolactic fermentation, lactic acid bacteria (mainly Oenococcus oeni) convert the malic acid into lactic acid:

Malic acid (2 carboxyl groups) → Lactic acid (1 carboxyl group) + CO₂

The result: the wine loses approximately 1–3 g/L of acidity (measured as total acidity) and tastes softer, rounder, and more harmonious. The pH rises slightly, which increases the wine's microbiological stability.

Effect on Taste and Aroma

The effects of malolactic fermentation go far beyond simple acid reduction:

Softer, rounder texture: The sharp, green malic acid is replaced by the milder, creamier lactic acid. The wine feels smoother and less angular on the palate. This is particularly important in cool climate regions, where grapes naturally have high levels of malic acid.

Buttery, creamy aromas: As a by-product of MLF, diacetyl is produced – a compound with a buttery, milky aroma. In moderate amounts, diacetyl gives the wine pleasant butter, cream, and brioche notes. Too much diacetyl, however, can come across as cheesy or rancid.

Complexity and depth: MLF contributes to so-called secondary aromas – those aromas that do not originate from the grape but arise through winemaking. In addition to butter, notes of yoghurt, cream, hazelnut, or toast may also appear.

Fruit transformation: MLF can alter the primary fruit character of the wine. Fresh, green fruit aromas (green apple, lemon) become softer and tend towards riper, yellower fruit (ripe apple, peach). In red wines, the red fruit notes become rounder and more velvety.

Stability: A fully completed MLF prevents the process from occurring spontaneously in the bottle at a later stage, which would cause unwanted cloudiness, carbon dioxide, and off-aromas. MLF is therefore also an important stabilisation measure.

When is MLF Carried Out?

In almost all red wines: MLF is practically standard for red wines. The reduction in acidity softens the tannins and makes the wine more supple. Red grapes often have high malic acid levels that, without MLF, would result in overly acidic, angular wines.

In many high-quality white wines: MLF is particularly widespread in Chardonnay. The buttery, creamy notes of a classic Burgundy or Californian Chardonnay come directly from MLF (combined with lees ageing and often barrique maturation). MLF is also frequently used for other Pinot varieties (Pinot Blanc, Pinot Gris).

Rarely in aromatic white wines: For grape varieties prized for their fresh acidity and crisp fruit aromas – such as Riesling, Sauvignon Blanc, Grüner Veltliner – MLF is usually avoided or blocked. These wines would lose their characteristic vibrancy from the acid reduction.

In sparkling wines: MLF is carried out in most base wines before the second fermentation takes place, preventing unwanted carbonation in the bottle. Some Champagne producers, however, deliberately forgo MLF in order to preserve freshness.

Controlling MLF

Winemakers can deliberately steer or prevent MLF:

Spontaneous MLF: After alcoholic fermentation, naturally occurring lactic acid bacteria initiate MLF when conditions are right (temperature above 18°C, pH not too low, low SO₂). This can take weeks or months and is difficult to control.

Inoculation with bacterial cultures: Modern winemakers often inoculate the wine with selected Oenococcus oeni strains. This starts MLF reliably, speeds it up, and reduces the risk of off-aromas from unwanted bacterial strains.

Preventing MLF: Where MLF is undesirable, it can be blocked through sulphiting (SO₂ addition), chilling (below 15°C), or filtration of the bacteria. With many fresh white wines, winemakers actively work against MLF to preserve lively acidity.

Partial MLF: Some winemakers allow only part of the wine to undergo MLF and then blend it with wine that has not undergone MLF. This allows for a tailor-made balance between freshness and creaminess. The same technique is also applied to sparkling wines.

MLF and Barrique

MLF is often carried out in barrique barrels, especially for high-quality Chardonnay or Pinot Noir. The warm temperatures in the small barrel promote bacterial activity, and simultaneous lees ageing (with bâtonnage) results in a particularly creamy, complex texture.

The combination of MLF, lees ageing, and barrique maturation is the winning formula of many great white wines. The buttery-creamy notes from MLF blend with the brioche aromas from lees ageing and the vanilla-toast notes from the wood, creating a harmonious whole.

Timing of MLF

MLF can take place at various points:

During alcoholic fermentation: With co-inoculation, yeast and bacteria are added simultaneously. MLF runs in parallel with fermentation. This saves time but may result in less pronounced buttery aromas (less diacetyl).

After alcoholic fermentation: The classic timing. After fermentation is complete, the wine is warmed or inoculated, and MLF proceeds over weeks or months. This produces the most intense MLF aromas.

In spring: Traditionally, MLF is often carried out in spring, when cellar temperatures naturally rise. "When the vines flower, the wine begins to work" is an old winemaker's saying.

Risks and Challenges

MLF also carries risks:

Too much diacetyl: Excessive diacetyl production leads to unpleasant cheesy, rancid, or artificially buttery aromas. This occurs with certain bacterial strains or unfavourable conditions.

Mousiness: Unwanted bacteria can cause the wine fault known as "mousiness" – an unpleasant, musty aftertaste reminiscent of a mouse cage. This is a particular risk in wines with a high pH and without SO₂ protection.

Biogenic amines: Some lactic acid bacteria produce biogenic amines such as histamine, which can trigger headaches or intolerances in sensitive individuals.

Loss of aroma: In aromatic grape varieties, MLF can dampen characteristic primary aromas. A Riesling that has undergone MLF loses its typical citrus freshness and can taste flat.

For these reasons, controlling MLF and selecting the right bacterial strains is critical to wine quality.

Recognising MLF in the Glass

Wines that have undergone full MLF show typical characteristics:

• Softer, rounder texture – less acid-driven
• Creamy, buttery aromas – especially in Chardonnay
• Less green, more ripe fruit aromas
• Harmonious, integrated acidity – not sharp or angular

Wines without MLF show, by contrast:

• Crisper, more vibrant acidity – fresher, more invigorating
• Green fruit aromas – green apple, lime, lemon
• Cleaner, more direct fruitiness – without creamy notes

In white wines from cool regions (Germany, Loire, northern Italy), the likelihood of MLF having been carried out is low. In white wines from warmer regions (California, Australia, southern France), it is more common.

Frequently Asked Questions

Is MLF the same as fermentation? No, the term "malolactic fermentation" is misleading. It is not a fermentation (conversion of sugar into alcohol) but rather an acid reduction by bacteria. Nevertheless, the term has become internationally established.

Does MLF make wine sweeter? Indirectly, yes – by reducing acidity, the same amount of residual sugar or fruit tastes sweeter. MLF itself does not produce sugar. The perceived sweetness is an effect of the acid-sweetness balance.

Why is MLF not used for Riesling? Riesling lives from its vibrant acidity, which harmonises perfectly with its fruitiness and mineral notes. MLF would destroy this freshness and make the wine taste flat. High acidity is a quality marker in Riesling, not a problem.

Can all bacteria carry out MLF? No, only specific lactic acid bacteria species – primarily Oenococcus oeni – are suitable for wine production. Other bacteria can cause wine faults. This is why modern winemakers often use selected pure cultures.

Does MLF always happen automatically? No, MLF requires the right conditions: sufficient warmth (above 18°C), not too low a pH, little SO₂, and the presence of bacteria. Winemakers must actively decide whether to encourage or prevent it.