Malolactic Fermentation: The Second Transformation

What Is Malolactic Fermentation?

Malolactic Fermentation — abbreviated MLF and sometimes called "the secondary fermentation" or "malo" — is a biochemical transformation that occurs in nearly all red wines and many whites. Despite the word "fermentation," it is not caused by yeast and does not produce alcohol. Instead, lactic acid bacteria (LAB), primarily Oenococcus oeni, convert Malic Acid into Lactic Acid and carbon dioxide.

The reaction looks simple:

L-malic acid → L-lactic acid + CO₂

But its consequences for wine style are profound. Malic acid is sharp and green-fruited — think of biting into an unripe apple. Lactic acid is the same acid found in yogurt and cultured butter: rounder, softer, and creamier. When MLF runs to completion, the total acidity of the wine decreases, the pH rises slightly, and the texture changes from angular to smooth.

The Chemistry in More Depth

Grapes naturally contain two primary acids: Tartaric Acid and Malic Acid. Tartaric acid is unique to grapes and is chemically stable — bacteria cannot easily metabolize it. Malic acid, on the other hand, is the preferred substrate of lactic acid bacteria.

The key enzyme in MLF is malolactic enzyme (a decarboxylase), which the bacteria use to convert malic acid while simultaneously expelling CO₂ as a gas. The process is technically a deacidification because total titratable acidity falls as the more acidic malic acid is replaced by the weaker lactic acid.

Additionally, LAB produce a compound called diacetyl as a secondary by-product of citric acid metabolism. Diacetyl is the chemical responsible for the butter and cream aromas that are the hallmark of heavily malolactic wines. At low concentrations (under 1 mg/L), diacetyl is subtle and pleasant. At higher levels (above 5 mg/L), it can dominate and make a wine smell like movie-theater popcorn butter — a style that was fashionable in 1990s California but has fallen out of favor.

Which Wines Undergo MLF?

The decision to allow or block MLF is one of the most consequential choices a winemaker makes, and it divides wines into two broad camps.

Wines That Undergo Full MLF

Red wines: Almost universally undergo complete MLF. Red wines begin with higher pH and lower total acid than whites, making them naturally hospitable to LAB. Tannin structure, rather than acidity, provides their architectural backbone, so softening the acid is typically beneficial. A young Cabernet Sauvignon from Bordeaux or Napa Valley that has not completed MLF can taste aggressively sour and unbalanced; after MLF, it integrates and feels complete.

Oaked Chardonnay: The prototypical example of MLF in white wine. Wines from Bourgogne undergo complete MLF as a matter of tradition and style — it is part of what gives Meursault and Puligny-Montrachet their famous silky, nutty richness. California Chardonnay producers in the 1980s and 1990s enthusiastically adopted full MLF, sometimes producing wines that were almost excessively round and buttery.

Wines That Block MLF

Crisp whites and aromatic varieties: Wines built around high, vivid acidity — Riesling, Sauvignon Blanc, Gewürztraminer, Grüner Veltliner — typically block MLF entirely. These grapes derive much of their character from malic-acid-driven crispness and bright fruit. Allowing MLF would flatten the very quality that makes them distinctive. Winemakers block MLF by adding Sulfites (sulfur dioxide), keeping the wine cool, filtering early, and bottling promptly.

Champagne base wines: The base wines for Traditional Method Sparkling production in Champagne are intentionally high-acid. MLF is traditionally blocked to preserve the sharp, taut backbone that undergirds Champagne's aging potential and the focused, crisp palate the region aims for in its non-vintage blends. However, in the 21st century some prestige Champagne houses (notably Krug) have continued their tradition of blocking MLF, while others have moved to full MLF for stylistic reasons.

Partial MLF: Some winemakers deliberately allow MLF in only a portion of their wine — perhaps 30–50% — then blend the mlf'd and non-mlf'd portions to achieve a textural middle ground: some creaminess without losing freshness. This is a nuanced tool that requires careful monitoring.

How Winemakers Control MLF

Encouraging MLF

To encourage MLF, winemakers: - Keep the cellar warm (above 18°C; bacteria become sluggish below 15°C). - Minimize sulfite additions after primary fermentation (SO₂ inhibits LAB). - Inoculate with commercial LAB cultures (O. oeni strains) for predictability. - Maintain pH above 3.2 (very low pH slows or prevents bacterial activity). - Avoid filtration until MLF is complete.

Blocking MLF

To prevent MLF: - Add sulfur dioxide immediately after primary fermentation. - Keep the wine cold (below 13°C) in tank. - Filter the wine through tight (0.45-micron) membranes that remove bacteria. - Bottle quickly, before bacterial populations can establish.

Detecting MLF: Paper Chromatography

The traditional and still widely used method for confirming MLF completion is paper chromatography. A small wine sample is spotted onto a specially treated chromatography paper alongside reference acids. After development in a solvent system, distinct spots appear for malic acid, lactic acid, tartaric acid, and citric acid. When malic acid is absent from the developed paper — replaced by the lactic acid spot — MLF is complete.

Modern wineries also use enzymatic assays and HPLC (high-performance liquid chromatography) for more precise measurement. But for small producers, the paper chromatography test remains a practical, inexpensive, and reliable field method.

Sensory Fingerprints of MLF

The easiest way to understand MLF is to taste wines that clearly show it versus wines that block it. Side-by-side comparisons reveal:

For Rich White styles, these MLF characteristics are desirable features. For Crisp White and Aromatic White styles, they would be considered faults.

MLF and Wine Stability

Beyond flavor, MLF plays an important role in wine stability. Malic acid can be a substrate for spoilage organisms later in a wine's life — if unwanted bacteria trigger spontaneous MLF in a bottled wine, the CO₂ produced causes the wine to become slightly fizzy and the flavors to shift in unpredictable ways. By ensuring MLF completes before bottling, the winemaker eliminates this risk.

This is one reason why red wines and full-bodied whites are typically allowed to run MLF to completion even if the stylistic impact is secondary — it is simply good cellar hygiene.

The Trend Toward Freshness

Since roughly 2010, consumer and critical preference has shifted markedly toward wines with less MLF influence. Heavily buttery Chardonnay fell out of fashion as drinkers sought more energy and precision. In Napa Valley, producers began harvesting earlier, using less new oak, and in some cases blocking or reducing MLF to recapture freshness. Even in Bourgogne, there is ongoing discussion about how much MLF character is a bug rather than a feature of the house style.

Understanding MLF allows wine drinkers to decode this debate — to recognize that when a critic calls a Chardonnay "too broad" or "lacking freshness," what they often mean is that the wine carries too much of the lactic, buttery character that full MLF imparts.

Malolactic fermentation is, ultimately, a tool. Like all tools, its value depends entirely on whether it is the right instrument for the job at hand.