noun From malo- (malic acid, from Latin malum, "apple") + lactic (from Latin lac, "milk") + fermentation

Malolactic fermentation /ˌmæloʊˈlæktɪk/

A secondary biological process in winemaking in which lactic acid bacteria (primarily Oenococcus oeni) convert sharp-tasting malic acid into softer lactic acid and carbon dioxide. Also called malolactic conversion (MLF) or "malo."

Which wines undergo MLF

Category Examples Effect on style

Almost always undergoes MLF Barolo, Bordeaux, Burgundy reds, Shiraz, Rioja — all major red wines Softens tannin perception, adds body, reduces green or hard acid edge.

Usually undergoes MLF Chardonnay (full-bodied), white Burgundy, Viognier, Roussanne Creamy texture, buttery or toasty notes, reduced sharpness; particularly prominent in oaked Chardonnay.

Usually avoids MLF Riesling, Sauvignon Blanc, Pinot Grigio, Champagne base wine, Chablis (often partial) Acidity preserved; sharper, crisper style; primary fruit and floral aromas retained.

Variable or partial Chablis, Champagne, some rosé Winemaker choice; partial MLF can add complexity without full textural change.

What MLF changes in wine

The conversion of malic acid (apple-like, sharp) to lactic acid (yoghurt-like, soft) changes three things perceptibly. First, acidity drops — the wine tastes less sharp and its pH rises. Second, texture becomes rounder and creamier — particularly noticeable in white wines. Third, flavour complexity changes: MLF reduces primary fruit intensity slightly but adds secondary notes — cream, butter, brioche — through diacetyl and other by-products.

For red wines, these changes are desirable almost universally: softer acidity integrates better with tannin, and the textural shift complements the wine's structure. For whites, the decision is more nuanced — full-bodied Chardonnay benefits; aromatic, high-acid whites (Riesling, Sauvignon Blanc) would lose their defining freshness. For how MLF fits into the complete process of making wine, see the How wine is made guide.

Sources: OIV (oiv.int) — oenological practices and lactic bacteria standards. See also: WSET (winemaking curriculum, Level 3 and 4), UC Davis Viticulture & Enology (MLF microbiology research), AWRI (malolactic bacteria and wine stability).

Questions

About malolactic fermentation.

Why does MLF make wine taste buttery?

A by-product of MLF is diacetyl, the compound responsible for the buttery or creamy note in many full-bodied Chardonnays. Diacetyl is produced by the lactic acid bacteria during conversion. Its concentration depends on how MLF is managed: slow, warm MLF in barrel tends to produce more diacetyl than fast, tank-based MLF. Winemakers can reduce diacetyl levels by racking soon after MLF completes. Some producers deliberately encourage the character (Australian and Californian oaked Chardonnay); others avoid it (Chablis, Burgundy).

Is MLF the same as alcoholic fermentation?

No — they are completely separate processes. Alcoholic fermentation converts sugar to alcohol via yeast. MLF converts malic acid to lactic acid via bacteria. They can happen sequentially (alcoholic first, then MLF) or simultaneously. In red wine, they often overlap. In white wine, the winemaker typically completes alcoholic fermentation first, then decides whether to allow or block MLF. See the fermentation glossary entry for a full explanation of alcoholic fermentation.

How do winemakers prevent MLF if they don't want it?

Several techniques prevent or inhibit MLF: adding sulphur dioxide (which inhibits lactic acid bacteria), keeping cellar temperatures below 15 °C, sterile filtration, keeping pH low, and using micro-oxygenation. The decision is important because if MLF occurs spontaneously after bottling it causes serious faults — cloudiness, off-flavours and unwanted CO₂. Wines like Riesling, Sauvignon Blanc and Champagne base wines are carefully managed to prevent this.

Does MLF affect wine acidity?

Yes — malic acid has a harder, sharper taste than lactic acid. The conversion reduces a wine's total acidity and changes its perceived acidity significantly. The pH rises (becomes less acidic). For wines that depend on bright acidity as a feature — Riesling, Champagne base wines — this is why MLF is avoided. For tannic reds, the softer acidity helps integrate the wine. The acidity glossary entry explains how acidity is measured and why it matters to wine structure.

What does malolactic fermentation do to Chardonnay specifically?

Malolactic fermentation has a more pronounced effect on Chardonnay than on almost any other white variety, because Chardonnay is a neutral grape — it does not have strong aromatic compounds to mask MLF's textural influence. When Chardonnay undergoes MLF (as most oaked Burgundy and California Chardonnay does), malic acid converts to lactic acid, producing a softer, rounder, creamier mouthfeel. The by-product diacetyl adds butter, cream or toast notes — the "buttery Chardonnay" associated with New World styles. Unoaked Chablis often goes through partial or no MLF to retain high acidity and mineral freshness. The contrast between an oaked, MLF Chardonnay (rich, creamy, vanilla) and an unoaked one (crisp, citrus, mineral) is almost entirely driven by these two decisions. See the Chardonnay style guide for a full breakdown.

Do natural wines always undergo MLF?

Most do — natural winemakers generally let MLF happen spontaneously without inoculating with commercial bacterial cultures. This is consistent with the minimal-intervention philosophy: no added bacteria, no agents to prevent it. However, some natural wine producers working in hot climates where acidity is already low may prefer to avoid MLF to retain freshness. The interaction between sulphites and MLF is relevant: low-sulphite wines (typical in natural winemaking) are more susceptible to spontaneous MLF. For more on natural wine practices, see the natural wine and sulphites article.