Transform & Preserve

Malolactic Fermentation

A bacterial conversion that softens sharp malic acid into rounder lactic acid in wine and cider.

Malolactic fermentation is a secondary process where lactic acid bacteria convert tart malic acid into softer lactic acid, lowering perceived acidity and adding body and buttery notes. It commonly follows alcoholic fermentation in red wines and many whites like Chardonnay, as well as some ciders. The diacetyl it can produce is the source of the classic buttery aroma in oaked Chardonnay.

Winemakers shorthand it as MLF, but the name is slightly misleading: malolactic fermentation is not a fermentation of sugar at all. It is a bacterial decarboxylation carried out most reliably by Oenococcus oeni, in which L-malic acid loses a molecule of carbon dioxide and becomes L-lactic acid. That single chemical step is what gives most dry red wines, barrel-aged Chardonnays, and traditional dry ciders their rounded, creamy mouthfeel, and it typically trims titratable acidity by 1–3 g/L while lifting pH by roughly 0.1–0.3 units — modest numbers on paper, a perceptible softening on the palate.

The sensory signature beyond acidity is diacetyl, the buttery, butterscotch-scented intermediate that most strains produce in measurable amounts. Heavy in a buttery Chardonnay, faint in a Pinot Noir run through a neutral strain, diacetyl sits alongside other bacterial metabolites — yogurt, hazelnut, fresh bread crust — that add savory complexity to wine that would otherwise read as fruit and tannin alone. MLF also stabilizes the wine microbiologically: once the malic acid is gone, the food source for Brettanomyces and many spoilage organisms disappears with it.

In practice, the winemaker or cider maker chooses when and how to run MLF, in what vessel, and with which strain, balancing the desire for softness and complexity against the risk of volatile acidity, mousy off-aromas, and the sulfite schedule. It is one of the few winemaking steps where a single temperature reading or a missed pH measurement can decide whether the wine improves for the year ahead or stalls out and has to be restarted.

Difficulty: Hard

Types & varieties

Spontaneous (native) MLF

Relies on indigenous lactic acid bacteria already in the cellar and on the fruit; slower and less predictable, but prized by some natural-wine makers for added complexity.

Inoculated post-fermentation MLF

Commercial O. oeni culture added after alcoholic fermentation is dry and the lees have been stirred; the most common modern approach.

Co-inoculation

O. oeni added 24–48 hours after yeast pitch, so MLF runs in parallel with alcoholic fermentation; faster, lower VA risk, but tighter on timing and nutrition.

High-diacetyl strains

Selected for pronounced buttery character, typical of oaked Chardonnay and certain richer red styles.

Low-diacetyl / neutral strains

Selected for softness and mouthfeel without overt butterscotch, suited to Pinot Noir and fruit-forward reds where the fruit must lead.

How to do it

1
Finish primary alcoholic fermentation
Allow the yeast-driven fermentation to run to dryness (Brix or specific gravity stable for 48 hours, residual sugar under 2 g/L). Racking off heavy gross lees is optional but reduces competing microflora and sulfide precursors that can derail the bacteria.
2
Assess readiness for MLF
Measure and record pH, titratable acidity, free and total SO2, alcohol, and residual sugar. Conditions should fall within roughly pH 3.2–3.6, free SO2 near zero, alcohol under 15%, and a cellar temperature that can be held at 18–22 °C.
3
Inoculate the culture
Rehydrate the selected O. oeni strain according to the supplier's protocol (typically in chlorine-free warm water with a nutrient packet) and add to the wine. For co-inoculation, dose 24–48 hours after pitching yeast instead, accepting that the bacteria will share the ferment with the yeast.
4
Hold temperature and keep bacteria in suspension
Maintain 18–22 °C throughout. Stir or punch down the lees weekly to keep the bacteria in suspension and to feed them amino acids; avoid splashing and oxygen ingress once MLF is underway.
5
Monitor malic acid breakdown
Run paper chromatography every 3–5 days, or use an enzymatic L-malic assay. Track diacetyl, sensory softness on the palate, and any rise in volatile acidity as supporting indicators that the conversion is actually progressing.
6
Confirm MLF is complete
Consider MLF finished when L-malic acid is below ~0.2 g/L — the paper chromatogram should show no clear malic spot. Retest after 48 hours to rule out a stalled restart, and taste for the softer, rounder acidity the conversion should deliver.
7
Stabilize and rack
Sulfite to 30–40 ppm total SO2 (or to the target free SO2 for the pH and style), then rack off the lees within a week. Cold-stabilize, fine, or filter as the style requires before moving the wine to aging or bottling.

Beyond the Wine Cellar

The same bacterial conversion shows up in a handful of other fermented foods, though the term is almost always discussed in a wine or cider context. The most notable crossover is in Belgian-style sour beers, where the conversion runs in lambic and gueuze alongside Brettanomyces and Pediococcus, contributing a lactic backbone rather than a buttery overtone. MLF can also occur in some traditional vinegars during aging.

A common source of confusion is the distinction between MLF and keeving in cider. Keeving is a pectin-driven enzymatic depectinization of the juice followed by a natural malolactic step — the bacteria convert the malic acid that is already present, but only after a nitrogen- and pectin-starved ferment has set the stage. It is not the same as inoculating an O. oeni culture into a finished cider.

In lambic and gueuze, MLF runs spontaneously alongside Brettanomyces, contributing lactic acidity rather than diacetyl.
In some traditional vinegars, residual malic acid is converted during barrel aging.
Keeving in cider uses pectin-driven nutrient stripping to provoke an indirect, secondary MLF — do not confuse the two processes.

Common uses

Deacidify high-acid musts and ciders, especially from cool climates or sharp cider apple varieties where the raw malic acid would read as harsh.Soften mouthfeel and add textural roundness to red wines and barrel-aged whites.Impart buttery, nutty, or yogurt-like complexity through diacetyl and other LAB metabolites.Microbiologically stabilize wine by exhausting the residual malic acid that other spoilage organisms could feed on.Build the soft acid profile expected in traditional English and French dry ciders.

Tips & pitfalls

Confirm alcoholic fermentation is truly dry (under 2 g/L residual sugar) and the yeast is in decline before inoculating; leftover sugar invites Brettanomyces and a volatile-acidity spike.
Hold free SO2 at zero during MLF — even small additions can stall the culture. Save the sulfite for after malic acid is below 0.2 g/L.
Track degradation with paper chromatography every few days, or send for enzymatic analysis; taste alone is unreliable until late in the process.
Keep the cellar in the high teens to low 20s °C. A cold cellar under 15 °C is the most common reason for a stuck MLF.
If pH is below 3.1 or alcohol is above 15%, expect a hard start — switch to co-inoculation or a strain specifically selected for harsh conditions.
Watch for volatile acidity creeping above 0.4 g/L acetic and for mousy off-aromas (ethyl tetrahydropyridine) — both are signs that heterofermentative LAB have taken over.
Once MLF is complete, sulfite promptly to 30–40 ppm total SO2 and rack off heavy lees within a week. Do not leave the wine on thick MLF lees for months without monitoring for spoilage.

Good to know

Primary organism: Oenococcus oeni, a lactic acid bacterium; Lactobacillus and Pediococcus species can also perform the conversion but are generally treated as spoilage risks in wine.
Biochemical reaction: L-malic acid → L-lactic acid + CO2 — a decarboxylation, not a true fermentation of sugar.
Effect on acidity: Reduces titratable acidity by roughly 1–3 g/L and raises pH by about 0.1–0.3 units.
Optimal temperature: 18–22 °C (64–72 °F); activity slows markedly below 15 °C or above 25 °C.
Optimal pH window: pH 3.2–3.6; below pH 3.0 the conversion is slow, erratic, or fails to start.
Alcohol tolerance: Most commercial O. oeni strains handle up to ~14–15% ABV; higher proofs risk a stuck MLF.
SO2 sensitivity: Free SO2 above ~10–15 mg/L (and any molecular SO2) inhibits or kills the culture; sulfite only after MLF is confirmed complete.
Signature byproduct: Diacetyl (2,3-butanedione), a buttery, butterscotch-scented intermediate that defines the MLF aroma.