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Cellari The Winemaker's Corner

Winemaking guide · Malolactic fermentation (MLF)

Malolactic fermentation: how to do it and when it's done

Malolactic fermentation (MLF) is the secondary fermentation in wine: lactic acid bacteria turn sharp malic acid into soft lactic acid. Every gram per liter of malic acid that turns into lactic lowers total acidity by about 0.56 g/L. Run it at 18–22 °C (64–72 °F) with no SO₂. It’s complete when malic acid is down to 0.1 g/L or less.

By the Cellari team Written with AI assistance Published

EXIT MALIC EMPTY Malic's all gone. Lights out! Cabernet 2026 MLF ✓ Stable z z Z
Shift's over: there's no malic acid left to eat. Add SO₂, and the bacteria have no fuel left in the bottle.

What happens to the wine during malolactic fermentation

Grapes carry two main acids: tartaric and malic (the sharp acid of green apples). Lactic acid bacteria, mostly Oenococcus oeni, break malic acid down into lactic acid (the acid in milk) and CO₂. You’ll also see it called MLF, malo, or secondary fermentation.

The difference is the number of acid groups: malic acid has two, lactic acid has one. When malic turns into lactic, half of its acidity leaves with the CO₂.

Malic acid COOH H⁺ CH₂ CH–OH COOH H⁺ C₄H₆O₅ 134 g/mol 2 acid groups Lactic acid CH₃ CH–OH COOH H⁺ C₃H₆O₃ 90 g/mol 1 acid group CO₂ 44 g/mol Leaves as gas: the tiny bubbles O. oeni bacteria malolactic enzyme 134 g malic → 90 g lactic + 44 g CO₂ · so each gram gives 0.67 g lactic
Two hydrogen ions (H⁺) go in, one comes out. That's why total acidity drops and pH rises.
HOOC–CH₂–CHOH–COOH → CH₃–CHOH–COOH + CO₂

Malic acid → lactic acid + carbon dioxide. In tartaric acid units, a gram of malic acid counts as 1.12 g of acidity. After the fermentation, that same gram counts as only 0.56.

Why do malolactic fermentation

1. Microbial stability: the main reason

Malic acid left in the wine is food for bacteria. They can wake up in the bottle and run the fermentation there: gas, haze, sediment, and sometimes a cork pushed out. Once the malic acid is used up in the tank and you’ve added SO₂, they have nothing left to run on.

2. Lower acidity, higher pH

Total acidity (TA, in g/L as tartaric acid) drops and pH rises. Exactly how much depends on the wine, so measure at the end. In a tart wine from a cool region, that’s a plus. In warm regions such as Israel, where wine pH is often high already, you have to manage it (more on that below).

3. Mouthfeel

The green bite of malic acid gives way to the softer acidity of lactic, and the wine feels rounder.

What malolactic fermentation does to flavor and aroma

The green-apple tartness goes. Some wines also pick up a buttery smell: that’s diacetyl, which the bacteria make from the citric acid in the wine. In a delicate white you can pick it up at 0.2 mg/L; in a full-bodied red, only at around 2.8.

1–4: buttery, round Over 5–7: masks the fruit 0 1 2 3 4 5 6 7 8 Diacetyl, mg/L Chardonnay 0.2 Pinot Noir 0.9 Cabernet Sauvignon 2.8
Diacetyl detection thresholds from Lallemand: the level at which tasters start to notice butter. How much butter ends up in your wine depends on the bacteria strain and the conditions.

For more butter (barrel-fermented Chardonnay, for example): add the bacteria after the alcoholic fermentation, use a strain that makes diacetyl, keep lees contact short, run the MLF cool and slow, and add SO₂ as soon as it’s done.

For less: add the bacteria 24 hours after the yeast (the yeast break diacetyl down), use a low-diacetyl strain, keep the wine on its lees for a long time without stirring, and run the MLF warm and short. At high pH, don’t put off the SO₂.

Wild or commercial bacteria

Spontaneous MLF relies on the bacteria that come in on the grapes and the equipment. After crushing there are only thousands to tens of thousands per mL, and the fermentation starts once they reach about a million. Until then, spoilage bacteria such as Lactobacillus and Pediococcus can take over and produce volatile acidity (a vinegar smell).

A commercial culture of Oenococcus oeni puts millions per mL into the wine from the start, so MLF starts fast and finishes more predictably. Pick the strain to suit the wine: tolerant of alcohol, low pH or SO₂, or a low-diacetyl strain.

For home batches: the small VP41 packet, for example, is sized for 250 L (66 US gal), and an opened packet has to be used right away. Split one with other winemakers, or buy a home-size pack.

What the bacteria need: pH, temperature, alcohol and SO₂

MLF bacteria are far more sensitive than yeast. Any one factor on its own slows them down, and several tough ones together can stop them. The table pulls together ranges from the AWRI, university extension services and bacteria producers. The sources don’t agree exactly, so the limits are rounded.

FactorGood rangeHard (slow)LimitNote
pH3.3–3.53.1–3.3below 3.1Above 3.5, spoilage bacteria thrive too
Temperature18–22 °C16–18 or 22–25 °Cbelow 16, above 25 °CAbove 25 °C the risk of volatile acidity rises
Alcoholup to 13%13–15%above 15%The AWRI calls above 14% unfavorable. Tolerant strains such as VP41 are rated up to 16%
Free SO₂up to 5 mg/L5–10above 10Best: not measurable at all
Total SO₂up to 30 mg/L30–40above 4050–60 can stop MLF completely
Malic acid0.8–5 g/Lbelow 0.8above 5: may not finishLittle malic: a hard start
NutrientsFine yeast leesDemanding yeast, low-nitrogen juice—DAP doesn't help. Dedicated MLF nutrients exist

Alcohol in % by volume. 18–22 °C is 64–72 °F; 16 °C is 61 °F; 25 °C is 77 °F. The factors affect each other: at high alcohol, stay at the cool end of the range, 18–20 °C.

Traffic light: is your wine ready for the bacteria?

  • pH MLF will go easily, but so will spoilage bacteria. Work clean and finish fast.
  • Temperature Good range.
  • Alcohol Hard. Use an alcohol-tolerant strain and hold at 18–20 °C.
  • Free SO₂ Not measurable. Excellent.
  • Total SO₂ Good range.

You can start. Follow the advice for each yellow factor and keep a close eye on it.

Based on the ranges in the table. Molecular SO₂ is worked out from free SO₂ and pH with the same formula as the Cellari SO₂ calculator. According to the AWRI, 0.3–0.5 mg/L molecular is already lethal to the bacteria.

When to inoculate for MLF

Inoculating means adding the bacteria culture. There are four possible timings, and the most common is after the alcoholic fermentation ends (sequential inoculation).

Sugar Alcohol Alcoholic fermentation sugar falls, alcohol rises End of alcoholic fermentation 1. With the yeast before fermentation 2. Co-inoculation 24–48 h after the yeast 3. Near the end a little sugar left 4. After fermentation sequential, most common MLF on sugar MLF MLF MLF 0 1 2 3 4 5 6 weeks from adding yeast
Schematic only: how long each fermentation takes varies from wine to wine. The yellow dot is when the bacteria go in; the bar is the malolactic fermentation.

How to inoculate for MLF, step by step

O. oeni bacteria 20 °C 1 · Rehydrate chlorine-free water, ≤ 15 min pour Free SO₂: 0 2 · Add and stir gently, full vessel, no SO₂ M M M L L L malic lactic 3 · Bacteria at work 18–22 °C, two to six weeks
From the packet into the wine. Exact directions vary by strain, so read the product sheet for the culture you bought.
  1. Test the wine and enter the results in the traffic light above. If anything shows red, fix it first or pick a tolerant strain.
  2. Bring it to temperature: 18–22 °C (64–72 °F). For VP41: 17–25 °C in reds, 16–20 °C in whites and rosés. An unheated cellar in winter can drop below 16 °C (61 °F), and there the bacteria all but stop.
  3. Rehydrate, if the strain calls for it. Clean, chlorine-free water, 20 times the weight of the powder, at 20 °C (68 °F), for no more than 15 minutes. Many strains can go straight in from the packet.
  4. Add and stir gently, without splashing in air, in a full vessel with an airlock.
  5. No SO₂ and no sorbate. SO₂ stops the bacteria. Sorbate (potassium sorbate) together with lactic acid bacteria gives a smell of geranium leaves.
  6. Stir the lees once a week to bring the bacteria and their food back into the wine. Skip this if you want as little diacetyl as possible.
  7. Monitor. Lallemand recommends measuring malic acid every 2–4 days. At home, run the first paper chromatography test 2–3 weeks after inoculating, then once a week.

How to tell malolactic fermentation is under way

Tiny bubbles in the demijohn are a hint, not proof, and tasting isn’t enough either. Measure:

Paper chromatography: cheap and simple

You spot a little wine and some reference solutions onto special paper and stand it in a jar of solvent. The solvent climbs the paper and carries each acid to a different height. Once the paper dries, the acids show up as yellow spots. The kit is cheap and suits home winemakers.

Before: inoculation day After 4 weeks solvent front solvent front Tartaric Malic Lactic Wine Tartaric Malic Lactic Wine gone
Lactic climbs highest, malic sits in the middle, tartaric stays low. When the malic spot in the wine lane disappears, less than about 100 mg/L of malic acid is left: that's the limit of what the test can detect. Tartaric doesn't change in this fermentation.

Careful: the solvent is toxic (butanol and formic acid). Work somewhere well ventilated and wear gloves.

Enzymatic malic acid test

Gives you a number in g/L and picks up very low levels. This is how wineries decide MLF is done. There are home kits, and wine labs will test a sample for you.

pH and total acidity

Falling TA and rising pH show the fermentation is moving, but not that it’s finished.

When is malolactic fermentation complete, and what to do right away

According to the AWRI, MLF counts as complete at 0.1 g/L malic acid or less, and the ideal is “not detected”, usually under 0.05 on an enzymatic test. For home wine, WineMaker says it’s enough for the malic spot to be gone from the chromatogram, as long as the wine gets SO₂ and is kept cool. It usually takes 2–6 weeks, and months if the wine is cold.

  1. Add SO₂ the same day. The AWRI recommends 40–50 mg/L to kill off the remaining bacteria. After that, keep free SO₂ at the level the new, higher pH calls for.
  2. Measure pH again and adjust the acidity if needed. Now, not halfway through the fermentation.
  3. Rack the wine off its lees, and keep it below 18 °C (64 °F).

Worked example: what happens to 2.5 g/L of malic acid

  • Cabernet Sauvignon
  • Demijohn 20 L (5 gal)
  • Malic 2.5 g/L
  • TA 6.5 g/L
  • pH 3.6

Molecular weights: malic 134.09 (two acid groups), lactic 90.08 (one), tartaric 150.09 (two, so 75.05 per group).

  1. One gram of malic acid as tartaric acidity: (1 ÷ 134.09) × 2 × 75.05 = 1.12 g. After the fermentation one group is left: 0.56 g. The drop: 0.56 per gram.
  2. Lactic acid formed: 90.08 ÷ 134.09 = 0.67 g per gram of malic, so 2.5 × 0.67 ≈ 1.68 g/L.
  3. Drop in acidity: 2.5 × 0.56 = 1.40 g/L. Total acidity: 6.5 − 1.4 = 5.1 g/L.
  4. CO₂ given off: 2.5 × 44.01 ÷ 134.09 ≈ 0.82 g/L, or about 16 g of CO₂ for the whole demijohn.
  5. Say you measure pH 3.8 at the end. The target for a red, 0.5 mg/L molecular, rises from 0.5 × (1 + 103.6 − 1.81) ≈ 31 to 0.5 × (1 + 103.8 − 1.81) ≈ 49 mg/L free SO₂.

Acidity drops by 1.4 g/L, to 5.1

This is a theoretical calculation. Wine isn’t just water with acids in it, so measure TA and pH at the end. You can’t work out the pH in advance.

Work out the SO₂ for your wine after MLF. Enter the new pH, your free SO₂ and the volume.

Open the free SO₂ calculator

If pH has climbed too high, correct it with tartaric acid using the acid adjustment calculator, and only then work out the final SO₂. The full explanation of SO₂ by pH is in the guide how much sulfite to add to wine.

In warm regions such as Israel: high pH, hot summers

Reds from warm regions often reach pH 3.6–3.9 before MLF even starts, and above 3.5 spoilage bacteria are comfortable too. Work clean, finish fast with a commercial culture, and add SO₂ on the day it’s done.

Don’t lower the pH below 3.3 before MLF, and don’t adjust acid halfway through. And in a hot summer, in storage or in shipping, wine that still has malic acid in it can start fermenting in the bottle.

When not to do malolactic fermentation

Crisp whites and rosés, such as Sauvignon Blanc or Riesling: acidity and fruit are the point, and butter gets in the way.

Wine with very high pH and little malic acid: MLF will push the pH higher still and call for more SO₂. Some winemakers skip it here. Our advice: skip it only if you can keep it blocked all the way into the bottle. Without sterile filtration, we’d rather complete the MLF and fix the acidity afterwards.

Wine that hasn’t gone through MLF has to stay blocked, in the bottle too. Here’s how to block it:

Don’t blend wine that has been through MLF with wine that hasn’t: the blend gets both the bacteria and the malic acid. Lysozyme and bentonite cancel each other out, so leave two weeks between them.

Stuck malolactic fermentation and off-aromas

If a month has gone by and nothing is moving, check in this order:

What to check What to do
Temperature below 16 °C (61 °F) Warm it to 18–22 °C. The most common cause in winter.
pH below 3.3 Use a strain tolerant of low pH. Below 3.1 MLF rarely starts.
Total SO₂ above 30 Use an SO₂-tolerant strain and an acclimatized culture (see below).
Alcohol above 13% Use an alcohol-tolerant strain, ideally after acclimatizing it.
Fatty acids from the yeast Yeast hulls, 0.2 g/L, before adding fresh bacteria.
Lack of nutrients A dedicated bacteria nutrient. DAP doesn’t help them.

Rescue with an acclimatized culture: grow the bacteria in diluted juice with no SO₂, add an equal amount of the problem wine so they adapt, and only then add them to the whole tank. The AWRI has the full protocol.

Aromas that come from bacteria

Smell or taste What causes it
Vinegar (volatile acidity) Bacteria that ate leftover sugar; heat above 25 °C (77 °F)
Mousy taint Bacteria growing at high pH with little SO₂
Bitterness in red wine Spoilage bacteria that turn glycerol into acrolein, which binds to phenolics
Thick, oily wine Pediococcus producing polysaccharides (ropiness)

The defense is almost always the same: pH below 3.5, clean work, a commercial culture, a fast finish, and SO₂ right at the end.

Common mistakes

Frequently asked questions

What happens if I bottle before MLF is done?

The bacteria carry on in the bottle: gas, haze and sediment. If you have to bottle, block it: enough free SO₂ for 0.8 mg/L molecular, or sterile filtration.

Do white wines go through malolactic fermentation?

Some do, especially barrel-fermented Chardonnay. Crisp whites usually don’t, and below pH 3.1 MLF rarely starts anyway.

Is secondary fermentation the same as MLF?

In a still wine, usually yes. In home winemaking, “secondary” can also just mean the slow stage after you rack into a carboy, and MLF often runs during it. In sparkling wine it means something else: yeast fermenting added sugar to make bubbles.

Should I leave the wine on the yeast lees?

Fine lees feed the bacteria and lower diacetyl. Gross lees are better racked off after the alcoholic fermentation.

Glossary

Malolactic fermentation, MLF
Bacteria turn malic acid (the green-apple acid) into lactic acid (the acid in milk).
Lactic acid bacteria (LAB)
The group that includes O. oeni, the bacterium in commercial cultures, as well as Lactobacillus and Pediococcus.
Inoculation
Adding a culture. Co-inoculation: bacteria during the alcoholic fermentation. Sequential: after it.
TA
Total acidity, in g/L as tartaric acid.
Lysozyme
An enzyme from egg white that blocks MLF.
Lees
Dead yeast that settles at the bottom after fermentation.

More guides: How much sulfite to add to wine, and when, a cold stability guide (tartrate crystals, KHT) and a heat stability guide (proteins).

Calculators for this guide

Sources