Study and careerWinemaking & vineyard
Malolactic fermentation: the complete guide
What malolactic fermentation does to acidity, texture and aroma, which wines go through it, how winemakers start or stop it, and the terms to know for exams.
Updated October 2026
Rewritten by Alper Billik, Advanced Sommelier
5 min read
Malolactic fermentation is the reason a red wine feels smooth rather than sharp, and the reason some Chardonnay smells of butter. Wine students meet it in every course; wine lovers taste it every week without knowing its name. Here is the full picture. (For the short, feeling-first version, read malolactic fermentation is basically a hug.)
The short answer: malolactic fermentation, MLF or “malo”, is when lactic acid bacteria, mainly Oenococcus oeni, turn the grape's sharp malic acid into softer lactic acid and a little carbon dioxide. The wine becomes less tart, rounder and more stable, and it can pick up a buttery note. It happens in virtually all red wines and in some whites and sparkling wines.
What actually happens
- Malic acid is the green-apple acid of the grape. Lactic acid is the gentler acid of yoghurt.
- It is bacteria, not yeast. That is why many teachers prefer the term malolactic conversion: no sugar is turned into alcohol.
- Acidity falls and pH rises slightly, because malic acid has two acid groups and lactic acid only one.
- Timing: most often after the alcoholic fermentation (sequential), sometimes alongside it (co-inoculation).
The Australian Wine Research Institute (AWRI) sums up its three jobs: it lowers acidity, it stabilises the wine by removing malic acid that microbes could otherwise feed on, and it shapes aroma and flavour.
What it does to the taste
- Softer acidity: less of the green-apple bite.
- Rounder texture: the wine feels fuller and creamier.
- Butter: the bacteria make diacetyl from the grape's citric acid. In small amounts it reads as creamy or butterscotch; in large amounts, as cinema-popcorn butter. Strong oak or very fruity wine can mask it.
- In reds: some bacterial strains lift red- and dark-berry fruit.
Which wines go through it
| Wine | Malo? | Why |
|---|---|---|
| Almost all reds | Yes | Softer, rounder and stable in the bottle |
| Rich Chardonnay (Burgundy and many New World regions) | Often | Creamy texture, complexity, sometimes butter |
| Chablis | Often, even without oak | Louis Michel, for example, lets it happen naturally in stainless steel |
| Champagne and other sparkling bases | Depends on the house | A style choice: softer and rounder, or sharper and fresher |
| Riesling, Sauvignon Blanc, Gewurztraminer, Pinot Grigio | Usually not | To keep crisp acidity and pure fruit aromas |
- Almost all reds
- Malo?Yes
- WhySofter, rounder and stable in the bottle
- Rich Chardonnay (Burgundy and many New World regions)
- Malo?Often
- WhyCreamy texture, complexity, sometimes butter
- Chablis
- Malo?Often, even without oak
- WhyLouis Michel, for example, lets it happen naturally in stainless steel
- Champagne and other sparkling bases
- Malo?Depends on the house
- WhyA style choice: softer and rounder, or sharper and fresher
- Riesling, Sauvignon Blanc, Gewurztraminer, Pinot Grigio
- Malo?Usually not
- WhyTo keep crisp acidity and pure fruit aromas
So malo does not always mean butter. A Chablis can go through it and still taste of lemon and stone; the buttery Chardonnays usually combine malo with oak and long lees contact. More on that in malolactic fermentation in Chardonnay.
How winemakers start it
- Wild: wait for the bacteria already in the cellar. Traditional, but slow and less predictable.
- Inoculated: add a selected bacterial strain. After the alcoholic fermentation is the most common timing; with co-inoculation, bacteria go in 24 to 48 hours after the yeast.
- Give the bacteria a chance: they are fragile, especially in white wines.
| Condition (white and sparkling wines) | Favourable for malo | Harsh for malo |
|---|---|---|
| Temperature | 18–22°C | Below 12°C |
| pH | Above 3.3 | Below 3.1 |
| Total sulphur dioxide | Below 30 mg/L | Above 60 mg/L |
| Alcohol | Below 12% | Above 14% |
- Temperature
- Favourable for malo18–22°C
- Harsh for maloBelow 12°C
- pH
- Favourable for maloAbove 3.3
- Harsh for maloBelow 3.1
- Total sulphur dioxide
- Favourable for maloBelow 30 mg/L
- Harsh for maloAbove 60 mg/L
- Alcohol
- Favourable for maloBelow 12%
- Harsh for maloAbove 14%
Source: AWRI fact sheet on MLF in white and sparkling wines. The bacteria are especially sensitive to free sulphur dioxide: the AWRI gives 0.3–0.5 mg/L of molecular SO2 as lethal to wine bacteria.
How winemakers stop it
- Keep it cold after fermentation.
- Add sulphur dioxide early.
- Filter the bacteria out with a fine, sterile filtration before bottling.
- Or go halfway: let part of the wine go through malo and blend it with the rest, to fine-tune acidity.
Butter on demand
Winemakers can push diacetyl up or down. From the AWRI's own table:
- More butter: a longer malo, a cooler one (18°C rather than 25°C), fewer bacteria at the start, lower pH, and certain strains.
- Less butter: lots of bacteria at the start, a warmer malo, no oxygen, and long contact with the yeast lees afterwards, which reduces diacetyl.
When malo goes wrong
If malic acid is left in a wine and bacteria get in, malo can start in the bottle. The AWRI warns this can lead to spoilage, spritz and unwanted flavours: a red that fizzes when it shouldn't, a cloudy bottle, flavours that weren't there before. That is why a wine is usually either fully through malo or carefully protected against it. Lactic acid bacteria can also cause the mousy taint, more likely in wines with low sulphite and low acidity: see natural wine vs regular wine.
Terms to know for exams
- Malolactic fermentation / conversion (MLF): malic acid to lactic acid by bacteria.
- Oenococcus oeni: the main bacterium.
- Diacetyl: the buttery compound.
- Sequential inoculation / co-inoculation: bacteria added after, or during, the alcoholic fermentation.
- Blocked malo: stopped on purpose, to keep a wine crisp.
Test yourself with the Level 1 practice test, or see where malo fits among the other steps in viticulture and vinification for Level 1.
Questions people ask
Does malolactic fermentation make wine sweeter?
No. It doesn't involve sugar. It swaps a sharp acid for a softer one, so the wine can seem rounder, but it is not sweeter.
Is malo the same as the second fermentation in Champagne?
No. Champagne's second fermentation is yeast fermenting added sugar in the bottle to make bubbles. Malo is bacteria converting acid, and in Champagne it happens, or not, in the base wine before bottling.
Do all red wines go through malolactic fermentation?
Virtually all, according to the AWRI. It softens them and makes them stable in the bottle.
How can I tell if a wine went through malo?
Softer acidity, a rounder feel and sometimes a buttery note are clues. Labels rarely say; producers' technical sheets often do.
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