Malolactic Fermentation: What It Is and Why It Matters

Malolactic fermentation (MLF, or “malo”) is a step in winemaking where bacteria turn sharp malic acid, the acid you taste in green apples, into softer lactic acid. It makes a wine taste rounder and less tart, makes it more stable in the bottle, and is the reason some Chardonnay tastes of butter. Almost every red wine goes through it, while for whites it is a style choice. If you have ever wondered why one Chardonnay is creamy and another is crisp, this is a big part of the answer.

What malolactic fermentation is

Penn State Extension’s enology educator Molly Kelly describes MLF as a change in which lactic acid bacteria convert L-malic acid into L-lactic acid and carbon dioxide. The bacterium that does most of the work is Oenococcus oeni. Other lactic acid bacteria (Lactobacillus and Pediococcus) can carry it out too, but Penn State says O. oeni is the usual choice because it copes with low pH, high alcohol and sulfur dioxide (SO2). The Australian Wine Research Institute (AWRI) adds that it is also valued for producing desirable sensory attributes, and warns that some strains of Pediococcus and Lactobacillus are potential spoilage organisms.

Despite the name, it is not a fermentation in the usual sense. Decanter points out that no yeast is involved, and Wine Folly calls it “not technically a fermentation”: it is a bacterial conversion that happens after, or alongside, the main alcoholic fermentation, in which yeast turns grape sugar into alcohol.

What it does to a wine

The AWRI sums up MLF’s role in three parts: it lowers acidity, it stabilizes the wine by removing malic acid, and it shapes aroma and flavor.

Effect	What happens	Source
Softer acidity	Sharp malic acid becomes milder lactic acid; Decanter notes the pH also rises	Penn State, Decanter
Stability	Malic acid is gone, so bacteria and yeast can’t feed on it later and cause spritz or off-flavors	AWRI
Butter	O. oeni makes diacetyl, the buttery compound	AWRI, Wine Folly
Texture	Sensory words linked to MLF include “fuller” and “rounded”	AWRI
Fruit	Some O. oeni strains enhance red-berry and dark-berry notes in reds	AWRI

Softer acid. Oregon State University (OSU) Extension enologist James Osborne calls MLF essential for balance in cool-climate wines with high malic acid. If you find some wines too sharp, our guide to why wine tastes sour explains how acidity shows up in the glass.

Stability. This is the reason reds almost always go through it. The AWRI explains that leftover malic acid can feed yeast and bacteria later, which may lead to spoilage, a light fizz (“spritz”) and unwanted flavors. Decanter adds that finishing MLF in the cellar prevents it from starting spontaneously after bottling.

Flavor. The AWRI lists descriptors such as buttery, vanilla-like, nutty, spicy, fruity and toasty for MLF’s influence. In our view, the most useful takeaway for shoppers is simple: malo makes a wine taste softer and fuller, and sometimes buttery.

Diacetyl and buttery Chardonnay

The buttery note has a name: diacetyl. The AWRI says O. oeni forms it by breaking down citric acid during MLF, and that it gives wine its buttery or butterscotch aroma and flavor. Wine Folly makes the same point about Chardonnay and stresses that the butter comes from malolactic fermentation, not from oak aging. Oak adds its own notes, such as vanilla from vanillin.

How much butter you notice depends on the dose and on the rest of the wine. The AWRI says high levels come across as overtly buttery and are considered undesirable, and that strong oak or very fruity character can mask it. OSU puts the line for objectionable diacetyl at about 5 mg/L, while lower levels can be welcome, depending on the style.

Winemakers can push diacetyl up or down. According to the AWRI:

• Bacterial strain. Some O. oeni strains produce a lot of diacetyl, others very little.
• Speed. A longer MLF tends to leave more diacetyl; a faster one less.
• Temperature. MLF at 18°C (64.4°F) gave more diacetyl than at 25°C (77°F).
• Lees contact. Long contact with yeast lees (the spent yeast at the bottom of the tank) lowers it.
• Timing. When bacteria are added with the yeast (co-inoculation), the yeast can convert diacetyl into compounds with little smell, so the wine ends up less buttery.

OSU’s guide agrees on that last point and adds that when a buttery character is the goal, winemakers do better with a traditional sequential MLF (after alcoholic fermentation) and a high-diacetyl strain.

Our pick if you want to see the difference for yourself: open a rich California Chardonnay next to a crisp, unoaked one.

Which wines usually go through it

Wine	Usual practice	Source
Red wines	Almost always	AWRI (“most red wines”, “virtually all red wines”)
Chardonnay, Viognier	Routinely	Decanter
Sparkling base wines	Sometimes, as a style choice	AWRI
Riesling, Sauvignon Blanc and other crisp aromatic whites	Usually avoided	Decanter
Reds from very hot climates with low acidity	May be better without it	Decanter

Reds. The AWRI’s red wine fact sheet says almost all Australian reds undergo MLF and 74% are inoculated with selected bacterial cultures (citing its 2019 winery practices survey). Decanter notes that many reds go through it in barrel, which can bring out smoky and spicy notes alongside softer acid and brighter fruit. See our guide to types of red wine for the main styles.

Whites and sparkling wines. For whites, the AWRI says the decision is “generally based on stylistic criteria,” such as lowering acidity or adding complexity and mouthfeel. It is also harder to do: white and sparkling base wines tend to have low pH (2.9 to 3.4), more SO2 and cooler cellar temperatures, all of which stress the bacteria. Decanter says MLF is generally undesirable in zingy, high-acid aromatic whites such as Riesling and Sauvignon Blanc, while Chardonnay and Viognier routinely go through it.

There are no fixed rules. Decanter recalls that in the very acidic 2014 vintage, some Bordeaux producers put Sémillon and Sauvignon Blanc through MLF, which is rare for white Bordeaux. Our guide to types of white wine groups whites by body, which roughly follows this split: the fuller, creamier whites are the ones most likely to have gone through malo.

When and how winemakers block it

A winemaker who wants to keep a wine crisp has a few tools. Decanter lists adding SO2 after alcoholic fermentation or using the enzyme lysozyme. The extension guides explain why these work: the bacteria are fussy.

• SO2. Penn State says total SO2 above 50 ppm generally limits lactic acid bacteria. The AWRI’s red wine sheet says more than 50 to 60 mg/L may stop MLF completely.
• Low pH. Penn State reports minimal growth at pH 3.0. The AWRI rates pH below 3.1 as “harsh” for MLF in whites.
• Alcohol. Penn State says growth slows or stops at about 13.5% alcohol, though the AWRI notes that alcohol-tolerant strains exist for big reds.
• Cold. Decanter says the bacteria like temperatures above 16°C (60.8°F). Penn State notes MLF runs faster at 20°C (68°F) and above than at 15°C (59°F) and below.

OSU adds that sequential MLF makes it easier to block malo in wines where the producer wants to keep the acidity.

How winemakers make sure it happens

When malo is wanted, the goal is a clean, complete conversion. Decanter explains that bacteria living in a cellar can start MLF on their own, and that in the past winemakers often had to wait for the spring after harvest, when cellars warmed up. Today, the AWRI says, winemakers commonly add a selected bacterial culture because wild MLF is unpredictable.

OSU’s checklist for a successful MLF includes a temperature of 18 to 22°C (64.4 to 71.6°F), free SO2 below 5 mg/L, pH 3.20 to 3.50 and alcohol below 13.5%. MLF counts as finished when malic acid falls below about 30 to 50 mg/L, which OSU says should be confirmed by lab analysis before SO2 is added. The AWRI aims for “not detected,” usually under 0.05 g/L.

Timing matters for safety too. Penn State warns that the longer MLF takes to start, the greater the risk of Brettanomyces, a spoilage yeast. The AWRI notes that delays in stabilizing the wine after MLF can raise volatile acidity (a vinegar-like note). In our view, this is one reason home winemakers should follow a tested protocol rather than leave malo to chance.

How to tell if a wine went through malo

Labels rarely say so. A few clues help:

• “Buttery” or “creamy” on a back label usually points to malo in a white.
• Oak aging. Decanter says wines typically aged in oak undergo MLF.
• Texture. Decanter mentions a creamy, oily texture and rounder acidity, and a higher pH if the winery publishes technical data.
• Style words. “Crisp,” “zesty” or “unoaked” on a white often means the winemaker kept the malic acid. We’d treat these as hints, not guarantees.

Our view: knowing about malo is the easiest way to predict whether a white will be crisp or creamy before you buy it. If the tech sheet on a winery website mentions “100% malolactic,” expect a softer, rounder wine; “no malolactic” signals a sharper, fresher one.