18–22°C and <5 mg/L SO2, Malolactic Fermentation for Home Winemakers
Malolactic fermentation is the bacterial conversion of malic acid to lactic acid, and it’s the reason some wines taste round and creamy while others stay sharp and crisp. We’re talking softer acidity, a fuller mouthfeel, and sometimes that buttery note you either love or side-eye in a glass of Chardonnay. Most reds go through it. Some whites skip it on purpose.
Table of Contents
How does malolactic fermentation work, chemically speaking?
Here’s the science part, and we promise it’s not as dense as it sounds. Malolactic fermentation (everyone just calls it MLF, because wine people love a good acronym) happens when lactic acid bacteria get hold of the sharp, green-apple-tasting malic acid in wine and convert it into lactic acid, the softer, creamier acid you’d find in milk or yogurt. The process is called decarboxylation, which just means a carbon dioxide molecule gets knocked loose during the conversion. That’s why you’ll sometimes see tiny bubbles or hear faint fizzing in a barrel going through MLF. It’s not sparkling wine sneaking up on you. It’s just the bacteria doing their thing.
The measurable result is a real shift in the wine’s chemistry. Malolactic fermentation typically drops titratable acidity by about 1 to 3 grams per liter and nudges pH up by roughly 0.3 units. That might sound small on paper, but in a glass, it’s the difference between a wine that snaps at your tongue and one that settles in smooth.
The bacteria responsible are called lactic acid bacteria, or LAB for short, and one species does most of the heavy lifting:
Oenococcus oeni is the primary workhorse because it tolerates the low pH and higher alcohol levels found in wine better than most other bacteria.
Lactobacillus and Pediococcus species can also carry out MLF, but they’re less predictable and more often associated with off-flavors.
Climate shifts are pushing some winemakers toward Lactiplantibacillus plantarum and other alternative strains, since recent research points to hotter growing seasons producing higher-sugar, lower-acid grapes that call for more stress-tolerant bacteria.
Strain selection matters because not every bacterial strain behaves the same way once it hits your wine. Some finish fast and clean. Others stall out if the temperature or pH isn’t in their comfort zone, which is exactly why winemakers (hobbyists included) spend so much time thinking about the conditions we’ll get into below.
What does MLF actually do to flavor and mouthfeel?
This is the part most beginner winemakers actually care about: what’s going to happen to the wine in my glass? The acidity drop we mentioned earlier doesn’t just read as a number on a lab sheet. It changes how the wine physically feels on your tongue. Lower titratable acidity means less of that mouth-puckering tartness, and the higher pH softens the overall sharpness, giving you a rounder, sometimes almost silky texture.
Then there’s diacetyl, the compound most responsible for that buttery, popcorn-like aroma people either associate with a classic baked Chardonnay or avoid like a bad date. Diacetyl forms as a byproduct of the bacterial conversion, and how much you get (and whether it reads as charming or cloying) depends on a few variables:
Strain choice matters because some Oenococcus oeni strains produce noticeably more diacetyl than others.
Lees contact (leaving the wine on the spent yeast cells after fermentation) can either boost or soften diacetyl perception depending on timing.
Temperature and timing during MLF shift how much diacetyl survives into the finished wine, since some of it breaks down naturally over time.
One of the most useful numbers to know here: detection thresholds for diacetyl sit around 0.2 milligrams per liter in white wines and 2.8 milligrams per liter in reds, meaning whites show buttery character at much lower concentrations. Push past roughly 5 milligrams per liter in either style, and most tasters start calling it a flaw rather than a feature.
That threshold gap is why a barely-there buttery note can make a Chardonnay sing while the same compound at a higher level makes it taste like movie theater popcorn. It’s also why winemakers aiming for a leaner, more mineral style actively manage diacetyl down, while those chasing a rich, buttery Chardonnay lean into it.
Which wines typically go through malolactic fermentation?
Not every wine needs or wants this process, and knowing which ones usually get it (and which ones usually skip it) helps you understand what’s in your glass before you even taste it.
Nearly all red wines go through MLF because the softer, rounder texture it creates plays well with tannins and body.
Many Chardonnays go through full or partial MLF, which is where that classic buttery, creamy character comes from.
Aromatic whites like Riesling, Sauvignon Blanc, and Gewürztraminer usually skip MLF on purpose, since winemakers want to preserve the crisp, zesty acidity that defines those styles.
Climate and the grape’s natural malic acid levels also factor into the decision. Grapes grown in cooler regions tend to retain more malic acid at harvest, which gives winemakers more raw material to convert and more reason to run MLF for balance. Warmer-climate grapes start with less malic acid to begin with, so the acid-softening effect of MLF is less necessary, and sometimes skipped entirely to preserve what little freshness remains.
It really comes down to a trade-off between freshness and roundness. A crisp, high-acid white can feel thin if pushed through full MLF, while a naturally tannic, higher-acid red almost always benefits from the smoothing effect. There’s no universal rule here, just a style decision every winemaker makes on purpose (or sometimes, Mother Nature makes for them).
Sequential or co-inoculation: which timing works best?
Once you’ve decided a wine is getting MLF, the next question is timing: do you let the bacteria go to work after alcoholic fermentation finishes, or do you introduce them at the same time as the yeast? Both approaches work, but they come with different timelines and trade-offs.
Sequential inoculation means adding the ML bacteria after alcoholic fermentation is complete. This is the traditional route and typically takes about 20 to 30 days to finish. Many hobbyists prefer it because it gives you full control: you can taste and adjust before introducing the bacteria, and there’s less risk of competition between yeast and LAB.
Co-inoculation means adding ML bacteria alongside the yeast, right at the start. This speeds things up considerably, with MLF sometimes wrapping up in as little as 3 to 7 days. It also lets you add protective sulfur dioxide earlier, which lowers the risk of spoilage organisms taking hold during that vulnerable window.
Strain compatibility checks matter either way, but especially with co-inoculation, since some yeast strains actively inhibit certain ML bacteria. Checking supplier pairing charts before you commit saves you from a stuck fermentation down the road.
Pro Tip: If you’re new to MLF, start with sequential inoculation on your first batch. It’s slower, but it gives you more checkpoints to catch problems before they snowball.
Commercial starter cultures are worth the investment for anyone starting out, and most of the hobbyists we’ve taught lean on them for good reason. A lab-cultured strain gives you predictable timing, known flavor characteristics, and far less guesswork than hoping wild bacteria show up and behave themselves. Wild, uninoculated MLF can absolutely happen on its own, but it’s unpredictable, and the risk of off-flavors or spoilage organisms hijacking the process is a real concern for anyone without years of experience reading a wine’s behavior.
If you want to understand how the yeast-driven stage before MLF actually works, our guide to how alcoholic fermentation happens covers that groundwork in plain language.
What temperature, pH, and SO2 levels does MLF need?
MLF isn’t a “set it and forget it” process. The bacteria are picky about their environment, and getting a few key numbers right makes the difference between a smooth fermentation and a stuck one.
Temperature should sit between 18 and 22 degrees Celsius (64 to 71 degrees Fahrenheit) for reliable bacterial activity.
Cold temperatures below roughly 16 degrees Celsius (61 degrees Fahrenheit) can stall Oenococcus oeni almost entirely, which is a common reason home winemakers find their MLF has quietly stopped.
Sulfur dioxide needs to stay low, since free SO2 is toxic to the bacteria at higher concentrations; most guidance recommends keeping it minimal until MLF wraps up.
pH and alcohol both push back against bacterial survival, so wines with very low pH or high alcohol may need acclimatized or more tolerant bacterial strains to get MLF moving at all.
Nutrients and lees contact give the bacteria something to work with, since LAB can struggle in a nutrient-stripped wine, and light lees contact during MLF often supplies the extra support they need.
A number worth writing on a sticky note: professional guidance recommends keeping free SO2 below roughly 5 milligrams per liter until malolactic fermentation finishes, since anything higher can shut the bacteria down before they get started.
Temperature control is genuinely the single biggest lever most hobbyist winemakers have, and it’s worth getting right before you worry about anything else on this list. If you want a deeper look at why temperature swings affect fermentation (and finished wine) so dramatically, our piece on wine temperature control breaks down the practical side of keeping your setup in range. For a broader primer on why acid levels matter so much to begin with, our guide to wine acidity is a good companion read. And if you’re trying to nail serving temperatures once the wine’s finished, this quick guide on wine service temperatures from one of our partners is a handy supplemental read.
How do you know when malolactic fermentation is actually done?
You can’t eyeball MLF completion. The wine might look perfectly still while bacteria are still quietly working, or it might look like it’s finished when it’s actually stuck. A couple of straightforward tests settle the question.
Paper chromatography is the budget-friendly option most home winemakers reach for first. You spot a sample of wine onto treated paper alongside a reference standard, let it develop, and compare the resulting spots. Malic acid and lactic acid separate into distinct positions, so a chromatogram showing no malic acid spot generally signals MLF is complete.
Enzymatic assays or lab testing give you a precise number instead of a visual read. Most guidance recommends targeting malic acid below roughly 30 to 50 milligrams per liter before adding sulfur dioxide or bottling, since residual malic acid above that threshold risks a surprise fermentation happening later, in the bottle.
If neither test shows progress after a few weeks at the right temperature, you’re likely looking at a stuck fermentation. Check your temperature first, then free SO2 levels, then consider whether the bacteria simply ran out of nutrients.
Chromatography is great for a quick gut check, but if you’re planning to bottle soon, a lab-verified enzymatic result is worth the extra step. Bottling before MLF is truly finished is one of the most common and most avoidable mistakes beginner winemakers make.
How do you fix a stalled or risky malolactic fermentation?
Even with good planning, things go sideways sometimes. The good news is that most MLF problems have known fixes, as long as you catch them early.
In-bottle MLF is the fault nobody wants: residual CO2, cloudiness, and off-flavors showing up after the wine’s already sealed. Prevent it by confirming malic acid is below threshold with a lab test (not just a visual chromatography read), double-checking your SO2 math actually binds enough free sulfur dioxide, and considering sterile filtration if you’re bottling on a tight timeline.
Volatile acidity and spoilage bacteria thrive in the same low-SO2 conditions that MLF needs, so sanitation is non-negotiable. If spoilage organisms take hold, a lysozyme treatment can knock them back, but wait at least one to two weeks before re-inoculating with fresh ML bacteria in red wines (longer for whites), since lysozyme can inactivate the new culture if you rush it.
A stalled MLF usually responds to warming the wine slightly, adding a nutrient supplement formulated for LAB, or re-inoculating with a fresh, more robust strain if the original culture simply didn’t survive the conditions.
Pro Tip: Keep a simple log of temperature, SO2 additions, and test results from day one. When something stalls, that log is the fastest way to figure out what actually went wrong instead of guessing.
What should a beginner’s first MLF trial actually look like?
We’ve taught enough beginner winemakers to know the gap between reading about MLF and actually running it yourself is real. Here’s a simple checklist for a first attempt that won’t leave you guessing:
Temperature control setup, even something as simple as a heating belt or a warm closet, to keep the must in that 18 to 22 degree Celsius range.
ML nutrient supplement on hand, since a nutrient-starved batch is a common reason first attempts stall.
A paper chromatography kit, which is inexpensive and gives you a visual way to track progress without sending samples to a lab every week.
A compatible yeast and ML bacteria pairing, checked against the supplier’s compatibility chart before you commit to co-inoculation.
A simple guided tasting exercise helps the chemistry actually click. Pull a small sample before MLF starts and taste it alongside a sample partway through (or a finished bottle of the same grape that skipped MLF entirely). The acidity shift and any buttery notes become obvious fast once you’re tasting side by side instead of relying on memory. It’s the same side-by-side approach used in tasting classes, because reading about acid conversion is one thing, and actually tasting the before-and-after is what makes it stick.
Does MLF change how a wine ages or holds up in the bottle?
Malolactic fermentation does more than adjust flavor. It also makes a wine more microbially stable once bottled. Malic acid is a food source certain spoilage bacteria can exploit later on, so removing it through MLF closes off that risk before the wine ever leaves the cellar. That stability benefit is arguably as important to professional winemakers as the sensory changes, since a wine that’s chemically unstable can develop unwanted fermentation activity months or years after bottling, long after anyone’s watching for it.
For aging potential specifically, the picture is more nuanced. The lower acidity that comes with MLF can make a wine feel more approachable sooner, but high-acid wines often age longer precisely because that acidity acts as a preservative. A red built for long cellaring might go through MLF for stability and texture while still retaining enough backbone acidity to hold up over years. A wine that skips MLF entirely, if it’s going to see extended aging, needs other structural elements, like tannin or alcohol, to carry it.
The takeaway: MLF trades some raw acid preservation for microbial security and immediate drinkability, which is exactly why winemakers treat the decision as a style choice rather than an automatic step.
Does it matter if MLF happens naturally or gets inoculated?
Spontaneous MLF, where native bacteria already present in the cellar or on the grapes kick off the conversion without any help, can produce genuinely complex, layered results. Some winemakers chase that complexity on purpose, treating an unpredictable native fermentation as part of their house style.
The catch is consistency. Wild MLF depends on whatever bacterial population happens to be floating around, and that population varies from vintage to vintage, cellar to cellar, even barrel to barrel. One batch might finish clean and interesting. Another, using the same grapes and the same process, might stall out or pick up unwanted flavors from a less desirable bacterial strain.
Inoculated MLF, using a known commercial Oenococcus oeni strain, trades some of that potential complexity for predictability. You know roughly how much diacetyl to expect, roughly how long the process will take, and you’re working with a strain that’s been tested for exactly this job. For anyone still learning to read a fermentation, that predictability is worth far more than the small chance of extra complexity from a wild population that might just as easily go wrong.
Are there labeling rules tied to malolactic fermentation?
Malolactic fermentation itself isn’t something that triggers special labeling requirements on a wine bottle. It’s a winemaking technique, not an additive or an allergen, so you won’t find “went through MLF” disclosed on a label the way sulfite warnings are. That said, the process intersects with labeling in one practical way: MLF affects the final acidity and sugar chemistry of a wine, and those downstream numbers matter if a wine is being labeled for sulfite content or marketed around a specific style claim.
If you’re making wine at home purely for personal use, labeling rules are mostly a non-issue. If you’re producing anything intended for sale, the labeling requirements come from your local alcohol regulatory body rather than from anything specific to the malolactic process, so it’s worth checking the rules that apply to your own situation and production scale rather than assuming a one-size-fits-all answer.
How did malolactic fermentation become standard winemaking practice?
Malolactic fermentation isn’t a modern invention dreamed up in a lab. Winemakers have been seeing wines “go through a second fermentation” for centuries, long before anyone understood the bacteria behind it. Historically, it often happened by accident, triggered by rising cellar temperatures in spring after a cold winter had kept bacteria dormant through the first fermentation. Winemakers noticed their wines softening and sometimes developing new aromas, without fully understanding why.
Once microbiologists identified Oenococcus oeni and mapped out the chemistry of the malic-to-lactic conversion, winemakers gained the ability to control the process on purpose rather than hoping it happened naturally. That shift, from accidental to intentional, is a big part of why MLF became a standard, deliberate step in red winemaking and in styles of white wine built around a rounder, creamier profile. What used to be a mysterious springtime event in the cellar is now a measurable, manageable part of the winemaking process, with known temperature ranges, known bacterial strains, and known chemical endpoints.
Why we favor predictable control over leaving it to chance
If you’re just starting out, we’ll say it plainly: choose inoculation and measurable controls over waiting to see what the cellar gives you. Spontaneous MLF has its romance, and plenty of seasoned winemakers chase that complexity on purpose. But a beginner’s first few batches are where you’re learning to read chemistry, not where you want an unpredictable wild bacterial population adding a variable you can’t yet diagnose.
A known strain, a tracked temperature, and a chromatography test you actually understand will teach you more in one batch than three unpredictable ones ever could. Once you’ve got a few successful fermentations under your belt and know what “normal” looks like, that’s the right time to start experimenting with letting nature take the wheel.
— Thomas
Ready to taste the difference malolactic fermentation makes?
Reading about diacetyl thresholds and TA shifts only gets you so far. The moment it actually clicks is when you taste a wine before and after that conversion happens side by side, which is exactly the kind of hands-on comparison we build into our classes.
Our Elements of Wine class walks beginners through fermentation basics without the textbook jargon, and our Elements of Tasting class is built around exactly this kind of guided, side-by-side comparison so you can train your palate to actually notice the buttery notes and softened acidity we’ve spent this whole article describing. Both are one-off purchases, no subscription, no pressure. If you’d rather learn with friends or want a session built around your own questions, our Guided Tasting and Custom Classes can be arranged for private groups. Head to our courses page to see what’s running and grab a spot.
FAQ
How do you trigger malolactic fermentation?
You trigger MLF by inoculating the wine with a commercial Oenococcus oeni culture, either right after alcoholic fermentation (sequential) or alongside the yeast (co-inoculation). Keeping the wine between 18 and 22 degrees Celsius and free SO2 low gives the bacteria the conditions they need to get moving.
Which wines go through malolactic fermentation?
Nearly all red wines go through MLF because the softer texture suits their tannin structure, and many Chardonnays go through it for that classic buttery character. Aromatic whites like Riesling and Sauvignon Blanc usually skip it to preserve their crisp, zesty acidity.
How do you stop malolactic fermentation?
MLF is typically stopped by chilling the wine below the bacteria’s active range, adding sulfur dioxide to suppress bacterial activity, or sterile filtering to remove the bacteria entirely. Confirming the wine’s malic acid level with a lab test first helps you know whether stopping now is even the right call.
How long does malolactic fermentation usually take?
Sequential inoculation, done after alcoholic fermentation finishes, typically takes about 20 to 30 days to complete. Co-inoculation, where bacteria are added alongside the yeast, can finish in as little as 3 to 7 days, though actual timing depends on temperature, strain, and the wine’s chemistry.
Sources
Recommended
Comments