What MLF Actually Does
Malolactic fermentation, usually shortened to MLF, is the conversion of sharper malic acid into softer lactic acid by lactic acid bacteria. In practical cellar terms, it rounds out acidity, reduces the green-apple edge common in young reds, and helps make the wine more microbiologically stable before bottling. Most red wines go through MLF. Many fresh whites do not.
The key point is this: MLF is not a side note after alcoholic fermentation. It is its own stage with its own risks, timelines, and management decisions. If you do not actively track it, you are gambling with stability.
When to Start It
Small wineries usually choose one of two approaches. The first is co-inoculation, where malolactic bacteria are added near the end of primary fermentation. This can shorten tank time and often leads to a smoother integration between yeast and bacteria. The second is sequential inoculation, where you wait until alcoholic fermentation is complete, rack if needed, and then inoculate. That approach gives you more control, but it often takes longer.
Either way, don't add sulfur dioxide before MLF is finished unless you intentionally want to stop it. SO₂ is one of the fastest ways to stall or kill your bacteria. If you inoculate and then make a routine sulfur addition out of habit, you can create weeks of delay and a lot of confusion.
The Conditions MLF Needs
MLF tends to move best when wine temperatures stay around 65–72°F (18–22°C). Below that, activity slows dramatically. pH also matters: lower-pH wines are harder on bacteria, while very high-pH wines increase microbial risk and require tighter management. Alcohol matters too. A big, high-alcohol red that finishes primary fermentation stressed and nutrient-depleted is not an easy environment for bacteria.
If you want MLF to finish on schedule, stack the deck in its favor. Keep the wine warm enough, avoid early sulfur additions, and choose a bacteria strain that fits your wine conditions rather than assuming any culture will work.
How to Monitor Progress
This is where many small producers get loose. They assume MLF is done because the wine tastes softer or because enough time has passed. That is not a real verification method. The correct approach is to test for malic acid using paper chromatography, enzymatic assay, or a lab panel. Your goal is to confirm that malic acid has dropped to essentially zero or to the level your lab defines as complete.
A simple weekly log should include: inoculation date, tank or barrel ID, wine temperature, malic test result, and sulfur status. If you are managing multiple lots, this is exactly the type of process that breaks down in spreadsheets. One missed note or one forgotten barrel can create expensive bottle instability later.
Common Reasons MLF Stalls
The usual causes are predictable: wine too cold, SO₂ added too early, pH too low, alcohol too high, or bacteria pitched into a harsh wine without enough support. Dirty cooperage and poor sanitation can also complicate the picture by introducing spoilage organisms that compete with the MLF culture.
If MLF appears stuck, don't guess. Verify the current malic level, check temperature, review every sulfur addition, and confirm whether the inoculation rate and strain were appropriate. Randomly warming the wine or adding another culture without diagnosis can waste time and money.
What to Do When It Finishes
Once MLF is verified complete, move quickly into your post-MLF stabilization workflow. Rack if needed, make the appropriate sulfur addition, and tighten up oxygen management. At that point, the wine is in a much better place for aging and bottling because you have removed a major source of microbial uncertainty.
For small wineries, the lesson is simple: treat MLF like an operating process, not a background event. Track it deliberately, confirm it with data, and close it out cleanly. That discipline protects wine quality and prevents one of the most frustrating avoidable problems in the cellar.