Malolactic fermentation: when to induce it, how to run it, and how to know it’s done

Malolactic fermentation is the second, quieter fermentation — bacteria, not yeast — that converts sharp malic acid into softer lactic acid. It is standard for nearly all reds and many barrel-fermented whites, and skipped on purpose for crisp aromatic whites. Run well it adds roundness, complexity and, above all, microbial stability. Run badly, or left half-finished, it is a bottling risk.

What MLF actually does

• Softens acidity — malic (sharp, "green apple") becomes lactic (soft), lowering total acidity and nudging pH up by ~0.1–0.3.
• Adds complexity — buttery and creamy notes (diacetyl) at low levels; textural roundness.
• Buys stability — once malic is consumed, there is no substrate left for a spontaneous MLF to erupt in bottle.

The conditions Oenococcus oeni wants

The bacteria are fussier than yeast. Give them the wrong environment and MLF stalls for months.

• Temperature around 18–20 °C — cold cellars are the usual reason it will not start.
• pH above ~3.2 — low pH strongly inhibits the bacteria.
• Low free SO₂ — this is the key one: SO₂ is antibacterial, so do not sulfite before the MLF you want.
• Moderate alcohol — very high alcohol stresses the culture; choose an alcohol-tolerant strain if needed.

The single most common mistake is adding SO₂ at the end of alcoholic fermentation out of habit — it blocks the malolactic you were counting on. Decide the sequence before the wine is dry.

Monitoring to the finish

You cannot judge MLF by taste or bubbles. Track malic acid directly — by enzymatic assay or paper chromatography — every week or two. MLF is complete when malic is at or below ~0.1–0.2 g/L. Only then is the wine microbiologically stable for that substrate.

Lock it in

When malic is gone, add SO₂ to protect the wine and stop any further bacterial activity, and consider cooling or racking off the lees. This is the first real free-SO₂ target of the wine’s post-ferment life.

Guidelines only — confirm strain instructions, temperatures and analytical targets with your supplier and lab.