Malolactic Fermentation Temperature Guide
Find the ideal temperature range for malolactic fermentation (MLF) and estimate completion time.
Critical for red wine softness and stability.
Malolactic fermentation (MLF) is a bacterial process that converts sharp malic acid into softer lactic acid, reducing wine acidity and adding buttery, creamy complexity. Temperature is the single most important factor in whether MLF succeeds.
The Temperature-Activity Relationship:
MLF bacteria (primarily Oenococcus oeni) have a narrow activity window:
| Temperature | MLF Activity |
|---|---|
| Below 12 °C (54 °F) | Dormant, MLF stops |
| 12–15 °C (54–59 °F) | Very slow, six weeks to three and a half months |
| 15–18 °C (59–64 °F) | Workable, reliable but unhurried |
| 18–22 °C (64–72 °F) | The sweet spot, three to four weeks |
| 22–25 °C (72–77 °F) | Fast, but watch for diacetyl and volatile acidity |
| Above 25 °C (77 °F) | Bacteria stressed, real risk of off-flavours |
Calculating Time to Complete MLF:
Estimated days = Base days at 20 °C × Temperature adjustment factor
Where base days at 20 °C ≈ 21 days (inoculated wine, healthy bacteria)
Temperature adjustment factor:
| Temperature | Factor | Days |
|---|---|---|
| 12 °C | × 5.0 | ~105, if it goes at all |
| 15 °C | × 2.0 | ~42 |
| 16 °C | × 1.8 | ~38 |
| 18 °C | × 1.3 | ~27 |
| 20 °C | × 1.0 | 21 |
| 22 °C | × 0.85 | ~18 |
| 25 °C | × 0.85 | ~18, no faster, just riskier |
Notice the curve flattens at the top. Past about 22 °C the bacteria are not meaningfully quicker, so the extra warmth buys nothing and costs you diacetyl control. This calculator interpolates between those anchor points.
Worked Example:
Cellar temperature is 16 °C. Inoculated with O. oeni starter culture.
Estimated completion = 21 × 1.8 = ~38 days
Monitor with paper chromatography or a malic acid test kit. The malic spot should disappear entirely, and “almost gone” is not done. A wine bottled with 0.3 g/L of malic still in it will finish the job in the bottle, and a bottle has nowhere to put the carbon dioxide.
Practical Tips:
- Heat the cellar or use a heating mat during cool months to keep wine above 15 °C
- Add MLB nutrients (MBR rehydration nutrients) to help bacteria survive high alcohol
- Wines above 14% ABV and below pH 3.2 are hostile to MLF: consider inoculating at end of primary
- Always add SO2 immediately after MLF completes to protect the wine
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