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 — weeks to months |
| 15–20 °C (59–68 °F) | Ideal range — reliable completion |
| 18–22 °C (64–72 °F) | Fastest completion — 2–4 weeks |
| Above 25 °C (77 °F) | Bacteria stressed — risk of off-flavors |
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 (approximate):
- 18 °C: × 1.3 → ~27 days
- 15 °C: × 2.0 → ~42 days
- 12 °C: × 5.0 → ~100 days or stalls
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 — target malic acid spot should disappear.
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