What MLF Actually Does
Malolactic fermentation is not technically a fermentation — it's a bacterial conversion. Lactic acid bacteria (LAB), primarily Oenococcus oeni, convert sharp malic acid into softer lactic acid, releasing CO₂ in the process. The result: a wine with lower perceived acidity, more textural richness, and greater microbiological stability.
For most red wines and barrel-fermented whites like Chardonnay, MLF is standard practice. For crisp whites — Sauvignon Blanc, Pinot Grigio, Riesling — it's usually blocked to preserve freshness. The decision depends on your style goal, not a rule.
When to Inoculate
You have two timing options: co-inoculation (adding LAB at the same time as yeast) or sequential inoculation (waiting until alcoholic fermentation finishes). Each has trade-offs.
Co-inoculation shortens total processing time and reduces the window for spoilage organisms to establish. Some commercial strains (like Lallemand's VP41) are specifically designed for it. The downside: if alcoholic fermentation stalls, you now have an actively fermenting bacterial population and a stressed yeast population competing for resources.
Sequential inoculation is lower risk. Wait until residual sugar is below 3 g/L, then inoculate. The bacteria enter a more hospitable environment — alcohol is already fixed, temperature has stabilized, and yeast hulls are available as nutrients. For small wineries without dedicated lab monitoring, sequential is usually the right call.
Conditions That Drive (or Kill) MLF
MLF is sensitive. The three biggest variables:
- Temperature: LAB want 65–72°F (18–22°C). Below 60°F, activity slows dramatically. Above 75°F, you risk off-flavor production. In an unheated cellar, MLF in winter can stall for weeks.
- SO₂: Free SO₂ above 15 mg/L will suppress or kill LAB. Before inoculating, test free SO₂ and adjust if needed. Many winemakers hold off on SO₂ additions until MLF is confirmed complete.
- pH: Below 3.2, MLF becomes very difficult. The bacteria struggle in high-acid environments. Above 3.5, it proceeds quickly but you also have elevated spoilage risk. Know your pH going in.
Monitoring Completion
The most common method is paper chromatography. You spot the wine on a chromatography sheet alongside standards for malic and lactic acid, develop it with solvent, and compare the bands. It's inexpensive, requires no equipment beyond a fume hood or outdoor space, and gives clear visual confirmation.
Enzymatic analysis (using a malic acid test kit) is more precise and will give you a number — typically below 0.1 g/L is considered complete. Some wineries use this as their final confirmation after chromatography shows completion.
Do not rely on CO₂ activity alone. Fine bubbles on a barrel bung or in a glass sample are suggestive, but not conclusive. Always confirm analytically before adding SO₂.
Common Problems and Fixes
MLF won't start. Check free SO₂ first — this is the most common cause. Then check temperature and pH. If all three look fine, try rehydrating a fresh LAB culture with Go-Ferm Protect Evolution before re-inoculating. Starter cultures (growing the bacteria in a small volume of juice or wine first) can improve success in difficult conditions.
MLF stalls mid-process. Most commonly a temperature drop. Move the wine to a warmer space or use a cellar heater. If temperature isn't the issue, check that free SO₂ hasn't crept up from an accidental addition. Nutrient deficiency can also stall MLF — NutriStart or OptiMalic can help.
Off-flavors during MLF. Mousy or geranium notes indicate a spoilage issue, not normal MLF. Mousiness comes from Brett or contaminated LAB strains and is extremely difficult to reverse. Geranium character comes from sorbic acid reacting with LAB — never add potassium sorbate to a wine that may undergo MLF. These are cellar hygiene problems, not MLF problems per se.
Blocking MLF When You Don't Want It
For whites where you want to preserve acidity, block MLF early. After alcoholic fermentation is confirmed complete, adjust free SO₂ to 30–35 mg/L (at your wine's pH), chill to below 55°F if possible, and sterile filter before bottling. Lysozyme (an enzyme that disrupts bacterial cell walls) can also be used mid-process to arrest MLF — useful if you want partial conversion for stylistic reasons.
The Bottom Line
MLF is one of the highest-leverage decisions you make post-harvest. Done well, it adds complexity and stability. Mismanaged, it introduces off-flavors, spoilage risk, and wasted time chasing a stalled fermentation. Monitor your SO₂, hold your cellar temperature, and confirm completion analytically — every time.
The winemakers who get MLF right consistently are the ones who track it consistently. Temperature logs, SO₂ test dates, inoculation records — that data pays for itself the first time it helps you catch a problem before it becomes a loss.