Malolactic Fermentation Explained (MLF)

Malolactic fermentation (MLF), or malolactic conversion, is a secondary fermentation carried out by lactic acid bacteria (LAB), most commonly Oenococcus oeni. Unlike primary alcoholic fermentation, MLF doesn't produce significant alcohol — instead, it converts one type of acid to another, fundamentally changing the wine's character.

The Chemistry

Malic acid (diprotic — two acidic hydrogen atoms) → Lactic acid (monoprotic — one acidic hydrogen atom)

COOH-CHOH-CH₂-COOH → COOH-CHOH-CH₃ + CO₂
  Malic acid              Lactic acid    Carbon dioxide

The conversion reduces total acidity because lactic acid is a weaker acid than malic acid. For every 1 g/L of malic acid converted, TA drops by approximately 0.67 g/L. pH rises by 0.1–0.3 units.

Key secondary product: Diacetyl — a butter-flavoured compound produced by LAB during MLF. At low concentrations, diacetyl adds complexity; at high concentrations, it produces an overwhelming butter/butterscotch character that many consider a fault.

When to Encourage MLF

All red wines

MLF is almost universally encouraged in red wines because:

• Reduces green, harsh malic acid → softer lactic acid
• Improves mouthfeel and complexity
• Diacetyl at low levels contributes savoury, spice complexity
• Enhances microbial stability (removes a substrate that unwanted bacteria could ferment later in bottle)

Chardonnay (classic Burgundy, California style)

Rich, full-bodied Chardonnay benefits from MLF: rounder mouthfeel, creamy texture, buttery complexity. The famous Meursault or California Chardonnay butter character comes directly from diacetyl and lactic acid.

High-acid varieties in cool climates

Riesling, Sauvignon Blanc, Pinot Grigio at 8–10 g/L TA may benefit from partial MLF to reduce harshness — though full MLF is usually avoided to preserve aromatic freshness.

When to Prevent MLF

Crisp aromatic whites

Riesling, Sauvignon Blanc, Pinot Grigio, Gewurztraminer — the defining character of these wines is fresh acidity and aromatic vibrancy. MLF flattens the acid and mutes aromatics. Always prevent MLF in these styles.

Low-acid musts

If your grapes are already low in acidity (pH above 3.6, TA below 6 g/L), MLF will make the wine flabby and microbiologically unstable. Prevent it.

Wines destined for early bottling

A wine with incomplete, uncertain MLF that is bottled early risks completing MLF in bottle — producing CO₂, off-flavours, and potential bottle spoilage. Either confirm MLF is done or prevent it entirely before bottling.

Conditions for Successful MLF

LAB are more sensitive to their environment than Saccharomyces. The conditions must be right:

Preventing MLF

Add SO₂ (20–30 mg/L free SO₂) immediately after primary fermentation ends. Keep the wine cool (below 15°C where possible). Monitor for any signs of CO₂ activity over the following weeks. If wild LAB is present in your winery, prevention requires vigilance.

Lysozyme (an enzyme from egg whites) can also be used to inhibit LAB at 250–500 mg/L without the off-flavours of SO₂.

Inoculating for MLF

Commercial LAB starters are the most reliable way to ensure MLF happens predictably:

Popular commercial starters:

• Chr. Hansen Viniflora CH11: robust, fast, produces consistent diacetyl character
• Lallemand Lalvin VP41: popular across reds and full whites; low diacetyl production
• Enartis PN4: good for difficult conditions (lower pH, higher alcohol)
• Chr. Hansen Viniflora Oenos: clean, low-diacetyl, reliable

Inoculation protocol:

1. Rehydrate starter in warm water (25–30°C) per manufacturer instructions
2. Add immediately after alcoholic fermentation completes (while wine is still warm, LAB nutrients from yeast autolysis present)
3. Ensure free SO₂ is below 10 mg/L before inoculating
4. Maintain temperature at 18–22°C throughout

Allow 2–8 weeks for MLF to complete, depending on conditions.

Testing for Completion

Paper chromatography (malic acid detection)

The traditional method. Apply a small amount of wine to a chromatography strip alongside reference standards (malic, lactic, tartaric acid). After developing in solvent:

• Malic acid shows as a yellow spot (toxic-smelling solvent — do outdoors)
• If malic acid spot is absent and lactic acid spot is present, MLF is complete

Enzymatic test kit

Commercial enzyme kits (e.g. Vintessential) precisely measure residual malic acid. More expensive but more precise than paper chromatography.

Visual signs (not reliable alone)

CO₂ production during MLF is minimal — tiny bubbles, slightly hazy wine. Not a reliable confirmation.

Never bottle before confirming MLF is complete if you've encouraged it. Always confirm chemically.

What to Do After MLF

Immediately after MLF is confirmed complete:

1. Rack the wine off the lees
2. Add SO₂ to 25–30 mg/L free (the wine is now unprotected)
3. Seal the vessel tightly — no oxygen from here
4. Monitor temperature — keep cool to slow any further microbial activity

The post-MLF window before SO₂ addition is the wine's most vulnerable moment for oxidation and spoilage.