Wine · Terminology
Malolactic conversion
Bacteria turning sharp malic acid into softer lactic acid — the process behind buttery Chardonnay.
Malolactic conversion — MLF, malo — is the conversion of malic acid into lactic acid by lactic acid bacteria, chiefly Oenococcus oeni. It is not a fermentation in the strict sense (no sugar, no alcohol produced), which is why the modern term prefers conversion.
Its effects are threefold: the wine becomes softer, because lactic acid is less sharp than malic; it becomes more stable, because the malic that bacteria would otherwise attack in bottle is gone; and it may pick up diacetyl, the compound that smells of butter and cream.
In practice
- Standard and near-universal for red wine.
- Optional for whites — full malo gives buttery Chardonnay; blocking it keeps a Riesling or Sauvignon Blanc crisp and appley; partial malo splits the difference.
- Blocked by cool temperatures, sulfur dioxide, filtration or simply keeping the wine off the bacteria's preferred conditions.
- Encouraged by warmth (18–22 °C), low sulfur and inoculation with selected bacterial cultures.
Where you'll meet it
- Buttery Chardonnay — malo plus oak plus lees stirring is the full recipe.
- Cool-climate reds, where softening a searing malic edge is essential.
- Champagne, where houses take opposite views: some block it for tension, others use it for roundness.
What changes
| Before MLF | After MLF | |
|---|---|---|
| Dominant acid | Malic (sharp, green apple) | Lactic (soft, milky) |
| Titratable acidity | Higher | Lower — by roughly 1–3 g/L |
| pH | Lower | Higher — a stability concern |
| Aroma | Fresh apple, citrus | Butter, cream, hazelnut (from diacetyl) |
| Microbial stability | At risk in bottle | Stable |
The diacetyl note is dose-dependent and partly reversible: stirring the lees afterwards actually reduces it, because yeast metabolise diacetyl.
The exam angle
- Name it correctly and describe all three consequences — softening, stabilisation, buttery aroma — not just the buttery one.
- Explain why a winemaker would block it: to keep a fresh, fruity, high-acid style in an aromatic white.
- Do not confuse the raised pH (a stability risk) with lower perceived acidity — examiners like the distinction.
Related terms
- Acidity — what is being converted
- Fermentation — the alcoholic one, before
- Bâtonnage — often done alongside
- Flabby — what over-softening risks