Malolactic Fermentation: The Second Fermentation Nobody Talks About
A second fermentation, done by bacteria, that turns sharp malic acid into soft lactic acid — why barrel-aged Chardonnay tastes buttery and most reds taste smooth.
Yeast does the first fermentation. Most people know about it: sugar in, alcohol out, that's wine. But there's a second fermentation that almost no one outside winemaking knows exists. No yeast involved. No alcohol produced. It's done by bacteria — specifically Oenococcus oeni — and it transforms the wine completely.
This is malolactic fermentation (MLF), or “malo” in winemaker shorthand. It turns sharp, tart wine into smooth, silky wine. Almost every red wine in the world goes through it. About half of white wines do. Some, deliberately, don't.
Once you know about it, you can taste it in every glass.
The four chapters
01The Chemistry
Malic acid in, lactic acid out.
Leer el capítulo 1 →When It Happens
All reds, some whites, never sparkling base.
Leer el capítulo 2 →How It Changes Flavor
Buttery Chardonnay, silky Malbec.
Leer el capítulo 3 →The Argentine Choice
Why most Mendoza reds go through MLF.
Leer el capítulo 4 →Respuestas rápidas
What is malolactic fermentation?
A secondary fermentation in wine, conducted by bacteria (not yeast), that converts sharp malic acid into softer lactic acid. The wine becomes smoother, rounder, and develops a subtle buttery note from a compound called diacetyl.
Do all wines go through malolactic fermentation?
No. Almost all red wines do, because they'd be too sharp otherwise. Many whites do (most Chardonnay), but many don't (Sauvignon Blanc, Riesling, Torrontés). Sparkling wines deliberately avoid it to preserve high acidity.
Why does barrel-aged Chardonnay taste buttery?
Mostly because of malolactic fermentation, which produces diacetyl — the same compound that gives butter its flavor. The barrel adds vanilla and toast, but the buttery note specifically comes from MLF.
Can you taste malolactic fermentation in red wine?
Subtly. In red wine, MLF mostly affects texture rather than flavor. Reds with MLF feel silky and round; reds without feel sharp and angular. The buttery diacetyl note is much less obvious in red than in white.
What is partial malolactic fermentation?
A modern technique where MLF is completed on only part of the wine (often 30–60%), then blended with the rest. This gives some softening without losing the wine's brightness — a middle path between “buttery” and “sharp.” Many modern Argentine Chardonnays use this approach.