What is Malolactic Fermentation?

Understanding Malolactic Fermentation

Malolactic fermentation (MLF or "malo") is a bacterial conversion—not a true fermentation, despite the misleading name—in which lactic acid bacteria, primarily Oenococcus oeni, metabolize sharp-tasting malic acid (the same acid that makes green apples tart) into softer, rounder lactic acid (the acid found in yogurt and milk), releasing carbon dioxide and a compound called diacetyl that produces the characteristic buttery, popcorn-like aroma in wines like oaked California Chardonnay. Though practiced intuitively by winemakers for centuries, the science was only formally identified in the 1930s by legendary Bordeaux oenologist Emile Peynaud, whose research revolutionized the understanding of wine stability and ushered in the modern era of controlled MLF. The process requires specific conditions—typically temperatures between 60-70°F, pH above 3.2, low free SO2, and ample nutrients—and can be triggered naturally by ambient bacteria in the cellar or induced with commercial bacterial cultures for greater predictability. MLF is nearly universal for red wines (where it also improves microbial stability and integrates tannins) and common for barrel-aged whites, but winemakers actively block it in crisp, aromatic whites like Riesling, Sauvignon Blanc, and unoaked Chardonnay to preserve the wine's natural tartness and fresh fruit character—typically by chilling the wine, raising SO2 levels, or filtering out the bacteria. Modern techniques include partial MLF (where only a percentage of the wine undergoes conversion, preserving some malic crispness while adding complexity), co-inoculation (starting MLF alongside primary fermentation to compress the winemaking timeline), and deliberate lees stirring during MLF to amplify creamy, textural mouthfeel.

Examples

• 1California oaked Chardonnay often undergoes 100% MLF for buttery character
• 2Most red wines complete MLF in the winter following harvest
• 3Riesling, Sauvignon Blanc, and Chablis typically block MLF to preserve acidity
• 4Champagne producers selectively allow or block MLF depending on house style
• 5Burgundian producers often perform MLF in barrel to integrate oak and lees character

Related Wine Terms