Malolactic Fermentation

Malolactic fermentation (MLF) is a bacterial conversion of malic acid to lactic acid and CO2 in wine, carried out by lactic acid bacteria (primarily Oenococcus oeni). It reduces wine acidity, produces diacetyl (buttery notes at low levels), and contributes textural softness and stability.

Malolactic fermentation is technically not a fermentation in the conventional sense (no sugar is consumed) but a deacidification reaction: the dicarboxylic malic acid (tart, sharp) is converted to the monocarboxylic lactic acid (softer, dairy-like) with release of CO2. This reduces total acidity and changes the qualitative character of the acid profile. For Chardonnay — where full MLF is nearly universal in Burgundy and many New World producers — the result is the characteristic buttery, creamy, soft-acid profile that distinguishes a full-malolactic Burgundy from a crisper, un-MLF Chablis style.

For dealcoholized wine production, the decision to carry out MLF in the base wine before dealcoholization has significant sensory implications. A fully malolactic-fermented base wine brings lower acidity and a softer texture to the dealcoholized product — often desirable since dealcoholization can emphasize acidity (as the balancing alcohol is removed). However, for sparkling or aromatic wine styles where crisp acidity is desirable, blocking MLF (through early SO2 addition, low temperature, or filtration) produces a higher-acid, fresher style that may be preferable.

Diacetyl — the butter aroma compound — is produced as a minor byproduct of MLF and is normally metabolized by the bacteria to threshold levels. Under stress conditions or during partial MLF, diacetyl can accumulate to perceivable levels. In dealcoholized wine, any residual diacetyl from MLF is preserved through dealcoholization, making MLF management relevant to the quality of the finished zero-proof product.

A bacterial ecology insight: the primary MLF organism, Oenococcus oeni, is extraordinarily well-adapted to the hostile wine environment — tolerant of pH as low as 3.2, ethanol up to 15% ABV, and SO2 at concentrations used in normal winemaking. Its extreme stress tolerance makes it one of the most elegant examples of evolutionary adaptation to an artificial human-created environment in the microbial world.