Malolactic fermentation, often called MLF or malo, is not actually a fermentation in the usual sense. It is a bacterial conversion. Malic acid, which tastes sharp and green (think green apple), is converted by bacteria into lactic acid, which tastes softer and rounder (think milk). The result is a wine with lower apparent acidity and a creamier texture.
Almost every red wine goes through malolactic fermentation. The harsh malic acid in young red wine would make it unpleasant, so the winemaker encourages MLF to happen after the primary alcoholic fermentation is complete.
White wines are where the decision gets interesting. A winemaker can choose to allow MLF or block it, depending on the style they want.
Chardonnay is the most famous example. A Chardonnay that has gone through full MLF, especially one aged in oak, will be rich, buttery and full-bodied. Buttery is not a metaphor. MLF produces a compound called diacetyl, which is also responsible for the flavour of butter. When people describe a Chardonnay as buttery, they are identifying a wine that went through malo.
A Chardonnay with MLF blocked will be leaner, crisper, more acidic, with more apple and citrus fruit. Many modern winemakers, particularly in Burgundy, now deliberately block MLF to preserve freshness.
Riesling and Sauvignon Blanc almost never go through MLF. Both varieties rely on high acidity as part of their identity. Softening that acidity would remove much of what makes them distinctive.
The winery temperature matters. MLF bacteria are sensitive to cold. A winemaker who wants to block MLF can simply lower the cellar temperature.
