A secondary fermentation in winemaking in which lactic acid bacteria, principally Oenococcus oeni, convert sharp-tasting malic acid into softer lactic acid plus carbon dioxide. The process reduces the wine’s acidity and adds creamy, buttery complexity from the byproduct diacetyl. It is standard for most red wines and for fuller white styles such as oaked Chardonnay, but usually blocked in crisp whites like Sauvignon Blanc where fresh acidity is prized. Winemakers control it through temperature, sulphur dioxide levels, and inoculation with selected bacterial cultures. Beyond softening acidity, malolactic fermentation improves microbial stability, protecting the wine during ageing.