malolactic fermentation [ma-loh-LAK-tihk]

from

A biochemical reaction, sometimes called SECONDARY FERMENTATION, where bacteria converts malic acid into lactic acid and carbon dioxide—no alcohol is produced. Because lactic acid is milder than malic acid, wines that undergo this process become softer and smoother. In addition, malolactic fermentation produces diacetyl (or biacetyl), which resembles the smell of heated butter and adds complexity to wine. Malolactic fermentation is a positive event in some cases, and most high-quality red wines and some white wines (including white Burgundies and California Chardonnays) undergo it. On the downside, the fruitiness of wines undergoing this process is diminished, and sometimes off-odors can result. Many white wines need malic acid’s higher acidity to retain their crisp, lively character, and some are too delicate to withstand the potential off-odors that might be introduced. Many winemakers now encourage malolactic fermentation for some batches of their Chardonnay while inhibiting the process in others, thereby giving the final blend improved complexity while retaining fruitiness and higher acidity.