malolactic

A secondary fermentation undergone by most red wines and some whites by the action of bacteria similar to those responsible for transforming milk into buttermilk and yogurt. These bacteria consume the natural grape’s malic acid and convert it into the less acidic lactic acid, along with byproducts such as diacetyl, which is the artificial butter flavor used on movie popcorn. Malolactic diminishes wine’s freshness and fruitiness, lowers acid taste, and raises pH. These effects are minimized if malolactic completes during primary fermentation, and contact with yeast will absorb diacetyl. If by contrast malolactic fermentation is delayed until wine is barreled down, its action can soften oak tannins.