Another practice that is widely used is called malolactic fermentation. This second sort of fermentation is a softening conversion of acid in which the grapes’ harsher malic (think apples) acid is converted into softer, lactic (milky) acids. This is done by warming the liquid and adding lactic bacteria to the must after the main alcoholic fermentation.
Most red wines undergo malolactic fermentation, as well as many full bodied whites. Crisper, aromatic styles of white wines, however, do not. The effect of this process is a smoother, softer wine. The buttery quality found in many white wines can be attributed to malolactic fermentation.