Definition - What does Malic acid mean?
Malic acid is one of two major naturally occurring acids in wine grapes (and in other fruit). This acid presents itself as a pleasant, refreshing juicy taste, similar to biting into an apple. Malic acid is usually left in a wine to make it refreshing and tart. However, when there is too much malic acid, it is converted to lactic acid through malolactic fermentation, so that the wine will be smooth, full-bodied and complex.
Malic acid is naturally occurring, which means it is present in the vine and the wine grapes. The level of Malic acid present in the berries is dependent on the grape variety and the climate. Some grape varieties, such as Barbera, Carignan and Sylvaner are higher in naturally occurring Malic acids; in general, most reds have this acid in abundance while most whites do not.
WineFrog explains Malic acid
Malic acid is used by the grape vine to transport energy. It builds up in the berries during growth and is at it’s highest (up to 20 g/L) right before veraison (when the grapes change color). During the ripening period that follows, malic acid is lost through the respiration process. In colder climates, this process is slow while in warmer climates, this is a quick-paced process; malic acid is conserved in the cooler climates and lost in the warmer climates. Chablis, France (cool climate) will produce high acid wines; Napa Valley, California (warm climate) will produce low acid wines. As a result, at harvest time the malic acid levels could be as low as 1 g/L, and if the grapes are left on the vine until all of the malic acid is used up, they become over-ripe.
Malic acid is a biologically fragile wine acid; many types of bacteria can metabolize it. During naturally occurring alcoholic fermentation, 15% of the Malic acid is lost. The winemaker then has to decide if there is too much malic acid to produce the quality of wine he desires. If so, he initiates malolactic fermentation. Once the levels are acceptable to the winemaker, the wine has to be strained and any bacteria removed to ensure that the Malic acid does not undergo any further conversions.
For warm climate wines, the winemaker may need to take the removal of bacteria one step further due to the extreme low levels of malic acid. Some need to add additional chemicals to ensure that all of the bacteria are dead.
In the glass, malic acid is responsible for the tart, green apple taste. It is mellow, smooth and gives a persistent sourness to the wine. In addition, this acid enhances flavors and works to combine all of the different characteristics. For this reason, malic acid is desired by the winemakers to provide a bit of background structure and support.