Acids in Wine - Malic Acid

Malic Acid

Malic acid, along with tartaric acid, is one of the principal organic acids found in wine grapes. It is found in nearly every fruit and berry plant, but is most often associated with green apples, the flavor it most readily projects in wine. Its name comes from the Latin malum meaning “apple”. In the grape vine, malic acid is involved in several processes which are essential for the health and sustainability of the vine. Its chemical structure allows it to participate in enzymatic reactions that transport energy throughout the vine. Its concentration varies depending on the grape variety, with some varieties, such as Barbera, Carignan and Sylvaner, being naturally deposed to high levels. The levels of malic acid in grape berries are at their peak just before veraison, when they can be found in concentrations as high as 20 g/l. As the vine progresses through the ripening stage, malic acid is metabolized in the process of respiration, and by harvest, its concentration could be as low as 1 to 9 g/l. The respiratory loss of malic acid is more pronounced in warmer climates. When all the malic acid is used up in the grape, it is considered “over-ripe” or senescent. Winemakers must compensate for this loss by adding extraneous acid at the winery in a process known as acidification.

Malic acid can be further reduced during the winemaking process through malolactic fermentation or MLF. In this process, bacteria convert the stronger malic acid into the softer lactic acid; formally, malic acid is polyprotic (contributes multiple protons, here two), while lactic acid is monoprotic (contributes one proton), and thus has only half the effect on acidity (pH); also, the first acidity constant (pK_a) of malic acid (3.4 at room temperature) is lower than the (single) acidity constant of lactic acid (3.86 at room temperature), indicating stronger acidity. Thus after MLF, wine has a higher pH (less acidic), and a different mouthfeel.

The bacteria behind this process can be found naturally in the winery, in cooperages, which make oak wine barrels that will house a population of the bacteria or they can be introduced by the winemaker with a cultured specimen. For some wines, the conversion of malic into lactic acid can be beneficial, especially if the wine has excessive levels of malic acid. For other wines, such as Chenin blanc and Riesling, it produces off flavors in the wine (such as the buttery smell of diacetyl) that would not be appealing for that variety. In general, red wines are more often put through MLF than whites, which means a higher likelihood of finding malic acid in white wines (though notable exceptions, such as oaked Chardonnay, are often put through MLF).