While the word “acid” evokes images of car batteries rather than a refreshing beverage, acids play an important role in both the making and tasting of wine. The two major acids, tartaric and malic, are both naturally present in the grapes as they first develop on the vine.
Tartaric acid, Unlike malic acid, is not so common in the plant world and, outside the tropics, is almost exclusive to the grape vine. One of the ways that archeologists have been able to identify ancient wine-producing sites is through traces of tartaric acid. Because tartaric acid is unique to the grape vine, residue of tartaric crystals in amphora and other containers provides an indication of a winemaking culture. Depending on the varietal of the grape and the temperature, tartaric acids can sometimes crystalize in the wine. When this happens, the crystals sink to the bottom of the bottles, where they resemble broken glass or “wine diamonds.” Although they are perfectly harmless and tasteless, consumers object to these crystals, and so most winemakers try to remove excess tartaric acid before bottling.
Malic acid is found in just about every fruit and berry plant and is most commonly associated with apples, which is where it derives its name, malum, the Latin word for apple. It is that sharp astringent taste of green apples which is most recognizable in an acidic wine. The malic acids play an important role in the growth of the vine, providing energy during photosynthesis and, at veraison, metabolizing into sugar. It is the malic acid which provides the sharpness to the flavors and which balances the sweetness of any residual sugars, as well as complementing the bitterness of the tannins in red wine. Too much malic acid in a wine will make it tart and unpleasantly astringent, and too little will make the wine taste flabby and dead. The correct amount of malic acid is what provides the balance and harmony of a great wine.
Lactic acid. Unlike the other acids, is not found in the grapes, but is a product of a secondary fermentation. Most white wines and some red wines are not subject to the secondary fermentation, and therefore do not contain any lactic acid. Secondary, or Malolactic Fermentation, is the process by which certain bacteria convert the tart malic acids into the softer lactic acids. Lactic acid was first derived from sour milk in the eighteenth century, and the name comes from the Latin word for milk, lact. Most red wines and most Chardonnays undergo malolactic fermentation, partly to soften the tartness of the malic acids but also for the improved “mouth-feel” that results. Unlike, say, a Sauvignon Blanc, which retains the green-apple-sharpness of the malic acids, a Chardonnay following malolactic fermentation will have the softer, more “buttery” feel and flavor of the lactic acid.
Acetic acid is a product of the primary fermentation when the yeasts are converting the sugars into alcohol and carbon dioxide. If the wine is further exposed to oxygen, the alcohol will be converted by bacteria into ascetic acid and eventually into vinegar. Not a good acid.
In addition to balancing the taste of the sugars and tannins in wine to create the complex harmony that wine drinkers so enjoy, acids also play an important role in protecting and stabilizing the wine on its journey from the initial fermentation barrels to its happy arrival in our glass.