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Acidity in Wines 2--Acids and their Roles

        The major grape acids are tartaric and malic, with citric being among the minor ones. In other winemaking bases, malic or citric acids are usually dominant. When natural organic acids are absent or deficient in a base, a blend of tartaric, malic and citric acids is usually added, although certain bases make better tasting wine if only one acid--usually citric--is added. It is important to understand the roles of the major and minor acids in wine, if only to appreciate why their control is so desirable beyond the obvious considerations of taste.
        Generally speaking, some acid is desired--but not actually required--by the yeast. While it is true that acid is not required for yeast to reproduce and convert sugar into alcohol and CO2, it does seem to be desired. That said, it should be noted that acids play other, perhaps more important, roles in winemaking. Certainly they contribute to taste--not only the tartness possessed by most wines to varying degrees but also to complex flavors developed during aging. But their greatest role, it seems to me, comes from their ability to stop, or at least retard, the growth of many potentially harmful microorganisms that would spoil the wine itself.
        Tartaric acid is found in almost no fruit but the grape and here it is the predominate acid. It is important because it is the strongest and most voluminous acid present in grape wines and, with its potassium and calcium salts, largely controls the effective acidity (pH) of such wines. This, in turn, contributes to a wine's color, aseptic stability (resistance to bacterial infections) and taste. Tartaric deficiency can thereby contribute to many wine problems.
        Many non-grape wines are made with raisins or grape juice as a minor ingredient to add vinous qualities to the wine, the most notable being body and mouth-feel. Tartaric acid thereby becomes an important component of those wines, even if the major base ingredient contains little or no tartaric acid itself. The hardness in taste that tartaric is noted for can conflict with softer acids found in the non-grape bases.
        Malic acid is one of the most widespread acids among the many fruits and vegetables from which wines are made. In warmer climates less malic acid remains in the ripened fruit than in cooler climates, but in both climates malic decreases as the fruit ripens. An excess of malic tends to taste sharply "greenish," so reducing malic is often a major consideration in "toning down" or "smoothing out" and overly acidic must. One way to do this is to subject it to fermentation, as 20-30% of the harvested malic is respired during fermentation. If the fermented liquor (the wine) still contains too much malic, a malolactic fermentation can be encouraged whereby malic acid is converted to a weaker lactic acid. Some lactic acid is produced during regular fermentation, but malolactic fermentation (MLF) can reduce malic and increase lactic by a factor of five.
        Lactic acid, while certainly milder in taste than malic, can be a two-edged sword. On the one hand its mild sour taste counteracts the harsher tartness of malic, but on the other hand it can invite infection by certain lactic bacteria that produce odors suggestive of spoiled milk or sauerkraut. For this reason some winemakers dissuade MLF with the same vigor that others invite it. Another reason for dissuading MLF is that certain types of wine require it to attain their noted tastes.
        Citric acid, minor in grapes but major in many other fruits, is often added to wines to increase acidity, complement a specific flavor or prevent ferric hazes. In the grape, citric acid all but disappears during fermentation in much the same way that malic is reduced. It is reduced through normal fermentation and again during MLF. If added to an almost finished wine to increase acidity, citric acid gives the wine a freshness of flavor that seems (and is) artificial.
        Acetic acid is both volatile and odorous, detectable as the smell of vinegar. It is a natural component of most wines in very small quantities, but is formed quickly by certain bacteria exposed to air. Its sole role in wine is to spoil it.
        Succinic acid is a product of yeast fermentation and found in trace amounts in all wines. Its taste is a mixture of acid, salt and bitterness, and, while this description is rather indistinct, it is present in all wines and beers and contributes to total acidity. It is also considered superior to all other acids in its ability to produce rich, flavorful esters during the aging process.
        Citramilic, dimethylglyceric, galacturonic, glucuronic, gluconic, ketoglutaric, mucic, oxalic, and pyruvic acids are also found in grape and many other wines in trace amounts and contribute to total acidity.