Acids give wines their characteristic crisp, slightly tart taste. Alcohol, sugars, minerals, and other components moderate the sourness of acids and give wines balance. Some acids are naturally present in the base ingredients of wines, while others are byproducts of fermentation.
Natural acids have the freshest, purest acid tastes. Among grapes they are tartaric, malic and citric. Oxalic acid, found for example in rhubarb, is another natural acid. Fermentation acids have milder, more complex tastes. The major fermentation acids are lactic, succinic and acetic.
Acidity greatly influences the taste of wine. Therefore, winemakers need to understand the roles of the various acids, their occurance in common bases for making wines, their tastes, their sufficiency, how to measure them, and the principles for adjusting acids when necessary. These are the purposes to which this section is presented
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.
Emile Peynaud, in his seminal work Knowing and Making Wine (originally Connaissance et Travail du Vin in French, 1981, translated into English by Alan Spencer, 1984, John Wiley & Sons), offers the following experiment which is best suited to be administered by a winemaking club, society or guild. Prepare the following six buffered solutions with the same pH:
Taste each of the solutions in turn, clearing the palate between each tasting. The intensity of each will far exceed any taste perceived in an actual wine sample, but knowing the tastes will help the winemaker in future decisions regarding acidity.
Future considerations might well be shaped by realizing that tartaric produces the hardest taste, malic the most pronounced, citric the freshest, lactic the weakest, acetic the bitterest and most odorous, and succinic the saltiest. Decisions based on this knowledge might include:
Assuming you can measure the acid content of your finished but not yet bottled wine, how do you know if the total acids present are enough? This is a very subjective thing because some people like their wines acidic while others don't. With the right balance of acids, tannin, alcohol, and sweetness, even very acidic wines will taste less acidic than they actually are. There are, however, general guidelines that have long been accepted as "ballpark" territory for proper acidity of wines. These numbers reflect the total, titratable acids, or TA, as an expression of weight (grams per liter) or as a percentage of weight (divide grams by 10, so that 6.5 g/L equals 0.65% or 0.65 g/100mL). Remember, however, that guidelines are just that--guidelines--and are not rules and are not relevant to all wines.
Dry White Grape Wines..........0.65-0.75%
Sweet White Grape Wines.......0.70-0.85%
Dry Red Grape Wines.............0.60-0.70%
Sweet Red Grape Wines..........0.65-0.80%
Sherry Grape Wines.................0.50-0.60%
Non-Grape White Wines..........0.55-0.65%
Non-Grape Red Wines.............0.50-0.60%
Well balanced wines exceeding these guidelines may still be quite palatable, while wines possessing less than the recommended windows of acidity may, due to imbalance, seem too acidic to enjoy. There are few absolutes in wine.
Several inexpensive acidity test kits are available from your favorite homebrew/winemaking supply store for under $10 (mine cost under $7.00) that measure acidity by titration. Titration is the process of determining the concentration of a substance (in this case acids) in solution (in wine) by adding to it a standard reagent of known concentration in carefully measured amounts until a reaction of definite and known proportion is completed, as shown by a color change, and then calculating the unknown concentration.
The test is simple, effective and visually confirmative. Best of all, the math involved is grade-school simple.
There are many ways to increase acidity in musts. If one follows any of the recipes on this website, the recipe will attempt to take acid deficiencies into account by having you add a quantity of a specific acid, a blend of tartaric, malic and citric acids (called acid blend), or the juice of some citrus fruit (usually lemon or orange). A word to the wise here. Recipes are developed using base ingredients that may differ in natural sugar content or acidity from the same base ingredients you might encounter. In other words, a recipe that calls for a teaspoon of acid blend tells you that amount corrected the acidity in the base ingredients the recipe developer had at hand, but one teaspoon of acid blend may or may not be sufficient for the base ingredient you have at hand. The reasons for this are various, but basically they boil down to differences in varieties, differences in soils and the amount of water available to the host plants, differences in average day and night temperatures, differences in sunlight and intensity, and even differences in the maturity of the base ingredient when it was harvested. Therefore, the smart winemaker will measure the acidity of the must he or she is working with and correct the acidity according to what is required rather than what a recipe calls for.
Acid blend is commonly used to increase the acidity of a must. There are many formulations of these blends. Commercially, acid blends usually contain tartaric, malic and citric acids in a ratio of 40-40-20, 40-30-30, 50-30-20, or 50-25-25. You should ask your supplier what the specific ratio of the blend he sells is if you want precision in your acidity. Most blends, however, are 40-40-20, and adding 3.9 grams of this ratio blend will increase the acidity in a gallon of must approximately 0.1%. This same increase can be achieved by adding to a gallon of must 3.78 grams of tartaric acid (you can use 3.8 g/gal for the sake of simplicity). Fumaric acid in food grade powder form is sometimes used as a partial substitute for additions of tartaric acid because it tends to inhibit malo-lactic fermentation when used in the range of 1.5 to 5.7 grams per gallon (achieving an increase of 0.05 to 0.15% acidity). When used in large doses, however, it may affect flavor so testing its use in a sample of wine is wise before using it.
Citric acid added to a must before fermentation will largely be lost during fermentation. Thus, it is best to add it after all signs of fermentation have disappeared. Malic acid can be added anytime, but it too has a potential disadvantage. Malic acid buffers to a fairly high pH, so it should not be used if the intent is to increase acidity and / or lower the pH. In the latter case, tartaric acid is the additive of choice.
One can easily lower acidity in a finished wine through additions of calcium carbonate, acidex, or potassium bicarbonate, or through cold stabilization.
Calcium carbonate reacts preferentially with tartaric rather than malic acid, so one should not try to reduce acidity more thab 0.3 to 0.4% through its use. A dose of 2.5 grams per gallon of wine lowers TA about 0.1%. After its use, the wine should be bulk aged at least 6 months to allow calcium malate, a byproduct of calcium carbonate use, to precipitate from the wine. The wine should then be cold stabilized to ensure tartrate crystals do not precipitate out after bottling.
Potassium bicarbonate is used to deacidify a wine with a low pH (below 3.5), but should not be used to reduce acidity more than 0.3%. A dose of 3.4 grams per gallon of wine lowers acidity by about 0.1%. After use, the wine should be cold stabilized, as up to 30% of the potential acid reduction occurs during cold stabilization. It will cause a greater rise in pH than calcium carbonate for an equivalent reduction in acidity.
Finally, potassium bitartrate (a.k.a. Cream of Tartar) is used as a catalyst to help promote cold stabilization. It promotes the formation of tartrate crystals and is used at the rate of 2 to 5 grams per gallon, followed by vigorous stirring. Its use results in better and quicker stabilization, and these benefits will occur at slightly higher temperatures than without it.
The two measure of acidty are titratable acidity (TA) and the potential of hydrogen (pH). Titratable acidity relates to the amount of acid in solution as a percentage or as grams per liter; grams per liter (g/L) is obtained by multiplying percentage TA by 10, so that a TA of 0.80% becomes 8 g/L. In contrast, pH is related to an acid's strength in solution and is measured on a logarithmic scale; on the pH scale, 7 is neutral, numbers above 7 are alkaline and increase ascending, and numbers below 7 are acidic and increase descending. In other words, a pH of 9 is more alkaline than a pH of 8, while a pH of 4 is more acidic than a pH of 5. Because the measurements are logarithmic, a pH of 4 is 10 times more acidic that a pH of 5.
Although TA and pH are interrelated, they are not the same thing. A solution containing a specific quantity of a relatively weaker acid such as malic will have a different (higher) pH than a solution containing the same quantity of a stronger acid such as tartaric.
The pH of a solution is defined as the negative logarithm of the hydrogen ion concentration in gram-atoms per liter. Hydrogen ions (H+) are formed when a dissolved acid partially separates (dissociates) into hydrogen ions and and related anions (A-). The concentration of hydrogen ions largely determines the effects acids have on wine. A stronger acid such as tartaric dissociates more than a weaker acid such as lactic. Thus, the effective acidity of a solution depends on the concentration of all acids in the solution as well as their tendency to dissociate hydrogen ions. Effective acidity is measured as pH.
The measurable range of interest in acidity is a pH of approximately 2.5 to 4.5 for must and wine and a TA of 0.50-0.85%. We have already seen how to measure TA. Now we will look at measuring pH.
FOLLOW-ON SUBJECT IS UNDER CONSTRUCTION