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Copied verbatum, Acid section, Chapter 5, from "The Home Winemakers Manual".
Practically all of the acids in sound wine come directly from the grapes. However, very small quantities of several organic acids are produced during primary fermentation, and under adverse conditions, bacteria in wine can produce enough acetic acid to spoil good wine in a short time. In the United States, titratable acid in wine is expressed in grams of acid per 100 milliliters of wine, and titratable acid is calculated as if all of the different acids in the wine were tartaric acid.
The acid content of most finished table wine ranges from 0.55 to 0.85 percent. The desirable acid content depends on style and how much residual sugar is left in the wine. Ideally, the acid content of grapes should fall in the range from 0.65 to 0.85 grams per 100 milliliters (percent). However, grapes grown in cool climates often contain too much acid, and fruit grown in warm climates generally contains to little acid. One of the more important winemaking tasks consists of adjusting the starting acid content of the grapes before fermentation. The goal is to have just enough acid to produce a balanced wine.
Practically all of the acids found in sound wines are fixed acids. Most of the fixed acids originate in the grape juice, and these acids remain during fermentation and appear in the finished wine. Fixed acids are nonvolatile and nearly odorless. However, bacteria can produce acetic acid in wine, and acetic acid is different from other wine acids. Acetic acid is considered a volatile acid because it evaporates easily. Acetic acid has a distinctive odor, and it gives wine an unpleasant, hot aftertaste.
Acids Produce Hydrogen Ions
In water, some acid molecules ionize, and some acid molecules remain unchanged. Each ionized acid molecule splits into two separate pieces. One piece is a hydrogen atom (minus the electron), and the other piece is the remainder of the acid molecule. Both pieces have an electric charge, and both are called ions. A positive electric charge is carried by the hydrogen ion, and a negative charge is carried by the acid ion. The remainder of the acid molecules (the unionized molecules) remains unchanged in the water solution. Both tartaric and malic acids have two hydrogens that can ionize, and these two hydrogens (H) are shown in Figure 2.
Acids produce hydrogen ions in water solutions. However, the number of hydrogen ions produced can be large or small. The number of hydrogen ions depends on how much acid is present in the solution, and the number also depends on the strength of the acid.
In water, some acid molecules spontaneously split into positive and negative ions. However, many acid molecules remain unchanged. The fraction of acid molecules that ionize depends upon the strength of the acid. When practically all of the acid molecules ionize, the acid is called a “strong” acid. When only a few acid molecules ionize, the acid is called a “weak” acid. In other words, strong acids ionize completely, and weak acids only partially ionize.
Only a few acids are classified as strong. All of the organic acids found in wine are weak acids. However, some weak acids are stronger than others. Tartaric acid is a weak acid, and about one out of every 900 tartaric acid molecules ionizes in water. The other 899 molecules remain unchanged. Malic acid is weaker than tartaric acid. Only one out of every 2500 malic acid molecules ionizes in water. The other 2499 malic acid molecules remain unchanged. Tartaric acid is about 2.7 times stronger than malic acid because tartaric acid produces 2.7 times more hydrogen ions than an equal quantity of malic acid. Smaller quantities of a stronger acid can produce as many hydrogen ions as larger quantities of a weaker acid. Tartaric acid is considered the principal wine acid. It is the strongest of the wine acids, and generally more tartaric acid is present in wine.
Wine can be thought of as a simple, water-alcohol solution, and acids in wine behave much the same as they do in any other water solution. The number of hydrogen ions in a wine depends upon the quantity of acid, the strength of the acids and the quantities of potassium, sodium and calcium present in the wine.
Kinds of Acids
The tart taste of dry table wine is produced by the total quantity and the kinds of acids present. Tartaric and malic are the major wine acids. These two acids are present when the grapes are picked, and they are carried over through the fermentation process into the finished wine. Wine also contains small quantities of lactic, citric, succinic, acetic and several other organic acids as shown in Table 3. Some of these acids do not exist in the grapes. They are produced in small quantities by microorganisms throughout the winemaking process.
Malic acid and citric acid can be metabolized easily by microorganisms in the wine. Tartaric acid and succinic acid are more stable biologically, and they are seldom bothered by wine microbes. Even so, under certain conditions, tartaric acid can be attacked by microorganisms, and when this occurs, the wine is usually a catastrophic loss (see Chapter 13).
Few fruits other than grapes contain significant amounts of tartaric acid. One half to two thirds of the acid content of ripe grapes is tartaric acid, and it is the strongest of the grape acids. Tartaric acid is responsible for much of the tart taste of wine, and it contributes to both the biological stability and the longevity of wine.
The amount of tartaric acid in grapes remains practically constant throughout the ripening period. However, the situation in wine is different. The quantity of tartaric acid slowly decreases in wine by small amounts. Both potassium and calcium combine readily with tartaric acid and form potassium bitartrate and calcium tartrate compounds. Then crystals of these two materials precipitate out of the wine during fermentation. These tartrate materials can continue to precipitate for a long time, and aged wine usually contains about two thirds as much tartaric acid as the starting grapes because of tartrate precipitation. Unfortunately, these acid salts of potassium and calcium precipitate very slowly at normal cellar temperatures, and wine can contain excessive quantities of these materials even after many months of aging. Wineries use special wine treatments to speed up tartrate precipitation. Cooling the wine is the most commonly used procedure. Just cooling the wine to about 27 degrees causes excess potassium salts to precipitate out in a few days.
Tartaric acid is resistant to decomposition, and it is seldom attacked by wine microbes. This is why winemakers add tartaric acid to grapes deficient in acidity rather than using a less stable acid such as malic or citric. Most winemakers prefer the titratable acid to be about 0.7 percent for white grapes, and about 0.8 percent is preferred for white juice. When the titratable acid content falls below these levels, winemakers often add tartaric acid to the grapes or juice before they start fermentation.
Malic acid is prevalent in many types of fruit. This acid is responsible for the tart taste of green apples. Malic acid is one of the biologically fragile wine acids, and it is easily metabolized by several different types of wine bacteria. Unlike tartaric acid, the malic acid content of grapes decreases throughout the ripening process, and grapes are grown in hot climates contain little malic acid by harvest time.
Grapes grown in cool regions often contain too much acid. High acidity results in excessively tart wines, so the winemaker has a problem. During alcoholic fermentation, some malic acid is metabolized, and the malic acid content of the wine decreases about 15 percent. Malolactic fermentation (ML) can further reduce wine acidity. When wine goes through malolactic fermentation, bacteria convert the malic acid into lactic acid. Lactic acid is milder than malic acid, and ML fermentation is a standard procedure used to reduce the acidity of wines made from grapes grown in cool regions.
When grapes are grown in warm areas like southern California, the winemaking situation is much different. In warm regions, the grapes are usually deficient in acid, and removing malic acid by means of ML fermentation may not be a good idea. Now the problem becomes more complicated for the winemaker. Malic acid is not biologically stable, and when malic acid is deliberately retained to improve the acid balance of the wine, special steps may be needed to prevent ML fermentation from occurring after the wine is bottled. The winemaker can use a sterile filter and remove all of the bacteria from the wine before bottling, or he can add small quantities of fumaric acid to the wine. Small additions of fumaric acid can inhibit ML fermentation and make the wine stabile.
Only small amounts of citric acid are present in grapes. Only about 5 percent of the total acid is citric in sound grapes. Like malic acid, citric acid is easily converted into other materials by wine microorganisms. For example, citric acid can be fermented into lactic acid, and some types of lactic bacteria can ferment citric acid into acetic acid. Excessive amounts of acetic acid are never desirable in wine, so the citric acid into acetic acid fermentation can be a serious problem. This potential difficulty is why citric acid is seldom used to acidify must or juice before fermentation. Most winemakers consider the risk of producing excessive quantities of acetic acid too great.
The acetic acid risk is much smaller after wine has been clarified and stabilized, and winemakers often increase the acid content of finished white wines by adding small amounts of citric acid. Citric acid imparts a citric character that enhances the taste of many white and blush wines. However, citric acid is seldom used in red wine. The distinctive citric taste may not be appropriate for many types of red wine. In addition, the risk of biological instability is much greater in red wines.
Home winemaking shops sell a material called “acid blend.” Acid blend contains tartaric, malic and citric acids, and the three acids are in roughly equal proportions. Acid blend is often used in making fruit wines or wines made from grape concentrates. However, most winemakers will not add acid blend to grapes before fermentation because the citric acid in the acid blend might be converted into acetic acid. In addition, the lemon-like taste acid blend often imparts is not be suitable for many kinds of grape wines.
Succinic acid is formed by yeast, and small quantities of this acid are always produced during the primary fermentation. The production of succinic acid stops when alcoholic fermentation is complete. The flavor of succinic acid is a complex mixture of sour, salty and bitter tastes, and succinic acid is responsible for the special taste characteristics all fermented beverages have in common. Once formed, succinic acid is very stable, and it is seldom affected by bacterial action.
Lactic acid is the principal acid found in milk. Grapes contain very little lactic acid. All wines contain some lactic acid, and some wines can contain significant quantities. Lactic acid in wine is formed in three different ways. (1) A small amount is formed from sugar by yeast during primary fermentation. (2) Large amounts of lactic acid are formed from malic acid by bacteria during ML fermentation. (3) Both lactic and acetic acid can be produced by lactic bacteria from the sugars, glycerol and even tartaric acid in the wine. “Lactic souring” is the term used to describe wine when sugar is converted into lactic acid by bacteria. This type of souring is a form of gross wine spoilage. Lactic souring was a common winemaking problem before the use of sulfur dioxide became widespread, but it is seldom a problem today.
Lactic acid can exist in either a right-hand or left-hand form. Lactic acid produced by yeast occurs in the left-hand form, and lactic acid produced by bacteria occurs in the right-hand form. The right-hand form of lactic acid can be distinguished from the left-hand form in the laboratory very easily, so winemakers have a sensitive way of monitoring bacterial activity in wine simply by measuring the two forms of lactic acid.
All of the acids discussed above are fixed acids. Fixed acids have low vapor pressures, and they do not evaporate easily. When wine is boiled, the fixed acids do not boil away. All of the fixed acids remain in the wine container. Fixed acids do not have significant odors.
Acetic acid is different from fixed acids. Acetic acid has a high vapor pressure, and it is a volatile acid. Acetic acid evaporates very easily and has a distinctive odor. When wine containing acetic acid is boiled, the acetic acid quickly boils away. The acetic acid disappears into the air much the same as water and alcohol.
Sound grapes contain very little acetic acid. Just like lactic acid, acetic acid in wine is formed in several different ways. (1) Small amounts of acetic acid are formed by the yeast during alcoholic fermentation. (2) Some acetic acid is always formed during ML fermentation, and most of the acetic acid is formed by bacteria fermenting citric acid in the wine. (3) In stuck fermentations, lactic bacteria often convert residual sugar into acetic acid. (4) Vinegar bacteria (acetobacter) convert ethyl alcohol in the wine into acetic acid, and in the presence of air, acetobacter can produce large quantities of acetic acid.
The conversion of ethyl alcohol into acetic acid by vinegar bacteria is different from the other fermentation mechanisms discussed here. Vinegar formation is an oxidation process, and large quantities of acetic acid cannot be produced unless the bacteria have access to large quantities of air. Wine is not converted into vinegar when air is excluded, and this is why novice winemakers are cautioned to keep their wine containers completely filled and tightly sealed.
Acids in juice or wine occur in two forms. Some acid exists in a free form, and some acid combines with minerals to form acid salts. The acid salts of potassium, sodium and calcium are always prevalent in wine, and these acid salts are not stable. Potassium and calcium tartrates can precipitate out of the wine after a long time. In particular, potassium bitartrate can precipitate after the wine is bottled unless the winemaker specifically removes this material. When the tartrate precipitates out of the wine, crystals are formed in the bottle. The potassium bitartrate crystals are harmless (cream of tarter), but the deposits can cause unsightly hazes in the wine. Sometimes, large crystals are formed in the bottle, and the tartrate crystals are mistaken for “glass” particles by the consumer. Producing wines with such gross visual flaws is not good for business, and commercial wineries avoid these difficult public relation problems by “cold stabilizing” all their white and blush wines. The cold stabilization process removes the excess potassium bitartrate material.
Grape sugars consist mostly of two monosaccharides, glucose and fructose, and these two simple sugars occur in about equal proportions. Simple sugar molecules can combine and form larger sugar molecules called disaccharides and polysaccharides. Both glucose and fructose can be readily fermented, but most disaccharides and polysaccharides must be split into their smaller, simple sugar components before they can be readily converted into alcohol. Many large sugar molecules can be hydrolyzed and broken into smaller molecules by enzymes, acids or heat.
When sucrose (table sugar) is added to wine, it often produces strange flavors because many weeks may be required before the wine acids can hydrolyze all of the sucrose into glucose and fructose. Even in a warm cellar, the strange flavors can persist for several weeks. However, when all of the sucrose has been hydrolyzed into glucose and fructose, the strange flavor completely disappears, and the wine has a normal taste.
Organic acids produce the tart taste in table wines. Winemakers working with grapes grown in cold climates often encourage malolactic fermentation to reduce the acid content of their wines. Winemakers working with grapes grown in warm climates often add tartaric acid to the juice to increase the acid content of the finished wine. In either case, the winemaker is striving for just the right amount of acid to achieve a balanced wine.
Sometimes winemakers prefer to retain as much malic acid as possible in the wine, so they deliberately discourage ML fermentation. However, red wine is not biologically stable when malic acid is retained, and then the winemaker must take special precautions. Professional winemakers put wine containing malic acid through a sterile filter and remove the bacteria when the wine is bottled. Home winemakers prevent ML fermentation in the bottle by adding small amounts of fumaric acid.
Potassium bitartrate can precipitate out of wine very slowly, and unsightly bottle deposits are often formed when tartrates precipitate after the wine is bottled. Consequently, winemakers always use a cold stabilization procedure to remove excess tartrate materials from white and blush wines before these wines are bottled.
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