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 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. 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. 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 wine making 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. 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. Figure 2. When wine acids ionize, one or both of the hydrogens shown in bold type separate from the main acid structure.
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 tartaric acid molecules ionizes in water. The other molecules remain unchanged. Malic acid is weaker than tartaric acid. Only one out of every malic acid molecules ionizes in water. The other malic acid molecules remain unchanged. Tartaric acid is about 2. 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. 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 Some of these acids do not exist in the grapes.
They are produced in small quantities by microorganisms throughout the wine making 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 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. Tartaric Acid is a unique acid that is not commonly found in fruit, however it is a primary acid component in grapes.
It is one key acid that is monitored during the fermentation process as it plays a vital role in the stability of the wine and its pH levels. Tartaric acid combines with Potassium to create Potassium Titrate which then precipitate and causes the acidity to drop and the pH levels to rise. It is found in significant concentrations in grapes, but this can vary based on factors including the variety of grape and the vineyard soil content.
This led to different microbiological instabilities, which prompted different adjustments of winemaking practices. It included amongst others the application of cooling, acid additions and discerning sulphur dioxide management. The median pH of Australian wines was for example 3,45 in , but increased to 3,62 in During cool years the malic acid concentration can be twice as much.
Although a pH increase of 0,2 units may appear negligible, it can almost halve the acidity or quantity of hydrogen ions in a wine. It is important to be aware that higher pH grape juice is more prone to bacterial spoilage and the potential growth of Brettanomyces. Much higher K concentrations were observed in especially the warmer wine regions during the s.
As result of the present trend to harvest grapes earlier, less time is available for the accumulation of potassium in the berries. During the alcoholic fermentation, changes in the K and TA concentrations of the fermenting juice occur. Thereafter the concentration deceases as result of the precipitation of K-bitartate cream of tartar , which removes an H-ion from the solution.
This causes an increase in the pH of the fermenting juice. The tartaric acid concentration at the end of alcoholic fermentation is consequently nearly half the original tartaric acid concentration. A possibility exists that the formed alcohol may extract more K from the skins.
At that stage a pH decrease can also occur when malic acid is converted to the weaker lactic acid during malolactic fermentation MLF. Tartaric acid is frequently added to juice to increase its TA concentration. Tartaric acid is however a weak organic acid.
If it is added to a solution it dissociates in three different forms, namely tartaric acid H 2 T , bitartrate H 2 T — and tartrate T Different percentages of these three forms exist at different pHs. At the average of these two values, namely a pH of 3,65 there is a major shift between the percentages of these forms.
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