Sweet ⇄ Acid + Phenolics (Astringency and Bitterness).
Acids play an important role in the cellular and metabolic functions of the grape berry and in the color and texture of the fermented wine. The precursors of acid are formed in the leaves of the grape plant and are transported to the berries where they are synthesized to acids. Acid accumulation begins at the start of berry development and continues unabated until the beginning of the ripening process. Acid levels tend to vary acording to the controlling temperatures of the growing region; in warmer regions acid is used up during respiration, resulting in lower acidity levels in the fruit at harvest. Conversely, acid levels are higher and sugar levels lower in cooler-climate growing regions.
The primary acids found in grapes and fermented wine are tartaric, malic, and citric acids as well as the tartaric and malic derivatives potassium hydrogen tartrate (cream of tartar) and potassium hydrogen malate. Tartaric acid -- which occurs in nature in fruits such as grapes, bananas, and tamarinds -- represents between 50% and 66% of the acid content in a ripe berry and, as such, controls the acid content in the finished wine. The tartaric acid level falls off as the grape ripens but not as much as in the case of malic acid. Crystallized tartaric acid precipitates out of the wine during fermentation and can form crystals on the underside of the wine bottle cork if the wine is stored below 50ºF. Tartaric acid is resistant to attack by wine microbes (and thus lends ageability with lower spoilage risk to the finished product) and is the winemaker's material of choice if/when a decision is made to add acid to a wine.
Malic acid is the second most important contributor to grape acid levels with amounts ranging between 23% and 40% of the total acid content. The grape utilizes malic acid during respiration at a rate higher than for tartaric acid, leading to a higher ratio of tartaric-to-malic acid at harvest than at the earlier stages of fruiting. Unlike tartaric acid, malic acid can be metabolized by a number of organisms and winemakers take advantage of this fact to to reduce wine acidity through malolactic fermentation, a process wherein the bacteria convert the hard malic acid to the softer lactic acid and carbon dioxide. Malolactic fermentation increases the wines aging potential as the bacteria that metabolize the malic acid also scavenge remaining nutrients and, in so doing, reduce the potential for future microbial spoilage. Malolactic fermentation occurs post-alcoholic-fermentation and is automatic for most red wines and selected whites.
Acetic acid is produced during fermentation by the conversion of ethanol to acetic acid by a species of Acetobacter or from the actions on glucose by selected anaerobic bacteria. The acid is present in most wines at levels of approximately 0.5 g/L and is detectable by humans as a pungent odor at levels of 1.0 g/L and above. The legal limit for acetic acid in wine is 1.2 g/L in California and 1.4 g/L elsewhere in the U.S. Acetic acid boils off when heated and as such is referred to as volatile acidity.
The winemaker needs to know the acid content of the grape and must in order to: decide when to harvest; determine pre-fermentation must treatment; monitor wine stability; and comply with TTB requirements of 0.5% minimum acid levels. Total acidity is the sum of the hydrogen ions of both fixed and volatile acids that are present in the wine and, as such, is the most accurate representation of acid concentration. Total acidity is difficult to measure accurately, however, and so the more easily measurable titratable acidity (TA) is used as its proxy. Acids and bases neutralize each other to water so the acidity of a liquid can be approximated by determining the amount of an alkaline solution that is required to neutralize it to water. The acidity level revealed in this manner is called the substance's titratable acidity. Red table wines generally range between 0.6% and 0.7% TA as levels below 0.4% render the wine susceptible to infection and spoilage.
A second method for measuring the acidity of a wine is through observation of its pH (potential of hydrogen) level. The higher the number of hydrogen ions (H+) in a liquid, the more acidic it is while the higher the number of hydroxide ions (formed when an oxygen ion bonds to a hydrogen ion and represented as OH-) in the liquid, the higher its alkalinity. The pH scale (illustrated below) runs from 0-14 with acidic solutions falling below 7,
7 as a point of neutrality, and alkaline solutions falling between 7 and 14. A change of 1 unit on the scale represents a 10-fold change in pH.
The pH level of a wine affects the way it is perceived by the wine drinker as well as its reaction to micro-organisms. Low-pH wines are generally viewed as sour and render tannins more astringent but they also limit micro-organism growth. Higher pH provides a more favorable environment for micro-organism growth and reduces the functionality of sulfur application. White wine pH ranges between 3.0 and 3.3 while red wine pH falls between 3.3 and 3.5. Low pH values are often correlated with high TAs and vice versa.
To summarize, acid gives wine a tartness and freshness while countering the effect of sweetness and magnifying the astringency of tannins. If a wine has too much acid it will be puckery and sour; too little and it will be flat, flabby, and dull. As stated previously, wine balance is viewed as a key indicator of wine quality. If a wine has insufficient sugar in relation to its acids and phenols, it will present as harsh and acidic and will retard the evolution of flavors in the mouth. In such a case the winemaker may choose to de-acidify using potassium bicarbonate or calcium carbonate or dilute the wine with water or a low-acid wine. If the wine has too much sugar, it will be flabby and cloying and will not refresh the palate. In such a case the winemaker may choose to acidify the wine by adding tartaric, malic, or citric acid.
Whether naturally obtained or engineered, appropriate acidity is a key element of wine balance.
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