Handbook of Enology, Volume 2. Pascal Ribéreau-Gayon
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1.3 Different Types of Acidity
The fact that enologists need to distinguish between total acidity, pH, and volatile acidity demonstrates the importance of the concept of acidity in wine. This is due to the different organoleptic effects of these three types of acidity. Indeed, in any professional tasting, the total acidity, pH, and volatile acidity of the wine samples are always specified, together with the alcohol and residual sugar contents.
The importance of total acidity is obvious in connection with flavor balance:
Looking at this balance, it is understandable that dry white wines have a higher total acidity than red wines, in which phenols combine with acids to balance the sweet taste of the alcohols. Volatile acidity indicates possible microbial spoilage.
1.3.1 Total Acidity
Total acidity in must or wine, also known as “titratable acidity,” is determined by neutralization using a sodium hydroxide solution of known normality. The end point of the titration is still often determined by means of a colored reagent, such as bromothymol blue, which changes color at pH 7, or phenolphthalein, which changes color at pH 9. Using one colored reagent to define the end point of the titration rather than the other is a matter of choice. It is also perfectly conventional to use a pH meter and stop the total acidity assay of a wine at pH 7, and, indeed, this is mandatory in official analyses. At this pH, the conversion into salts of the second acid function of the diacids (malic and succinic) is not completed, while the neutralization of the phenol functions starts at pH 9.
The total acidity of must or wine takes into account all types of acids, i.e. inorganic acids such as phosphoric acid, organic acids including the main types described above, and amino acids whose contribution to titratable acidity is not very well known. The contribution of each type of acid to total acidity is determined by its strength, which defines its state of dissociation, as well as the degree to which it has combined to form salts. Among the organic acids, tartaric acid is mainly present in must and wine as a monopotassium acid salt, which still contributes toward total acidity. It should, however, be noted that must (an aqueous medium) and wine (a dilute alcohol medium), with the same acid composition and thus the same total acidity, do not have the same titration curve and, consequently, their acid–base buffer capacity is different.
Even using the latest techniques, it is difficult to predict the total acidity of a wine on the basis of the acidity of the must from which it is made, for a number of reasons.
Part of the original fruit acids may be consumed by yeasts and, especially, bacteria (see malolactic fermentation). On the other hand, yeasts and bacteria produce acids, e.g. succinic and lactic acids. Furthermore, acid salts become less soluble as a result of the increase in alcohol content. This is the case, in particular, of the monopotassium form of tartaric acid, which causes a decrease in total acidity on crystallization, as potassium bitartrate (potassium hydrogen tartrate [KHT]) still has a carboxylic acid function.
In calculating total acidity, a correction should be made to allow for the acidity contributed by sulfur dioxide and carbon dioxide. Sulfuric acid is much stronger
In fact, high concentrations of carbon dioxide tend to lead to overestimation of total acidity, especially in slightly effervescent (spritzy) wines and even more so in sparkling wines. This is also true of young wines, which always have a high CO2 content just after fermentation.
Wines must therefore be degassed prior to analyses of both total and volatile acidity.
1.3.2 Volatile Acidity
Volatile acidity in wine is a highly important physicochemical parameter to be monitored by analysis throughout the winemaking process. Although it is an integral part of total acidity, volatile acidity is clearly considered separately, even if it only represents a small fraction in quantitative terms.
This value has always been, quite justifiably, linked to wine quality. Indeed, when an enologist tastes a wine and decides there is excessive volatile acidity, this unfavorable assessment has a negative effect on the wine's value. This organoleptic characteristic is related to an abnormally high concentration of acetic acid, in particular, as well as a few homologous carboxylic acids. These compounds are distilled when wine is evaporated. Those which, on the contrary, remain in the residue constitute fixed acidity.
Volatile acidity in wine consists of free and salt forms of volatile acids. This explains why the official assay method for volatile acidity, by steam distillation, requires the salt fractions to be rendered free and volatile by acidifying the wine with tartaric acid (approximately 0.5 g per 20 ml). Tartaric acid is stronger than the volatile acids, so it displaces them from their salts.
In France, both total and volatile acidity are usually expressed in grams per liter of sulfuric acid. An appellation d'origine contrôlée (controlled designation of origin) wine is said to be “of commercial quality” if volatile acidity does not exceed 0.9 g/l of H2SO4, i.e. 1.35 g/l expressed as tartaric acid and 1.1 g/l as acetic acid. Acetic acid, the principal component of volatile acidity, is mainly formed during fermentation.
Alcoholic fermentation of grapes normally leads to the formation of 0.2–0.3 g/l (as H2SO4) of volatile acidity in the corresponding wine. The presence of oxygen always promotes the formation of acetic acid. Thus, this acid is formed both at the beginning of alcoholic fermentation and toward the end when the process slows down. In the same way, an increase in volatile acidity of 0.1–0.2 g/l (as H2SO4) is observed during malolactic fermentation. Work by Chauvet and Brechot (1982) established that acetic acid is formed during malolactic fermentation due to the breakdown of citric acid by lactic acid bacteria.
Abnormally high volatile acidity levels, however, are due to the breakdown of residual sugars, tartaric acid, and glycerol by anaerobic lactic acid bacteria. Aerobic acetic acid bacteria also produce acetic acid by oxidizing ethanol.
Finally, acescence in wine is linked to the presence of ethyl acetate, i.e. the ethyl ester of acetic acid, formed by the metabolism of aerobic acetic acid bacteria (Section 2.5.1).
1.3.3 Fixed Acidity
The fixed acidity of a wine is obtained by subtracting volatile acidity from total acidity. Total acidity represents all of the free acid functions, and volatile acidity includes the volatile acid functions, both free and in salt form. Strictly speaking, therefore, fixed acidity represents the free fixed acid functions plus the volatile acid functions in salt form (mainly acetic acid in salt form).
When fixed acidity is analyzed, there is a legal obligation to correct for sulfur dioxide and carbon dioxide. In practice, however, these two compounds have a similar effect on total acidity and volatile acidity, so the difference between total acidity and volatile acidity is approximately the same,