Glen Ferguson, Ph.D. and Ed Martin
Acidity is a defining characteristic of the wine. A detailed understanding of acidity is absent in many discussions. Acidity makes wine sapid and gives wines a freshness or brightness. If overdone, acidity can overwhelm other elements resulting in a tart and thin wine. A better understanding of wine's acidic taste can help winemakers understand the connections between the pH and TA measurements and the acid taste in wine.
Understanding the origin of acid taste requires understanding its molecular-scale origin. Acidity is a complex topic but simply put it is the propensity of a molecule containing hydrogen to dissociate into a proton (hydrogen nucleus) and a negatively charged molecule (anion) in water. When in water these molecules dissociate due to the energetic balance of the system. The proton and anion are more attracted to the water molecules than each other. Thus, the energy of the system is lower with charged protons and anions than undissociated molecules. A simple example is hydrochloric acid. The H–Cl molecule dissociates into H+ (proton) and Cl– (anion). Strictly speaking, this description is of Brønsted acidity. Other definitions, such as Lewis acidity, exist but they are not relevant to this discussion.
There are several methods of measuring acidity including pH and TA. The pH is a quantitative measurement of the number of protons in water. The “p” of pH is the –log10[C], where C is a number. In this case, the C is the concentration of protons in the solution referred to as “H,” giving pH. The reason for this transformation is simply to make the number easier to read. After conversion, the number is between 0 (acidic) to 14 (basic) with pure water at a pH of 7. Without the transformation, the number would be between 0 and 10,000,000,000,000. Wine typically has a pH between 3 and 4. Winemakers also measure acids as TA. This value is often termed total acidity but is usually titratable acidity. In a titration, the base removes protons from a solution. After removing the protons, one can calculate the total number of acid molecules in wine. Acids are either strong and weak. This determination is not a measure of acidic strength, though strong acids do have lower pH values for the same number of molecules. A strong acid, e.g., hydrochloric acid, completely dissociates into hydrogen ions and anions in water. Alternatively, weak acids, only partially dissociate. Wine acids are weak acids. Since not all acids are completely dissociated pH would only measure the protons and not the number of acidic molecules. However, TA is determined via titration in a wine lab. This technique does not quantify acid molecules bound to metal ions, such as potassium, and is better termed titratable acidity than total acidity. What does one taste when they taste the acid in wine? Is it possible to taste a proton? What about tasting the molecule that released the proton (the anion)? Can we taste the whole molecule? While it is true that acid modifies and interacts with other flavors in wine, let’s restrict the discussion to acid only. In Le Gout du Vin, by Emile Peynaud, several experiments are described that illustrate the taste of the principle wine acids: tartaric, lactic, citric, succinic, malic, and acetic. These experiments show that the TA is the most important measure of acidic taste and that the origin of acidic flavor is not the same for all wine acids. One of the most straightforward ways for acidic taste would be to taste the protons in an acidic solution. As we know, strong acids completely dissociate in water. If the acidic taste is that of protons, this solution should have an acidic taste. Adding hydrochloric acid to water results in a solution that has almost no flavor. This observation rules out protons as producing the taste of acid in wine, a surprising result.
Can we taste the anion or molecule without the proton? We know that bases can combine with protons and can be used to remove all free protons in a solution. If a weak acid is added to water and the protons are removed with a base, the solution contains the anions alone. Of the common wine acids, only succinic acid anions have a taste. No other acids had more than a feeble flavor. The succinic acid anion has an acidic flavor in wine, but other acids do not. This result is interesting because it shows that succinic acid taste would be determined directly by the pH in addition to the anion with the pH acting as a moderator of the flavor without direct influence.
Finally, we can explore the taste of the undissociated acid molecules in wine. This idea was tested by varying concentrations of weak acids in a solution but changing the pH. The acidic taste is stronger the higher the concentration of undissociated molecules. Thus, we can conclude for most acids it is the taste of the undissociated molecule that gives the acidic taste. This result again implicates pH as an acidic taste moderator. At lower pH, there will be less undissociated molecules with the implication there is a less acidic taste for most acids including tartaric! However, it is not clear that this would be perceptible over the pH range of wine. This above observation is complicated by the buffering capacity of wine. A buffer is a substance that resists a change in pH. While the taste of most wine acids is from the undissociated molecules, and the flavors associated with these acids are altered by the pH. This result indicates there is an effect of pH on the wines acidic taste, but this is not the primary cause of acidic taste. The caveat to this observation is that the buffering effect of wine could keep the pH stable as we drink it, i.e., protons could have a flavor in a buffered solution like wine. An unbuffered solution of strong acids would be very sensitive to dilution while a buffered solution would resist changes in pH. If a brave reader would like to spike a neutralized wine with hydrochloric acid and report the results, we would be interested to know the difference between the strong acid spiked wine and the control wine.
Another important measure is the intensity of the taste for each acid. The intensity of flavor of the acid anions or molecules is different. The order of flavor intensity of the main wine acids, added in equal parts to a solution, from most to least is malic, tartaric, citric, and lactic. This order gives some insight into why malolactic fermentation could have a marked effect on change in the acidic taste in wine.
The taste of acid is complex even under experimental conditions when ignoring other interactions in the wine. These experiments use solutions that are less complex than wine. While these simplifications are necessary to understand acidic taste, we cannot forget that wine is more complex and exciting than the experiments can capture. However, we can certainly learn something about acidic taste from reviewing the results. While it is safe to state that the TA is the most important consideration when thinking of acidic taste in wine the pH and types of acids also influence how the acids are perceived. While a trained palate can easily discern acidic taste, a scientific understanding can improve a winemaker’s craft by expanding their consideration of the relationship of pH and TA to acidic taste.