Acidity is a measurement of the quantity of acid present in a wine or must. This is not to be confused with the pH of a wine which is a measurement of the strength of those acids present.
Like pH, however, the acidity of a wine goes a long way in determining how a finished wine will taste, feel in your mouth, and how well it will age. Having a low acidity will result in a flat and boring wine while having too much acid can lead to tartness or even a sour wine.
However, if you peg the acidity just right you are much more likely to have flavors that pop, good mouthfeel, crispness in white wines, and decent age-ability. It takes careful measurements and a keen sense of taste for those acids produced by spoilage organisms. More on this soon.
What makes acidity tricky is that not all acids are measurable using the same laboratory methods. This is why acidity is described as three different entities, titratable, volatile, and total.
Titratable Acidity
Titratable acidity, sometimes referred to as fixed or erroneously total acidity, is a measurement of the total concentration of titratable acids and free hydrogen ions present in your wine. Total acidity and titratable acidity are not the same thing and the terms should not be used interchangeably.
What makes an acid titratable is its ability to be neutralized by adding a base, a laboratory process called titration. To measure this you would take a sample of wine and add to it a reagent, or indicator, which is a chemical that changes color when a specific pH is reached.
With this mixture in hand a base of known pH is then added to the mixture. At some point the base will raise the pH sufficiently to cause the reagent to change color, indicating that you’ve reached a specific pH. The amount of base added mixture required to turn the reagent to its indicator color is then translated to an amount titratable acids in the wine.
The most common titratable acids are tartaric, malic, citric, carbonic acid. These acids, along with many more in smaller quantities, either occur naturally in the grapes or are created through the fermentation process.
Volatile Acidity
Volatile acids are different from titratable acids in that they cannot be measure through a titration but must be quantified using a steam distillation process. In this a sample of wine is exposed to steam which in turn encourages the volatile acids to leave the wine.
The steam and the acids that leave the wine sample are then collected through condensation in another container. After a specific quantity of water and acids has been collected that mixture is then tested to determine the concentration of volatile acids. Thus volatile acidity is simply a measure of steam distillate acids present in a wine.
Volatile acids are produced through microbial action such as yeast fermentation, malolactic fermentation, and other fermentations carried out by spoilage organisms. The most prominent volatile acid present in wine is acetic acid. Also of importance is lactic acid.
Aside from lactic acid the presence of volatile acids in any appreciable quantity is a sign of spoilage. Acetic acid is produced by acetobacter as it ferments your nice fine wine into vinegar. Our palates are quite sensitive to the presence of volatile acids and for that reason we try to keep their concentrations as low as possible.
Total Acidity
As previously mentioned the total acidity of a wine is the combined sum of titratable and volatile acids present. To determine the total acidity of a wine you must first perform the titration to measure the titratable acids and the then the steam distillation of a wine sample to determine the concentrations of volatile acids.
After you’ve been through all that you can add the acidities together to obtain the total acidity. What does it tell you?
When you and I have a glass of wine our mouth is largely unable to tell the difference between titratable and volatile acids. If there is an overwhelming quantity of any single acid, say citric, we may be able to pick out their contribution to the wine. In the case of citric acid your wine may have citrus overtones to it. An over abundance of a volatile spoilage acid can relate off flavors and aromas to us. But even so it must be out of balance for us to notice this one particular acid.
Beyond having a single acid out of balance, it is the total acidity that we perceive. So if a wine is too tart the acidity is too high. If a wine is soft and flabby the acidity is too low.
To determine whether or not we’ve got our acidity pegged in a reasonable quantity we have to measure titratable and volatile acid concentrations to determine what the total amount of acid present is. From there we can decide what to do about it.
A malolactic fermentation will convert malic acid to lactic acid and reduce the titratable acidity in the process. Malic acid is a much harsher acid than lactic so in the end your wine will have less bite and take on the buttery smooth characteristics of lactic acid.
Photo of Steam Distillation Equipment By Izmaelt (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons
So, how in the heck do I do all of this testing at home? So far I have made kit wines which I assume have already been balanced. I am also planning to make some other types of fruit wines. How does acidity affect them?
Great questions Gary. I’ve had good luck using the Accuvin Titratable Acidity test kit. Volatile acidity testing is a little more complex to do so I’ll have to do some more research on testing methods and put together an article on this.
Acidity will affect fruit wines in the same way. If they’re to low they’ll taste weak or flabby, too high and they could be tart. I would think that a reputable recipe would take acidity into consideration when outlining the ingredients. Being able to test for it though may give you peace of mind. I’ll let you know what I find out.
Thanks Gary,
Matt