Acidity in wine grapes is the product of several organic acids naturally found in the fruit. Tartaric and Malic make up the lion's share of these acids. We mostly focus on them during winemaking. Depending on when the fruit was picked and how the wine was made, these acids will be present in varying amounts. The concentration of these acids determines how tart/sour the wine will be, as well as how long the wine will remain stable after bottling. As a result, adjusting the acidity of a wine/must involves lowering or raising these concentrations.
Acidity has several functions in a wine/must. The tartness helps to balance the fruity, sweet elements that would otherwise become too cloying if not present. Acidity also helps to trick our pallets into perceiving the alcohol in wine as a sweet component, rather than a burning sensation. It also creates a harsh environment that helps keep the wine from becoming spoiled by microorganisms (both from a pH perspective and because the acidity makes SO2 more effective). Finally, acidity in wine promotes good aging characteristics and helps ensure that the wine will hold up well in the bottle during its years of aging/storage.
There are two ways to look at acidity in a winemaking: TA or pH:
To illustrate how even a single added element can alter the perception of acidity, let's use the following example: you squeeze the juice of one lemon into a glass of water and taste it. At this point the combination of only lemon juice and water will be quite sour. However, if you add some sugar to it, the sharpness gets balanced out, you have lemonade. The amount of acid has not been altered, yet the perception of the acidity has shifted from sour to tart and refreshing.
This same type of modifying phenomenon is also taking place in the must/wine but in a much more complicated fashion. A complex variety of different compounds are responsible for not only altering the perception of the acidity, but for defining how the wine will react to any changes made to its acid structure. Because the ratio of these modifying compounds varies for each must/wine, it is difficult to predict exactly how the pH will shift as a result of a calculated acid addition or reduction.
While both TA and pH can be used to measure and discuss acidity, they are not directly related to each other in a predictable manner. If you add 1 g/L of Tartaric acid to a wine/must already containing 6 g/L, you TA test will clearly reflect this addition and let you know there is 7 g/L of acid present. However, when testing how the same sample for pH, it's highly unlikely to get a 'one-to-one' reaction. A pH of 3.7 will not go straight to 3.6. We could even get a final pH of anywhere from 3.65 to 3.5! That extra 1g/L has interacted and balanced out with all of the other elements in the must/wine, accounting for the final pH value. The only way to know for sure how the wine/must‟s pH will react to an acid adjustment is by doing a bench trial. When this is not possible it is recommended to make a partial addition, test/taste the results, and then add the balance if needed.
Generally, if either TA or pH is within the recommended ranges, the other will be as well. When adding acids, a wine will more gracefully absorb large acid additions during its earlier stages than it will as time goes by. Therefore, if you do need a large acid adjustment, we recommend making a relatively large correction early on (preferably in the must), so that you only need to make minor changes later on.
Acid additions become more apparent in wine as it ages and the fruitiness starts to fade. An adjustment that seemed just right early on might end up being a little too tart in six months to a year. As a result, when making large adjustments to your must or wine, it's best to use a conservative, two-step approach: make one addition now (in the middle range of what you are shooting for), then, if it needs it, add a little more later.
Another thing to consider: TA will drop 0.5-1.0 g/L (0.05-0.1%), or possibly even more, as a natural result of fermentation. Additionally, Malolactic Fermentation will lower your TA by another 1.5-4.0 g/L (0.15-0.4%). As a result, it's important to consider both the TA of the must prior to fermentation as well as the techniques you will be using when making additions at crush. If you are not sure of your approach, shoot for the 7g/L TA range for the must and go from there.
Whatever amount you decide on, it's a good idea to mix the acid in a small portion of juice, wine or even warm, chlorine-free water before adding it to the must or wine. You want to make sure that the acid crystals are completely dissolved before the addition goes into the wine so that your correction will be evenly and thoroughly mixed throughout the whole volume of liquid.
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