Learning about acidity in wine
Alright, I’m not really a winemaker yet, but after this past week trying to find more information about wine acidity on the Internet, I’m beginning to feel the frustrations of one. Last week, I ordered a digital pH meter which I’ll be using along with other tests to measure the acidity of the wine and whether I’ll need to balance it with additives. I was never much good at chemistry (that’s a whole ‘nother story) but I guess I’ll be learning it as I do this, so bear with me. I may not have some things exactly right, but hopefully, I’ll at least be on the right track. Besides, all I’m really trying to do is find more nifty gadgets to play—add a digital display to anything and it becomes fun.
The begin with, let’s look at some basic facts about acids in general:
- When an acid is dissolved in water, the molecules dissociate or break apart with a proton from the acid being donated to the water molecule to reach an equilibrium. I’m sure everyone remembers high-school chemistry but in case you don’t, here’s how that looks in a formula that’s totally obscure to most of us:
H2O + HA ↔ H3O+ + A-
- The constant that expresses this equilibrium is Ka known as the dissociation constant. The higher this constant, the stronger the acid. Simple enough, right?
- Since Ka is usually a small fractional number expressed in scientific notation, the constant pKa is often used as an easier form of this constant and is derived:
pKa = 10-Ka or pKa = -log10(Ka)
Okay, don’t glaze over just yet. Using this math is just something to justify the $40,000 you spent on college.
- pH is a measure of the acidity or alkalinity of a solution, based on the concentration of released hydrogen ions found in the solution. The lower the number, the higher the acidity.
- pH is a logarithmic scale much like the Richter scale used to measure earthquake intensity. So a pH of 3 is 10 times more acidic than a pH of 4.
Next we look at the characteristics of acid in wine:
- Wine is basically a water and alcohol solution in which acids behave much like they would in a water solution, only more expensively, sort of like how we behave when we put on an expensive nice suit.
- Too low a pH and the wine will be too tart.
- Too high a pH and the wine will be flat and flabby and may not be acidic enough to preserve the wine, which is probably okay since it wasn’t that good to begin with.
- For good wine stability the pH should be, at most, 3.3 for white wines, and the pH should be, at most, 3.5 for red wines.
- Tartaric acid is the main acid found in grape must along with malic acid; the proportions are dependent on the grape variety and where the grapes were grown. Colder climates produce more acidic grapes than warmer climates.
- Tartaric acid has a pKa of 2.98 ( we’re not accounting for the fact that tartaric acid also has a secondary dissociation constant pKa2,which we won’t use here because that’s even more confusing).
- During fermentation succinic, lactic, citric and acetic acids are formed in very small quantities.
- Tartaric acid in sufficient concentration will prevent the growth of microorganisms and prevent oxidation.
- Tartaric and malic acids are non-volatile meaning that they don’t evaporate. Acetic acid (vinegar) in wine is a volatile acid and is not something desirable, usually.
- Acceptable amounts of acetic acid or VA (volatile acidity) are between 0.03% to 0.06%.
Titratable Acidity or TA
In most technical data that I read about a wine, acidity is often referred to as a percentage concentration. The concentration of acid or percent acidity is different from pH in that it measures the amount of all the acid in a solution, not just the released hydrogen ions. In wine, this concentration is referred to as the titratable acidity or TA, which is a close approximation of total acidity. The TA is given as a percentage but is actually a measure of grams per 100 ml. So a TA of 0.62% is 0.62 grams/ml. or 6.2 grams/liter.
TA after fermentation assumes all the acid in a wine to be tartaric acid enabling the calculations to be consistent. A high TA of say, 0.9% would be too tart for most people unless balanced with sufficient residual sugars. A low TA of 0.45% or less in most wines would taste flat and wouldn’t have sufficient acid to prevent spoilage. High sugar wines such as late harvest varieties or Sauternes generally have high TA, usually higher than 1.0%, to balance the sweetness and as a side benefit, giving them longer cellar life.
Unfortunately, because pH can be said to measure the strength of acidity, not its concentration and TA measures the actual amount, these two measures are not the same and there is no direct correlation between them. That is, wine with low TA may still taste tart if the pH is low enough. The other tricky part of these measurements is that the actual amount of acid in a solution increases on a logarithmic curve as pH drops, so in order to decrease pH, I would need to add acid in greater amounts to achieve the desired results.
I googled until my fingers were sore and I couldn’t come up with a site that 1) fully explained how to make some correlation between TA and pH, 2) didn’t want to charge me a minimum of $16.95 to look at some published white paper from someone with multiple letters after their name, and 3) used the same constants that I mentioned above. That’s why all the information out on the web says it’s a trial and error method, much like R. Mondavi says that making fine wine is an art. Well, I’m pretty accomplished in the other arts, so this shouldn’t be so difficult, right?
The next installment of acidity will deal with determining TA in wine and its relationship with pH. If you notice anything seriously wrong in what I’ve said, please let me know what is wrong and why I didn’t understand it correctly. I’m still learning.