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A Brief Discussion of Acid (no not that kind)
Winemakers frequently talk about acidity, but don't much talk about what it really means. Herewith is a brief chemical description of acidity.
Simply, acidity measures the breakdown products of water in solution. Most folks know that water is a molecule comprised of 2 hydrogen atoms and one oxygen atom; i.e. H2O. At any given time a very small amount of water is broken down to H+ (hydrogen without its electron, which is negatively charged, hence the positive charge here), and OH- (the oxygen likes to keep the extra electron, thus the negative charge here). H+ is the active agent of acidity so the higher the H+ concentration the more acidic the solution, and since the pH is the negative log of the H+ concentration, the lower the pH, the more acidic the solution.
Before we get to wine, let's characterize the range a bit. Pure water is a ph of 7. Solutions with higher pH than 7 include your blood, which is just above at 7.2 or so, bleach at about 9, and strong bases like hydroxide can be above 13 and are extremely corrosive. On the other end, weak acids would be coffee at about pH 5, lime juice at about pH 2, stomach acid at between 1 and 2, then strong mineral acids like hydrochloric, again extremely corrosive, at less than one. Wine tends to fall in a range between 3 and 4; occasionally slightly below 3 with high acid Rieslings, or slightly above 4 with hot climate Syrah. From pH 3 to pH 4 is a tenfold reduction in H+, so this variation is quite significant.
One last piece of chemistry - you're loving this, right? pH is our way of measuring acid activity, as you now know, but in wine we also use another measure for acid, called titratable acidity (TA). This is essentially a measure of the concentration of the molecules in wine that contribute to the acid activity as measured by pH. These are the weak organic acids of wine, tartaric and malic acid, and if the wine undergoes malolactic fermentation, lactic acid as well. A higher TA does not always mean a lower pH as there are other factors, such as potassium exchange in the vine that can affect this relationship. There is however, an independent flavor contribution of these components.
OK, so why do we care? Actually there are huge effects of varying pH and, to a lesser extent, other acid measures in wine, not necessarily good or bad in any given case, but very important to characterizing a particular wine. Here are some of the issues:
Stability (both chemical and microbial):
Color perception and stability: all of the color in wine comes from three ring phenolic compounds called anthocyanins, and it turns out that only their positively charged (protonated) form is colored red, so higher H+ concentration (lower pH) intensifies red wine color. At wine's pH, only about 10% of the total anthocyanin in wine is protonated, so a very small pH variation can make a huge color difference. Without going into detail, this color is also far less susceptible to chemical degradation at lower pH. For more involved reasons, this is particularly significant in Pinot.
Longevity: as mentioned previously, oxidation reactions, which comprise the bulk of wine aging reactions, are inhibited by lower pH, so low pH wines tend to age more slowly, and be more resilient in less than perfect storage conditions.
Flavor: acid is tart, and here is where the TA can have an effect as well as the pH (the other effects are purely pH related). The brightness and tart balance of a wine comes from its acidity. This balance with alcohol, which is perceived as sweet, is very important to the flavor profile of a wine. Higher acids (both pH and TA have influence here) also tend to make the astringency of a wine more apparent, as does higher alcohol. Acidity also affects the way a wine shows wood aging. Higher acid wine tends to show wood more slowly and changes the choices a winemaker might made in that regard.
In any case, I hope we shed a small amount of light on this greatly important, and to some weird folks, fascinating, area of interest in the winemaking craft.