Last night, when I used this word at your table (I think the topic was how awesome the riesling is with pork belly), I loaded it with such obvious ardor that you must have wondered if acidity wasn't some weird byword for quality, that acidity=good.
And the equation might hold, at least for many sommeliers. Acid may be for us what capsaicin is for judges of chili cook-offs. Most of us are hooked on the stuff.
It is the acidity of orange juice that wakes you up in the morning. And it is the lime squeeze that enlivens your carnitas, the malt vinegar that keeps you motivated in front of your fish-n-chips. Acid in wine not only helps invigorate your food, but also works within the matrix of the wine itself to make you want you want to keep on drinking.
Acidity gives discipline and shape to a wine. Otherwise, the fruit flavors and alcohol would just laze around anywhere, directionless. Acid does the clutch work of tidying up.
A few types of acid may appear in a wine. Which ones, and their relative proportions, will have some bearing on its texture. Tartaric acid is the abundant workhorse acid, performing much of that task of structural stability. High levels of malic acid, will make a wine feel tart and fresh, like green apple, whereas lactic acid is rounder and less acute. Wines that have gone through a conversion called malolactic fermentation will have at least some of its malic turned into lactic (most reds see this conversion, as do some whites).
Certain wines, like Pinot Gris from Alsace, or some Northern Rhone whites, make stylish use of lower acidity to create attractive satiny textures. These wines are interesting exceptions to our earlier equation (and can be used to refute us junkie somms when we go on and on and on about acidity).
About the Author: Steven Grubbs is a sommelier and wine director at Empire State South (Atlanta, GA) and Five & Ten (Athens, GA). Ask him what to drink on Twitter, where he also accepts questions on tacos and manhood.