A wine club member emailed me today with the question in the title.
My response took just enough time that I thought it would be prudent and useful to post it here for everyone to read:
ACID IN WINE EXPLAINED, by Wes Hagen, Winemaker, J Wilkes Wines.
Wine has three basic forms of acid that are naturally occuring.
Tartaric (cream of tartar is made from wine tanks), malic (bright, apple-acid, like the stuff they put on sour ptch kids), and lactic acid–a creamy or more seamless, soft acid like in a buttery chardonnay.
Acid gives a wine ‘cut’ and ‘brightness’, without it the wine would be spineless and flabby. Acid also strips fat from your taste buds and allows you to taste food bite after bite when it’s washed down with wine.
Sugar/sweetness in wine cloys or covers up acidity, so a high acid wine can be balanced with a little or lot of residual sugar left in the wine.
Champagne has very high acidity, German rieslings and Sauvignon blanc usually have high acid, aromatic whites like Pinot blanc, Pinot Gris, Gewurtz, Chenin blanc have moderate to high acid.
Generally, the cooler the growing region and the earlier the grapes are picked, the higher the acid.
Acid also adds stability to a wine and requires less sulfur dioxide to be added for stability.
Acid is measured in wine by grams per liter of ‘titratable acidity’. Less than 4g/l would be soft like a Napa Cab, 5-7 grams is where most quality wines land, 7-9 grams is very high acid like in a German white wine.
pH is also used to measure hydrogen ion activity in wine, and is roughly analagous to acidity. Champagne is about 3.0 pH, and soft Napa/Paso reds can be about 4.0.
A good analogy is that ‘TA’ is the physical weight of acid in the liter of solution, the pH is how fast the hydrogen ions are whipping around in that solution. If a wine tank were instead a fish tank, the TA would be the weight of the fish, while the pH would be how fast the fish are swimming.
In the palate, acid is felt as a bright tingle, and may be more noticeable on the sides of the tongue, but every papillae on your tongue has a receptor for it.
Acid structures most human beverages. Orange juice without acid would not taste right, nor would a Coke that has gone flat, losing its fizz/acidity. In fact Coca-Cola has higher acidity when freshly opened than any wine in the world.
To get a sense of the extreme mouthfeel of acid, lemon juice is likely around 2.0 pH and 14-20g/l+ of acid, so that would give you a strong impression of how acid impacts your mouth.
Also, temperature impacts acid’s mouthfeel. The colder the beverage, the more noticeable the acidity, and the warmer the beverage, the less acid is perceived.
Hope that helps, if you have questions, please pose them in comments.