When considered as a whole, the volatile acids in wine make up a wine's volatile acidity. They include minuscule amounts of formic, propionic, and a few other acids, but without exception it is acetic acid (the same acid responsible for turning wine into vinegar) that plays the most important role in volatile acidity. Volatile acidity in minute proportions can actually give a refreshing lift to some heavy wines. To experience firsthand the potency of volatile acidity, open a bottle of Madeira—say, a 10-year-old Malmsey—and nose the wine. That nasal passage-piercing aroma is a result of volatile acidity.