Why Test for Volatile Acid?
High volatile acid content in wine is an indication that bacteria, especially Acetobacter, which spoil wine, are present; the wine may turn to vinegar. Some acetic acid (i.e. vinegar) is acceptable in wine, somewhat more so in older wines.
The government requires that wine be tested for volatile acids to protect customers; http://waterhouse.ucdavis.edu/winecomp/va.htm. Federal, California and European regulations have differing standards; California's are generally a little stricter.
The winemaker needs to know the volatile acidity to meet legal limits, to track the acidity during storage, as well as to watch for spoilage. He or she will typically check the acidity several times during the production process.
Volatile acids in wine include acetic, formic, and butyric acids, and other fatty acids. Lactic, succinic, sorbic acids are slightly steam-distillable, but are not considered part of the volatile acids; their contributions must be adjusted for. Carbon dioxide, from fermentation, and sulfurous acid, from wines high in sulfur dioxide, also affect the total acidity and can contribute to error.
Steam distillation of the volatile acids requires that steam be bubbled through the sample of wine to be tested. The volatile acids are vaporized and condensed in a Graham condenser. The distillate is collected and titrated to determine the total steam-distillable acidity.