Wine Arcana

Concept

Volatile acidity

Also known as VA

Volatile acidity is the portion of wine's acidity measured through acids that can be separated by steam distillation. Acetic acid, the principal acid in vinegar, dominates it. Winemakers often abbreviate the measurement to VA and report it as grams per litre of acetic-acid equivalent.

Small amounts arise during normal fermentation. Increasing VA can also signal unwanted microbial activity, making it useful both for following a wine's development and investigating spoilage. Its sensory effect depends on the other substances present as well as on the concentration of acetic acid.

Where it comes from

Yeast produce some acetic acid during alcoholic fermentation, with amounts varying among strains and fermentation conditions. Some lactic-acid bacteria produce it from sugars; metabolism of citric acid during malolactic conversion can also contribute. These routes help explain why a finished, sound wine contains measurable VA.

Damaged grapes give microorganisms access to juice before the main fermentation begins. Yeasts and bacteria growing on split or rotten fruit can produce acetic acid and other compounds before the winemaker receives the grapes. This is one reason fruit condition and the speed of processing matter. Botrytized sweet wines require particular care because damaged skins and concentrated juice create demanding microbial conditions.

After fermentation, acetic-acid bacteria can oxidise ethanol to acetic acid. They require oxygen, so air exposure and unfilled storage vessels can encourage their activity. Different organisms and substrates create different routes to VA: restricting air helps control acetic-acid bacteria, while sugar-metabolising organisms require attention to other cellar conditions as well.

Vinegar and solvent aromas

Acetic acid can smell vinegary and contribute a harsh, sour impression. Ethyl acetate, an ester, can add a solvent or nail-polish-remover note when elevated. At lower concentrations it can contribute to fruity aroma. The two compounds often occur together and are frequently grouped under “VA” in tasting conversation, although ethyl acetate is chemically an ester and is not itself an acid in the VA measurement.

Their proportions matter. Two wines with similar acetic acid can smell quite different if one contains more ethyl acetate. Sugar can mask some of the impression, while acidity and tannin affect the perceived harshness. The Australian Wine Research Institute (AWRI) therefore treats the sensory interpretation of VA as dependent on the wine matrix and the accompanying ethyl acetate.1. Adrian Coulter, “Volatile Acidity,” Australian & New Zealand Grapegrower & Winemaker 648 (January 2018), p. 16. A single analytical value cannot establish a universal boundary between complexity and a fault.

Reading the measurement

Laboratories may measure steam-distillable acidity or determine acetic acid directly. Acetic acid is the principal contributor, but the analytical terms are not perfectly interchangeable. The method and units belong with any comparison of results.

A rising measurement is particularly useful evidence because it prompts a search for a continuing microbial process. Sound fruit, controlled oxygen exposure, appropriate sulfur dioxide, and microbial control help prevent further formation. Removing microorganisms through suitable filtration can address the source of a problem, while the acetic acid already dissolved in the wine remains. Its removal requires a different treatment, such as specialised membrane processing.

Sources