Wine spoilage is legally defined by volatile acidity, largely composed of acetic acid. Ezekeil Neeley, 2004
Volatile acidity refers to the steam distillable acids present in wine, primarily acetic acid but also lactic, formic, butyric, and propionic acids. Commonly, these acids are measured by Cash Still, though now they can be measured by gas chromatography, HPLC or enzymatic methods. The average level of acetic acid in a new dry table wine is less than 400 mg/L, though levels may range from undetectable up to 3g/L.
U.S. legal limits of Volatile Acidity:
Red Table Wine 1.2 g/L
White Table Wine 1.1 g/L
The aroma threshold for acetic acid in red wine varies from 600 mg/L and 900 mg/L, depending on the variety and style. While acetic acid is generally considered a spoilage product (vinegar), some winemakers seek a low or barely detectible level of acetic acid to add to the perceived complexity of a wine. In addition, the production of acetic acid will result in the concomitant formation of other, sometimes unpleasant, aroma compounds (see ethyl acetate and acetaldehyde). These compounds have much lower sensory threshold than acetic acid—both acetaldehyde and ethyl acetate are detectable at less than 200 mg/L in wine. In addition to the undesirable aromas, both acetic acid and acetaldehyde are toxic to Saccharomyces cerevisiae and may lead to stuck fermentations.
- The amount of volatile acidity found in sound grapes is negligible. It is a byproduct of microbial metabolism.
- Acetic acid bacteria (e.g. Acetobacter aceti which is used to make vinegar) is able to convert both glucose and ethanol to acetic acid.
- Yeast found in the vineyard—Kloeckera, Hansenula, and Metschnikowia—are able to produce large amounts of acetic acid and ethyl acetate early in a fermentation, but his generally occurs only with damaged grapes. This conversion can be prevented by the addition of sulfites at crush.
- Most lactic acid bacteria will produce acetic acid from glucose if they are present when there is still significant amounts of sugar.
- Of wine yeast, Saccharomyces strains will produce varying amounts, while Brettanomyces is a strong producer of acetic acid.
- Dessert wines produced from botrysized (noble rot) grapes often have higher levels of acetic acid. The Botrytis mold breaks open the grape skins, allowing the co-infection of the grapes with yeast or bacteria, mentioned above, that produce acetic acid.
Acetic acid bacteria require oxygen to grow, therefore, elimination of any air in wine containers and sulfur dioxide addition will limit their growth. Likewise, rejection of moldy grapes will prevent possible problems. Use of sulfur dioxide and inoculation with a low-V.A. producing strain of Saccharomyces may deter acetic acid producing yeast.
A relatively new method for removal of volatile acidity from a wine is reverse osmosis. Blending may also help—a wine with high V.A. can be filtered (to remove the microbe responsible) and blended with a low V.A. wine, so that the acetic acid level is below the sensory threshold.
Ezekeil Neeley, 2004