Volatile Acidity—Acetic Acid
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.
Origins
- 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.
Prevention
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.
Treatment
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
Minor