I have recently had the unlucky chance of discovering a relatively high amount of volatile acid (VA) in one of the pear wines I produced. Although the wine is still usable with a small carbon treatment and some blending, it is not something winemakers ever like dealing with.
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. The average level of acetic acid in a new dry fruit 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
Fruit Wine: 1.1 g/L
The aroma threshold for acetic acid in a red fruit wine varies from 600 mg/L and 900 mg/L, depending on the variety and style. While acetic acid is generally considered bad (vinegar), some winemakers seek a low or barely detectible level of acetic acid to add to the perceived complexity of a wine. Too much acetic acid production will result in the formation of other, sometimes unpleasant, aromas like ethyl acetate and acetaldehyde. In addition to the undesirable aromas, both acetic acid and acetaldehyde are toxic to Saccharomyces cerevisiae and may lead to stuck fermentations which is a whole other subject of which I will touch on in the future..
Origins
- The amount of volatile acidity found in sound fruit is negligible. It is a by-product of microbial metabolism.
- Acetic acid bacteria is able to convert both glucose and ethanol to acetic acid.
- Most lactic acid bacteria will produce acetic acid from glucose if they are present when there is still significant amounts of sugar.
- Wine yeast, such as Saccharomyces strains will produce varying amounts, while Brettanomyces is a strong producer of acetic acid.
- Dessert fruit wines often have higher levels of acetic acid. The yeast needs to work very hard at fermenting in a high sugar medium and the extra stress often encourages the production of 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 fruit 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. The main prevention method is to ensure that the fermentation runs a healthy course and that enough nutrient is present in the wine throughout.
Treatment
A relatively new method for removal of volatile acidity from a wine is reverse osmosis. Blending may also help—a wine with high VA 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.
If the VA problem is minor, a fair amount of the off flavour can be reduced or eliminated with strong fining treatment such as PVPP or Activated Carbon, although this will not guarantee that the wine will be free of its unpleasant taste.
The best treatment is its prevention.
Happy fruit winemaking!