Current Vineyard and Cellar Events: Sources of volatile acid formation in wine and potential control measurements

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Current Vineyard and Cellar Events
Sources of volatile acid formation in wine and potential control measurements

Charl Theron
Acetic acid contributes largely to the spoilage of wine known as volatile acid or "va". It could have any one of several causes. It would be impractical, however, to try and eliminate all the possible sources and a better approach by far is to address the most critical causes.
Since acetic acid constitutes more than 96% of the volatile acid content of wine, this is the most important compound to address. In reduced concentrations volatile acid may be complementary to wine quality, but quantities of volatile acid that exceed 0,6 g/l usually reduce overall quality and the typical vinegary flavour becomes noticeable.
A questionnaire that was sent to cellars in New Zealand and USA revealed some interesting information:

Discernible amounts of volatile acid occurred across the entire spectrum of vinification.
Approximately 55% of the cellars experienced problems with volatile acid after malolactic fermentation (MLF) and barrel storage.
However, problems with volatile acid also occurred before fermentation, during fermentation, during MLF and as a result of cellar practices.

Microbiological activity is by far the biggest culprit when it comes to the formation of volatile acid in wines. This could be triggered by yeasts, malolactic bacteria as well as acetic bacteria, all of which might play a role.
The so-called wild yeasts, Kloeckera, Candida and Hansenula species, receive very little attention from winemakers, but more than 1 g/l acetic acid may be formed by the yeasts.
Brettanomyces yeasts may also cause high levels of volatile acid, but they are known to correlate strictly with the presence of oxygen. Cellars that are aware of Brettanomyces contamination must therefore attempt to limit the potential oxygen, in other words exposure to air, as much as possible.
Apart from the fact that lactic acid bacteria may cause the transformation of malic acid to lactic acid, these bacteria may also form undesirable compounds. The lactic acid bacteria that are generally encountered in wine include Oenococcus oeni, Pediococcus and Lactobacillus. Discernible amounts of acetic acid are usually formed by lactic acid bacteria via the heterofermentative metabolism of grape sugar. Since Oenococcus and Lactobacillus both have this kind of metabolism, they are primarily responsible for acetic acid formation. The former usually brings about low concentrations of acetic acid, but the latter is able to produce considerably bigger amounts. This takes place especially when the must and the wine have a high pH.
The name acetic acid bacteria reflects their ability to form high concentrations of acetic acid. Initially the belief was held that to control these bacteria, it would suffice to prevent contact with air and to maintain sufficient levels of sulphur dioxide. It was recently proven, however, that these bacteria might cause the formation of acetic acid at various stages. The survival of acetic acid bacteria in the vineyard, the formation of acetic acid during fermentation and stuck fermentations are ample proof.
The biggest source of Acetobacter spoilage, however, is still wines that are matured in barrels for any length of time. While the amount of oxygen in barrel matured wine is sufficient to enable bacteria to survive, pumping over and racking can also result in bacterial growth.
On the other hand, there is proof of non-microbiological sources of acetic acid occurring in wines. The chemical hydrolysis of wood hemicellulose, for example, may result in the formation of acetic acid. Oxygen diffusion may also take place more rapidly when wood has a coarser grain. The origin of oak and cooperage activities may therefore influence the acetic acid levels in wine.
During the vinification process it is especially high risk must, risky vinification techniques and poorly managed cellar conditions that come into play.
High risk musts include high sugar levels combined with a pH above 3,5. The potentially detrimental implications may be reduced by taking the following steps:

Regular monitoring of sugar and pH qualities before the grapes are harvested.
Grapes that are at risk should not be harvested mechanically.
Reduce the pH levels through the judicious addition of acid.
Reduce the microbe population before fermentation through the effective settling of musts.
Pasteurisation of musts may be considered, if problems are expected.
If MLF is not required, it may be inhibited through the addition of lysozyme.

Current research indicates that spontaneous fermentations and low fermentation temperatures are more subject to the abnormal formation of acetic acid.
By adhering to the following cellar practices, the incidence of problems with volatile acid may be limited:

The regular topping up of barrels.
The use of barrels that have been properly cleaned.
The cellar equipment must be thoroughly cleaned after all usage.
Once barrels have been emptied, they must either be filled again immediately or cleaned straight away.
By maintaining high levels of humidity in barrel cellars, the drying out of barrels will be prevented, which might reduce the uptake of oxygen.

Reference:
Kramer, E L, Noonan, M J, 2004. Sources leading to development of acetic acid in wine - And potential options for its control. Wine Industry Journal 19(1): 39 - 46.
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