5.6.4 Procedures in case of contamination

What can be done if contamination by acetic acid bacteria occurs?

Good hygiene and oxygen management remain key to avoiding accidents with volatile acidity. It is therefore necessary to have controlled inputs and to apply a series of good practices:

Avoid having tanks that aren't fully filled and, when this is the case, inert the headspace
Top barrels up regularly
Clean containers rigorously
In the event of extreme contamination of a barrel, the best solution is to dispose of it. Otherwise, soda-based treatments exist but they are long, and require a lot of water and increased monitoring for the possible presence of residues.
It may also be necessary to adjust the free SO2 level.

What can be done if contamination by Brettanomyces bruxellensis occurs?

**Racking and Sulphiting

The usual method to manage these yeasts is to regularly reduce the population by racking. Since the lees are a food source for the yeasts and often contain large quantities of them, it is beneficial to remove them from the environment regularly. One option is to rack every 3 to 4 months, sometimes more often depending on the winery/vineyard. Another option is to do the first racking when the temperature starts to rise, just before the summer.

Racking is particularly effective if there is significant cloudiness, i.e. at the beginning of the ageing process. However, towards the end of the maturation process, when the wine is much clearer, collateral risks such as exposure to oxygen can erase the beneficial effect of clarification by facilitating the growth of B. bruxellensis. For more details on racking, see the section on common practices during ageing.
If any growth is observed, racking followed by rigorous disinfection (steaming + burning sulphur discs) usually stabilizes the situation. For more details on cleaning, see barrel ageing.

Sterile filtration

Filtration during the ageing process is also a solution to eliminate spoilage microorganisms. In view of these yeasts' small size, a very fine filtration is required. A cut-off point of at least 1 µm and preferably 0.65 µm is necessary. However, clarifying the wine prematurely is not always qualitative from an organoleptic standpoint. It also creates a microbiological vacuum that makes the conditions very favourable for recontamination. For example, if the barrels are not perfectly disinfected, recontamination is likely to occur rapidly. It is therefore a last-resort solution.

Chitosan

Chitosan has recently been authorized for use in organic agriculture. It is a chitin derivative produced by Aspergillus niger that is a natural, biodegradable solution.
A 4 g/hL dosage appears to be sufficient to drastically lower the population, although some remains suspended in the wine. However, there is no effect on ethyl-phenols if they are present.
For the moment, there are no studies showing the impact of chitosan on wine's organoleptic properties.

Fining

Fining followed by racking eliminates some of the population by clarifying the environment. For example, fining with egg white, which naturally contains lysozyme, eliminates some of the lactic bacteria population. In addition, the flocculation effect of the cells with the egg white proteins reduces the B. bruxellensis population. Other fining agents exist and can be equally or even more effective against B. bruxellensis.
Fining with fresh lees or yeast autolysates can reduce the ethyl-phenol content by about 20%. Note, however, that such methods should only be used on wines whose B. bruxellensis population has already been lowered, otherwise, there is a risk of causing renewed yeast growth and aggravating the situation.
Finally, blending with another batch can lower the concentration and potentially bring it below the perception threshold. This blending should obviously only be done when the yeast population has been eliminated.

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