Vinegar bacteria must have access to large quantities of air to oxidize the alcohol into acetic acid. Colonies of bacteria often develop as thick, wrinkled mats on the surface of the wine where they have direct access to the air. However, vinegar bacteria can and do exist in the bulk of the wine. Barrels infected with vinegar bacteria should be taken out of service because disinfecting materials like wood is nearly impossible. Acetobacter activity is controlled by keeping the sulfur dioxide content of the wine at a reasonable level and by keeping wine containers completely full and tightly sealed.
Malolactic fermentation reduces wine acidity and improves the long term stability of red wine.
ML fermentation produces the butter-like quality prevalent in heavier style Chardonnay wines, and it adds pleasing complexity to red wines. ML fermentation is generally encouraged in red wines because red wine containing malic acid is biologically unstable. Besides yeast, many other microorganisms exist in wine. These organisms can ferment normal wine components into significantly different materials, and the products of some of these other fermentations can reduce wine quality. When winemakers are aware of the microorganisms present, they can take effective measures to prevent spoilage.