Sources

• The aerobic microorganisms either come from the grapes themselves or are added by contaminated processing equipment or storage containers. Exposure to air gives the bacteria a ready source of oxygen, enabling them to grow and spoil the wine.

Types

• Yeasts and acetic acid bacteria are the usual causes of bacterial contamination in wine. Candida and related types of yeast form a film on the wine's surface, while acetic acid bacteria use oxygen to convert the ethanol in wine to acetic acid.

Effects

• A study by E.J. Bartowsky et al. in the 2003 issue of "Letters in Applied Microbiology" found that acetic acid bacteria adversely affected both the smell and taste of contaminated wine. Yeast contamination can also cause unpleasant odors.

Prevention

• According to Dr. Murli Dharmadhikari of the Midwest Grape and Wine Institute, keeping wine containers full is one of the best ways to prevent contamination by aerobic bacteria. Other precautions include storing wine horizontally and not exposing cold wine to air.