Bacteria play both helpful and harmful roles in wine. Beneficial lactic-acid bacteria drive malolactic fermentation, softening sharp malic acid into rounder lactic acid. Harmful bacteria, such as acetic-acid bacteria, can spoil wine by producing vinegar (volatile acidity) if exposed to oxygen. Winemakers manage bacteria through hygiene, sulfur dioxide, temperature and timing to encourage the good and suppress the bad.
WHY IT MATTERS FOR COLLECTORS
Bacterial spoilage — a vinegary or 'nail-polish' edge — is a fault that worsens with poor storage. A trace can add complexity, but obvious volatile acidity in a young wine is a warning sign.
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