Lactic bacteria are useful in winemaking as they effect the conversion of malic to lactic acid, which is highly desirable in some wine styles. However, in light, fruity white wines, this conversion (the malo-lactic fermentation or MLF) is undesirable and must be prevented by:

• The addition of sufficient sulphur dioxide (20 mg/l free)
• Clarification
• Good winery hygiene

Care must be taken that the MLF does not occur in the bottle, as this leads to unpleasant smells and a clouding of the wine.In wines with low acidity, lactic bacteria can cause a breakdown of tartaric acid, which is highly undesirable. In wines with low acidity and low sulphur dioxide, the lactic bacteria can exude a mucilaginous substance (rather like the jelly in frog’s spawn) that gives the wines the appearance of flowing like rope (ropey wines!).