Most of the lactic acid present in the wine is produced during the malolactic fermentation with the transformation of L-malic acid into L-lactic acid, so that more than 75% of the lactic acid present in the wine is the L isomer. D-lactic acid, on the other hand, is associated with the metabolism of glucose (and other hexose sugars) by the same lactic bacteria (mainly leuconostoc and lactobacillus). A presence of D-lactic acid greater than 0.3 g/L is an indication of bacterial contamination, since these bacteria compete with the yeasts for sugars, until inhibiting alcoholic fermentation.