This is a secondary fermentation that occurs after sugars are consumed in the primary fermentation. Unlike the primary fermentation, which is performed by yeast and results in the production of alcohol, the malolactic fermentation is strictly bacterial and results in a decrease in total acidity of the wine. For centuries, it was observed that the onset of spring was often accompanied by the development of small bubbles and a haze in wine. Originally believed to somehow be related to the renewal of plant life, it is now known to be the result of bacterial activity brought about by warmer temperatures. In many instances, the bacterial activity was a malolactic fermentation which produced a superior wine.
Malolactic fermentation offers three advantages to the winemaker.
First, by converting malic acid, which has two acid groups, to lactic acid, which has one acid group, the total acidity of the wine is reduced. Second, the bacteria that perform the fermentation are nutritionally fastidious and consume many trace nutrients in the must. This helps stabilize the wine, preventing the growth of undesirable microorganisms and spoilage of the wine during storage. Finally, the fermentation can be accompanied by the formation of new flavor compounds, contributing to the complexity of the wine. They are strictly fermentative and are considered aerotolerant anaerobes.