Introduction

Introduction

Lactic acid bacteria found in wine can convert the dicarboxylic acid malate to the monocarboxylic acid lactate and carbon dioxide and use this process to generate energy. This is referred to as the malolactic fermentation as electron movements and proton fixation is indeed involved in the formation of lactate but this is more correctly viewed as a conversion than an actual fermentation. The malolactic fermentation can be conducted by several members of the lactic acid bacteria, Lactobacillus, Oenococcus and Pediococcusalthough not all strains are equally proficient in this process.  Some wine pediococci have been identified that are incapable of conversion of malate to lactate. The malolactic conversion requires the presence of manganese (Mn+2) and the availability of NAD+ and generally occurs after the exponential growth phase of the organism.

ML_conversion

The conversion of malate to lactate has several consequences for wine production. First the fixing of a proton on lactate rather than malate leads to a decrease in the free hydrogen ion content and an increase in pH. This change in pH can in turn impact aging rates and potential of the wine. The conversion of malate to lactate can alter the perception of the acidity, depending upon the matrix. Lactate may be perceived as less tart and more sour than malate. The growth of the malolactic bacteria can lead to the formation of other compounds that have a flavor-impact such as the buttery diacetyl and some off-characters. The consumption of nutrients by the bacteria renders the wine more microbially stable.  If the malolactic fermentation occurs in the bottle the wine will be cloudy from the bacterial growth and contain some carbon dioxide or be effervescent. Both of these may be considered defects by the consumer. Therefore it is desirable to have the malolactic conversion occur prior to bottling.