Malolactic Fermenation

In many cases the most important consequence of the malolactic conversion is the reduction of acidity that brings the acid content of the wine in balance with the ethanol level. Immobilized enzymes and immobilized cell systems have been developed that can result in the conversion of malate to lactate without need to grow or cultivate the lactic acid bacteria. The yeast Schizosaccharomyces pombe can ferment malate to ethanol and use of this organism as an acid reducer in wine has likewise been explored.

The malolactic fermentation can be monitored microscopically by looking for the presence of bacteria and taking care to know that the bacteria seen are lactic acid bacteria and not acetic acid bacteria or bacilli. The cell morphology seen can sometimes be used to determine if Lactobacillus, Oenococcus and/or Pediococcus is present. The malolactic fermentation can also be monitored by assessing changes in the levels of acid species present.

The successful progression of the malolactic fermentation depends in large part upon decisions made during the alcoholic fermentation.  Some yeast strains are more inhibitory of the lactic acid bacteria than other strains either because they make more inhibitory compounds or are less likely to lyse and release nutrients post-fermentation. If the malolactic conversion is desired then a yeast strain known to be compatible with the malolactic fermentation should be used.

The malolactic conversion may occur spontaneously before the yeast fermentation, may occur during the yeast fermentation or may happen well after the yeast fermentation is completed. If the juice has a sufficiently high pH and is held at warm temperatures (above 18°C/64°F) and no sulfite is used, the wild lactic acid bacteria present on the grapes may initiate growth and the malolactic conversion before the yeast are able to start fermentation. The conversion of malate to lactate may be completed before the onset of the alcoholic fermentation.

Inoculation with lactic acid bacteria as with yeast can be accomplished using an active dry preparation or by generating a starter inoculum pregrowing the lactic acid bacteria under permissive conditions, juice + wine mixture inoculated with an active malolactic fermentation. If a dried preparation is used, this can often be used directly without need for rehydration of the cells. If an inoculant is grown it is important to monitor what has actually grown up in the starter.

As with the yeast fermentation the malolactic fermentation may be spontaneous and caused by winery resident bacteria or those coming in on the grapes. Alternately commercial malolactic inocula may be used to initiate and conduct the malolactic fermentation. Commercial preparations are generally of Oenococcus oeni(formerly called Lecuonsotoc oenos). Although the direct consequences of the malolactic conversion are the same regardless of the organism involved, the spectrum of other flavor-active compounds produced may differ.

Several factors impact the initiation and progression of the malolactic fermentation. Temperature, pH, acidity, ethanol, sulfite and availability of nutrients are all important for the growth and metabolic activities of the lactic acid bacteria. The lactic acid bacteria are more fastidious in their growth requirements than the yeast. It can be challenging to get the malolactic conversion to occur at the desired time in the wine.

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.