Conversion of malic acid into lactic acid
Its progress can be easily analysed and controlled via paper chromatography.
The progress of malolactic fermentation (beginning, duration) varies widely according to the circumstances.
It can begin immediately after alcoholic fermentation and be completed in less than five days.
Or, sometimes it will take several weeks before beginning and decomposition of the malic acid can take several more weeks. In certain cases, malolactic fermentation does not take place at all and the wines are distilled without it. In order for malolactic fermentation to start, there must be a phase of bacterial growth.
Caused by bacteria belonging to the species Oenococcus oeni
Malolactic fermentation is carried out by bacteria belonging to the species Oenococcus oeni. These bacteria are present in the must and, in principle, only grow once alcoholic fermentation is completed.
At the end of alcoholic fermentation, yeast cells begin to die, ceasing to produce compounds that inhibit the bacteria and providing nutrients that will enable the bacteria to multiply.
It is essential that alcoholic and malolactic fermentation not overlap.
Premature bacteria growth in the presence of unfermented sugars would damage quality.
Again, perfect control of the alcoholic fermentation—and thereby the metabolism of the yeast—is decisive.
Malolactic fermentation: Mechanism for regulating residual acetaldehyde in the wine
Malolactic fermentation is not mandatory.
However, this process is often recommended because, as mentioned earlier, it yields somewhat rounder eaux-de-vie and reduces the level of acetaldehydes which may have been produced in too large quantity by the yeast.
Improved stability for wines awaiting distillation
Moreover, once this conversion has taken place, the wine being stored before distillation is more microbiologically stable (its malic acid, a primary substrate for numerous bacteria species, has been consumed).
Because of this waiting period and the absence of sulphur additions, the Cognac region must rely on acidic must, as the pH is an essential factor in natural preservation.