Alcoholic fermentation (AF) occurs in both barrel and small stainless steel or concrete tanks (between 10 and 50 hectoliters). In general AF takes place at temperatures between 15 and 18°C and can sometime last four or five weeks depending on the lots.
Fermentations are followed by daily checks of density and temperature to assure they are proceeding correctly. The greatest risk in sweet wines is indeed that the fermentation will be stuck before reaching the correct balance of alcohol and sugar.
Stuck fermentations can result from different factors: juice that is too sweet, inhibitor molecules from Botrytis cinerea, low temperature in the cave (AF occurs in November/December) etc,… When a lot slows down (yeasts transforming sugar into alcohol at a slower rate and decreasing temperature of the wine) we maintain the fermentations with small electric heaters placed in the tank.
There is a precise reason for this care: to avoid an increase of the volatile acidity produced by the yeasts and a decrease of the quality of the wine during this critical phase.
As densities approach 1050-1040, we conduct analysis to determine if the “point de mutage” has been reached.
If it has, we proceed to the mutage at low temperature: after stopping the yeast activity by cold (5°C), we rack the wine. Then we add a specific dose of sulfur dioxide (or SO2) to preserve the wine and avoid a secondary fermentation.
The majority of these lots will be transferred to barrels for aging.