In many instances, residual sugars are still present in press wines. Some intact berries will remain in the pomace and the sugars they contain are released during pressing. This is even more likely when whole bunches are vinified.
If only a few grams of sugar remain, light sulphiting at 0.5 g/hl is possible to slow down lactic bacteria development, which then enables the yeast to finish breaking down the sugar. Volatile acidity must be carefully monitored to avoid any problems.
If the quantity is higher, it is not usually a problem for the yeasts to return to alcoholic fermentation and finish breaking down the sugar. Sulphiting is then superfluous.
In some cases, if the residual sugar content does not go down, yeast has to be added to the wine using a starter vat. Temperature control and the addition of nutrients are then necessary for this procedure to succeed.