Sugar converts to alcohol during the fermentation stage of winemaking. Wine grapes already contain sugar in varying concentrations, but certain wines, such as Rieslings, require additional sugar to reach peak fermentation. Sugar is also added to increase alcohol content or to mask bitterness. Unintentionally sweet wine is, in most cases, the result of a stuck fermentation, when the yeast in the must is weakened by sugar, resulting in sweet wine and low alcohol content. In the opposite case -- high alcohol, sweet wine -- the yeast is used up, indicating that too much sugar was added during fermentation. A few simple tricks will reduce sweetness and rescue your wine.
Float your hydrometer on the surface of the must. The hydrometer approximates sugar levels in the must by determining the alcohol potential of the juice. Readings of 13 percent or higher on the potential alcohol scale indicate initial conditions where it's likely the sugar will overwhelm the yeast and stop fermentation.
Shake the must while moving it from the fermenter into a new container. Mix up a fresh yeast starter and allow it to develop in the fermenter.
Pour an equal portion of your must into the fermenter and mix with the yeast. Once fermentation is underway, add another equal amount of must. Continue adding equal quantities of must until your fermentation fully resumes.
Sweet Wine, High Alcohol
Measure your must's potential alcohol content. A reading of high alcohol and sweet wine indicates too much sugar was added, depleting the yeast. This is different from the low-alcohol stuck fermentation, where the sugar weakened or stunned the yeast.
Add dry wine or water gradually to reduce sugar levels in the must. Take multiple hydrometer readings until sugar levels fall.
Pour additional juice into the fermenter, if required. Adding too much water will dilute the acidity of the must; monitor and adjust for this by adding more juice.