Although grape brandy is most commonly added to produce fortified wines, the additional alcohol may also be neutral spirit that has been distilled from grapes, grain, sugar beets, or sugarcane. Regional appellation laws may dictate the types of spirit that are permitted for fortification.
The source of the additional alcohol and the method of its distillation can affect the flavor of the fortified wine. If neutral spirit is used, it will usually have been produced with a continuous still, rather than a pot still.
During the fermentation process, yeast cells in the must continue to convert sugar into alcohol until the must reaches an alcohol level of 16%–18%. At this level, the alcohol becomes toxic to the yeast and kills it. If fermentation is allowed to run to completion, the resulting wine will (in most cases) be low in sugar and will be considered a dry wine. The earlier in the fermentation process that alcohol is added, the sweeter the resulting wine will be. For drier fortified wine styles, such as sherry, the alcohol is added shortly before or after the end of the fermentation.
In the case of some fortified wine styles (such as late harvest and botrytized wine), a naturally high level of sugar will inhibit the yeast. This causes fermentation to stop before the wine can become dry.