Final answer:
The sweetness of a fortified wine depends on the timing of the fortification during the fermentation. Early fortification results in a sweet wine, while later fortification after all the sugar is fermented yields a dry wine.
Step-by-step explanation:
Whether a wine ends up sweet or dry depends on the point during fermentation that it is fortified. Fortification is the process of adding a distilled spirit, usually brandy, to wine. If wine is fortified early in the fermentation process, this means that not all the sugars provided by the grapes are converted to alcohol by the yeast, as the added alcohol kills the yeast off before they finish this process. As a result, the wine retains some of the residual sugars making it sweet. However, if fortification happens after the yeast has completely consumed the available sugars, the wine will have a dry taste. Therefore, the timing of fortification determines the sweetness of the wine.
In the fermentation of grape juice to produce wine, CO2 is a byproduct. This reaction is typically carried out by yeast under anaerobic conditions, where yeast converts the sugars from the grapes into ethyl alcohol. Ethanol levels can naturally reach around 12 percent before the yeast is inhibited by its toxicity.