Dessert wines and fortified wines aren’t mainstream beverages that you want to drink every day. Some dessert and fortified wines are much higher in alcohol than regular wines, and some of them are extremely sweet (and rare and expensive!). They’re the wine equivalent of really good candy.
Many wines enjoyed before dinner, as apéritif wines, or after dinner, as dessert wines, fall into the category of fortified wines (called liqueur wines by the European Union, or EU). Fortified wines all have alcohol added to them at some point in their production, giving them an alcohol content that ranges from 16 to 24 percent.
The point at which alcohol is added determines whether the wines are naturally sweet or dry :
When fortified with alcohol during fermentation, the wines are sweet, because the added alcohol stops fermentation, leaving natural, unfermented sugar in the wine. Port is the classic example of this process.
When fortified with alcohol after fermentation (after all the grape sugar has been converted to alcohol), the wines are dry (unless they’re subsequently sweetened). Sherry is the classic example of this process.
Some dessert wines don’t have added alcohol. Their sweetness occurs because the grapes are at the right place at the right time — when noble rot strikes. Noble rot is a fungus that infects ripe grapes in late autumn if a certain combination of humidity and sun is present. This fungus dehydrates the berries and concentrates their sugar and their flavors. The wine from these infected berries is sweet, amazingly rich, and complex beyond description. It can also be expensive at $100 a bottle or more.
Other dessert wines are sweet because winemakers pick very ripe (but not rotten) grapes and dry them before fermentation to concentrate their juice, or they let the grapes freeze on the vine in early winter. When the frozen grapes are harvested and pressed, most of the water in the berries separates out as ice. The sweet, concentrated juice that’s left to ferment makes a luscious sweet wine called Eiswein (literally, ice wine).