A fortified wine is one that receives an addition of brandy or a distilled spirit, such as cognac, during its production. This process fortifies the alcohol content of the wine.
In addition, the brandy or cognac has the power to kill the yeasts in the wine, interrupting the fermentation process. This means that some of the sugar that would be transformed into alcohol is still present in the drink, giving the wine a sweeter taste.
This process came about many years ago with one purpose in mind: to improve the preservation of wine by being able to store the bottle for much longer.
Even with the use of technologies to improve the preservation of the drink, fortified wine continues to be made because of its distinctive and attractive flavors.
Port wine is probably one of the best known fortified wines. Another Portuguese fortified wine is Madeira wine, which has the particularity of being an oxidized wine and subject to higher than normal temperatures, due to its aging in a stonemasonry system.
Another famous example is Spanish sherry wine. Curiously, this wine is made from white grapes, unlike Port and Madeira.
The process of fortifying sherry is also different, as in some cases it is done after fermentation. This is why sherry is a little drier than other fortified wines.