The original reason for fortifying wine was to preserve it. Alcohol is a natural preservative, and when added before the fermentation of wine is complete, it kills the yeast and leaves residual sugar behind. The end result is a wine that is both sweeter and stronger, normally containing about 20% alcohol by volume (ABV). Even though other preservation methods exist, fortification continues to be used because wine drinkers took a liking to this kind of wine.
By law, the term “fortified wine” is not permitted to appear on wine labels in the United States; consequently, these wines are usually called dessert wines. In Europe, they are called liqueur wines or fortified wines.
Under European legislation, Liqueur wine is a fortified wine with an actual alcoholic strength by volume of not less than 15 % vol. and not more than 22 % vol.; and an overall alcoholic strength by volume of not less than 17,5 % vol., except for certain quality liqueur wines. It must be obtained from grape must in fermentation or wine or a combination of the two. The added neutral alcohol must be of vine origin.
Apart from being an alcoholic drink in its own right, it used as wine base for alcoholic beverages such as cream liquors, RTDs ("alcopops"), as well as for health tonics and as flavouring for a wide range of food stuff including ice cream.
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 after the end of fermentation or soon before the end.
In the case of some fortified wine styles (such as late harvest and botrytized wines), a naturally high level of sugar will inhibit the yeast; this causes fermentation to stop before the wine can become dry.
Although grape brandy is most commonly added to produce fortified wines, the additional alcohol may be derived from a variety of other sources including sugarcane, sugar beets, and even petroleum. Regional appellation laws 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 wine.It is very popular amongst the poor. Spirits produced in a pot still tend to contain more impurities (flavor congeners), and these can impart distinct intense flavors. Most producers use neutral spirits and will use spirits produced with a continuous still.