Vin doux naturel

Fortified wine is a wine to which a distilled spirit, usually brandy, is added. Many different styles of fortified wine have been developed, including Port, Sherry, Madeira, Marsala, Commandaria wine and the aromatised wine Vermouth.

One reason for fortifying wine was to preserve it, since ethanol is a natural antiseptic. However, since the fortified wine-style started in about 1650, it is quite unlikely that this was the original reason for it. It is not until about 1850 that antiseptic action and even the function of yeasts were discovered. It is much more likely that the start of fortification was a fortunate accident. Even though other preservation methods now exist, fortification continues to be used because the process can add distinct flavors to the finished product.

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. For example, in the U.S. only spirits from grapes may be used.

The source of the additional alcohol and the method of its distillation can affect the flavour of the fortified wine. If neutral spirit is used, it will usually have been produced with a continuous still, rather than a pot still.

When added to wine before the fermentation process is complete, the alcohol in the distilled beverage 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).

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Wikipedia