Port wine (also known as Vinho do Porto, Porto, and usually simply port) is a Portuguese fortified wine produced exclusively in the Douro Valley in the northern provinces of Portugal. It is typically a sweet, red wine, often served as a dessert wine though it also comes in dry, semi-dry, and white varieties. The wine produced is then fortified by the addition of a neutral grape spirit known as aguardente in order to stop the fermentation, leaving residual sugar in the wine, and to boost the alcohol content. The fortification spirit is sometimes referred to as brandy but it bears little resemblance to commercial brandies. The wine is then stored and aged, in barrels stored in a cave, before being bottled. Over a hundred varieties of grapes are sanctioned for port production, although only five (Tinta Barroca, Tinta Cão, Tinta Roriz (Tempranillo), Touriga Francesa, and Touriga Nacional) are widely cultivated and used. Touriga Nacional is widely considered the most desirable port grape but the difficulty in growing it and the small yields cause Touriga Francesa to be the most widely planted grape. White ports are produced the same way as red ports, except that they use white grapes—Donzelinho Branco, Esgana-Cão, Folgasão, Gouveio, Malvasia Fina, Rabigato and Viosinho.