Fortified wines, also known as liqueur wines, are so called because they have been fortified (strengthened) by the addition of grape spirit. This addition increases the alcoholic strength and stabilizes the wine.
   The stage at which the spirit is added is critical to the style of fortified wine: if it is added before or during the fermentation process, the wine will be sweeter than if it is added at the end of the fermentation process. This is because the addition of the spirit stops the fermentation.
   It is widely believed that fortified wines can be kept for quite some time after they have been opened. However, this applies only to Madeira and to other wines that are exposed to heat during maturation. Other fortified wines should be treated in the same way as non-fortified wines, especially mature vintage ports. Delicate fresh styles such as Manzanilla and Fino sherry should be consumed as young as possible.
   Sherry is made in and around the town of Jerez de la Frontera in south-west Spain. The main grape variety is Palomino Fino. Pedro Ximenez – often referred to as PX – is also grown and is used mainly for sweetening wines.
   These base white wines are fermented to dryness before the spirit is added. The amount of spirit added affects the growth of a yeast called flor, which strongly influences the style of the wine. The yeasty flavor of flor is most evident in Fino and Manzanilla. Most sherries are between 15% and 18% alcohol by volume. Sweetness levels range from bone dry to very sweet.
   Sherries are matured in what is known as the solera system, which blends wine from different vintages to create a consistent style. This sherry is generally a non-vintage wine. More recently, age-dated wines have been permitted but these are still blends of several vintages. Traditionally drunk as an apéritif, sherry can be surprisingly versatile as a food wine, and starting at around 15% alcohol by volume, it is not much stronger than many table wines.
   Port is made in the Douro Valley in northern Portugal. It takes its name from the city of Oporto, the port from which wines are shipped around the world. Most ports are also aged in the city.
   Port is sweet because the grape spirit is added part-way through the fermentation, thus stopping the conversion of grape sugar to alcohol. It is therefore essential to extract color and tannins as quickly as possible before the spirit is added. This is why grapes were traditionally trodden by foot. Most ports are between 18% and 20% alcohol by volume. With port, it is the way the wines are matured that most influences the style.
   Most ports are red. There are more than 40 authorized black grape varieties but the most important ones are: Tinta Roriz (known as Tempranillo in Spain), Touriga Nacional, Touriga Francesa, Tinta Cao and Tinta Barroca. The main grape varieties in white port are white: Gouveio (Verdelho), Malvasia Fina, Rabigate, Viosinho and Donzelinho.

Port: styles

Madeira is made on the Portuguese-owned island of Madeira in the Atlantic Ocean. These wines are very resilient and long-living thanks to their high acidity and the way they are made. They even keep indefinitely after they have been opened. This is because the wines are exposed to heat during wine-making, so they are effectively 'baked' and thus protected from oxidation. This process is known as estufagem.
   The timing of the spirit addition varies but the better producers tend to add the spirit during fermentation, leaving the natural sweetness from the grapes. Lesser wines are usually fermented dry and then fortified and sweetened as necessary. Madeira may be aged further in casks and then blended or aged in a solera system similar to that used for sherry.
   The main grape varieties are Tinta Negra Mole, Sercial, Verdelho, Bual and Malvasia. Tinta Negra Mole is by far the most widely planted and is used for the less expensive blends that are labeled 'dry', 'medium dry', 'medium sweet', 'rich', or 'sweet'. The better quality and more expensive wines are made from at least 85 per cent of a single grape variety and the name of the variety is shown on the label. Each of the 'noble' varieties is associated with a particular level of sweetness, from driest to sweetest. Common to all Madeiras is their high acidity – much higher than in sherry – and a slightly caramelized flavor due to the heating process.
   Marsala is made on the Italian island of Sicily, around the coastal town of Marsala. It is made from the local grape varieties Catarratto, Grillo and Inzolia, fermented to dryness like sherry, then fortified. Sweet styles result from the addition of boiled down must or mistela (grape juice blended with spirit).
   Marsala is classified by color: ambra (amber), oro (gold) or rubino (ruby). Each color is made in secco (dry), semisecco (medium), and sweet styles. They are also classified into five categories according to how long the wine has been aged in cask. For example, Fino is matured for one year in cask, Superiore for two years, Vergine for five years.

Vins Doux Naturels: methods and styles

These sweet wines are made by adding spirit to partly fermented wine. The literal meaning of the name is 'naturally sweet wines'. White wines are usually made from the Muscat grape variety so have a strong flavor and taste. They include Muscat de Beaume-de-Venise from the Rhône Valley and Muscat de Rivesaltes from Languedoc-Roussillon.
   Chilled, they are good as apéritifs or with desserts. Red wines are usually made predominantly from Grenache, e.g. Maury and Banyuls from Roussillon and Rasteau from the Rhône. These red wines tend to go well with chocolate.