Fortification is one of the oldest methods of preserving wines so that they can be stored for long periods or for transportation. More than 4000 years ago Phoenician traders fortified wines to stabilise them before shipping them throughout the Mediterranean. Fortification involves the addition of a spirit or alcohol solution which raises the alcohol level of the wine to between 15% and 20% v/v. This precludes further microbial action so that not only does it stop yeast fermentation but also inhibits bacterial oxidation - very useful in the days before refrigeration and stainless steel containers.
The stage at which the spirit is added and the types of spirit used have a significant influence on the style of wine made. For vintage port and fino sherry styles where the purity of the fruit is to be highlighted SVR (spiritus vini rectificatus - spirit rectified from wine) is added which is almost (96%) pure spirit. For most other fortified wines brandy spirit is added which still contains 'impurities' that add further character and flavour to the wine.
Whether fortification occurs during or at the end of fermentation determines whether it is dry, semi-sweet or sweet. With Rutherglen Muscat or Topaque (Tokay) for example, fortification occurs after only a few days' fermentation so that considerable sugar remains giving them their luscious sweetness, whereas dry sherry is fortified after the initial fermentation is complete. After fortification the wine is left to mature in seasoned oak barrels, sometimes for decades, resulting in complex, aromatic wines with immense depth and concentration of flavour and colour. Vintage port and fino sherry styles are again the exception spending a maximum of two years in barrel before bottled - in the case of sherry to provide freshness - while vintage ports can continue to mature in bottle for decades unopened.
Resultant styles also depend upon the grape variety used. In Spain Palomino is used to make fino, amontillado, manzanilla and oloroso seco styles, whilst Pedro Ximenez makes gloriously sweet, concentrated sherries. On the island of Madeira the four styles it is famous for:- Sercial, Verdelho, Bual and Malmsey (Malvasia) are named after the varieties used. Marsala from Sicily uses the Cataratto Bianco or Grillo varieties.
Portugal is home to the two most famous fortified styles using red grapes - tawny and vintage ports. The port trade with Britain dates from the 17th century when it was at war with France and deprived of French wine. The English palate also tended towards sweet wine which can be more easily achieved with the fortification that helped preserve wines on the voyage from Porto and Bristol. Over a hundred different varieties are approved for port production in the Douro valley but the main varieties are Touriga Nacional, Tinta Cao, Touriga Francesa, Tinta Roriz (Tempranillo) and Tinta Barocca. Fortified wine can no longer be labelled as 'port' unless it comes from Portugal.
In Australia 'port' style wines generally use Grenache, Shiraz or Mourvedre (Mataro) while the incomparable fortifieds from Rutherglen use Muscat a Petit Grains for Muscat and Muscadelle for Topaque (Tokay). Australian 'sherry' style wines are now called Apera.