By Harold McGee
Fortified wines are so called because the strength of the base wine is boosted by the addition of distilled spirits to 18–20% alcohol, a level that prevents spoilage by vinegar bacteria and other microbes. Fortification appears to have begun in the sherry-producing region of Spain sometime before 1600. Winemakers take advantage of the stability of fortified wines by exposing them to the air for months or years. They thus make a virtue of the normally undesirable oxidative changes in flavor that come with keeping leftover wine. Most fortified wines keep well for weeks in an opened bottle or decanter.