This process is repeated with the remaining rows, though no more than one-third of each cask can be drawn off every three months. With the solera system, Brandy de Jerez bottled today will contain brandy that is decades old, perhaps even dating back to before the turn of the century. The final factor in the maturing of Brandy de Jerez is the use of Sherry barrels for aging. Usually, these oak butts, or botas , measure 500 liters in volume and are made of American oak, which is more porous than French or Spanish oak. This allows more rapid oxidation to take place, which softens the wine.
Additional oxidation is achieved when the barrels are topped with new brandy.
Sweeteners may legally be added, however. There are three levels of Brandy de Jerez, determined by age. Solera Gran Reservas receive 10 to 15 years of oak aging and would thus be comparable to XO Cognacs. America's fondness for cigars and its desire to drink less but higher-quality spirits has changed the makeup of Brandy de Jerez imports, says Julio Baguer, director of Wines of Spain, a government agency that promotes wine and spirits exports. While Solera represented 80 percent of the Spanish brandies imported into America 5 years ago, it now represents only 30 percent.