When fermentation has finished, the wine has reached a minimum of 13.5% alcohol; the wine is racked into 500 liter casks, but they are not filled to the top as they would be in almost any other wine region of the world. Inside the cask an unusual biological aging begins under what is known as the "veil of flor," a white cap resembling foam which forms on the surface of the wine. However, for this to occur the wine must possess between 15% and 17.5% alcohol, so in Jerez and Manzanilla the winemaker fortifies it with neutral grape brandy; in Montilla-Moriles this higher level of alcohol is reached naturally during fermentation as the Pedro Ximenez grape ripens to a higher level of sugars than Palomino. The cap of flor only forms in the very particular climate of the southwest of Andalucia; humidity is a fundamental factor, and the sherry casks are left open inside the bodega to promote flor growth. For the same reason the bodegas are not cellars but are instead at ground level. Flor is actually a form of yeast; it absorbs any remaining sugars in the wine while lowering volatile acidity and glycerine. At the same time it also increases aromatic esters and aldehydes that give sherry its characteristic aromas.
Each wine will become quite different according to their individual evolutions in the cask. Here the winemaker has many different classifications to choose from, deciding which will become the finest and most elegant wine. Those with an abundance of flor are destined to become "fino" sherries, but may become classified as amontillados instead, depending on their future aging. The casks which do not develop enough flor, or whose quality is otherwise insufficient, are used to make olorosos; they are fortified again up to 18% alcohol (flor can not survive at more than 17.5%) and are aged in separate casks.