Aging oxidative, oloroso wines

Oloroso wines are only obtained by oxidative aging, which is accomplished by fortifying the initial wine to an ethanol content of about 18 vol.% in order to prevent growth of flor yeasts. Under such conditions, oloroso wine acquires a dark colour by effect of the oxidation of phenol compounds. 

Amontillado wines are obtained in a two-step process involving biological aging under the same conditions to those of fino wines, followed by fortification and oxidative aging as in oloroso wines. Amontillado wines are thus the oldest and most valued of the three sherry types by virtue of their acquiring a more complex flavour than the other two. 

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