Why are olorosos fortified to 17% ABV?

Olorosos are fortified to 17% ABV primarily to initiate oxidative aging, as the yeast known as flor, which typically protects the wine from oxidation during the aging process in styles like Fino and Manzanilla, cannot survive in higher alcohol concentrations. Once the alcohol level exceeds approximately 16%, flor is no longer viable, allowing for exposure to oxygen, which is essential for the development of the rich, complex flavors characteristic of Oloroso sherries. This oxidative aging can greatly enhance flavors and aromas, developing nuttiness, richness, and complexity that define the Oloroso profile. Fortifying to this level ensures that the wine undergoes this specific aging process, distinguishing it from other styles that rely on flor for a different aging experience.