Flor is a naturally occurring layer of yeast (primarily Saccharomyces cerevisiae) that forms on the surface of certain Sherry wines, creating a protective white film that prevents oxidation while imparting distinctive flavors and aromas to the wine beneath. This beneficial yeast thrives in the unique conditions of Jerez's bodegas—specific humidity levels, temperature ranges, and the fortification to around 15-15.5% alcohol—and is essential for producing Fino and Manzanilla Sherries, which develop their characteristic fresh, saline, and nutty flavors through years of contact with the flor. The flor acts as a living blanket that consumes oxygen, glycerol, and acetic acid while producing aldehydes and other compounds that give these Sherries their bone-dry character and complex, yeasty aromas that distinguish them from oxidatively aged Sherries like Oloroso.