Flor is a strain of the yeast used in winemaking (saccharomyces cerevisiae) that can emerge on the surface of a wine once all the sugar has been converted to ethanol. Without any sugar to convert, the flor yeast metabolises carbon from the alcohol and creates acetaldehyde and acetic acid (and other compounds). Very specific conditions are required for flor yeast to grow and it is most famously found in Spain in Sherry production. In Fino Sherries, the wines are matured under a layer of Flor yeast and this gives the wines their distinctive nutty and briny character (sometimes described as ‘aldehydic’).
