1. Flor is a benevolent film-forming yeast which, in a winemaking context, can form on the top of partially-filled barrels of maturing sherry, protecting the wine from oxidising. It is typified by those yeasts native to Jerez.
2. A by-product of the action of flor yeast is the chemical acetaldeyde, which gives fino-style sherries their distinctive "salty" aroma. The presence of acetaldehyde causes the "sherry-like" nose of some unfortified wines which are made using flor or flor-like yeasts.
3. Flor feeds on glycerine, polyphenols, higher alcohols and volatile acidity.
4. It only develops in wine which is between 14.5 and 16.0% abv (and not, as per legend, due to a magical, ineffable quality in the Andalucian air). It forms on all sherry pre-fortification, so even some Oloroso Seco styles may have some flor characteristics.
5. It is used in the production of Fino, Manzanilla, Amontillado and Palo Cortado. In the case of Amontillado, the yeast either die away naturally after consuming all the glycerol, gradually exposing the wine to oxygen, or the wine is moved to a different solera mid-process. Palo Cortado makes use of the second technique only and so can be fuller-bodied than Amontillado.
(Sources: WSET materials, The Oxford Companion to Wine)
