Term (also flor yeast) for special yeasts of the species Saccharomyces bayanus, capensis, cheriensis, fermentati, montuliensis and rouxii. These are able to convert the sugar into alcohol in the first anaerobic phase (without oxygen) of fermentation. The flor yeasts can then switch the metabolism to aerobic (with oxygen), so that a waxy, initially white coating is formed from alcohol and oxygen, which coats the individual yeast cells. This causes the yeast cells to rise to the surface and form a centimetre-thick layer (film), which becomes increasingly brown in colour.
This protects the wine from air and prevents further oxidation. This requires nutrients, which is achieved by adding small quantities of wine from young vintages. In this way, the yeast culture can be kept alive for years. In Spain, this phenomenon is utilised in the Solera system in the production of the Fino and Manzanilla sherry varieties. The resulting flavours give the typical nutty aroma. An undesirable form of flor is called cream, which converts alcohol into acetic acid and acetic acid ethyl ester.
Process
However, the process can only take place at 14 to a maximum of 16% alcohol by volume. At higher alcohol contents, the flor yeasts die, which is deliberately controlled by spriting (adding alcohol) in the Oloroso sherry variety. The process is favoured by the special Andalusian climate. The dry air with very low humidity causes so much water to evaporate through the wooden barrels that the alcohol content is optimised. Below 14% vol. the wine would turn to vinegar. The dead cells slowly sink to the bottom of the barrel during the ageing process and are known in Spain as the "wine mother". The French Vin Jaune is also produced under similar conditions in the Jura region.
Further information
For information on the production of alcoholic beverages, see Champagne (sparkling wines), Distillation (distillates), Speciality wines, Spirits (types), Winemaking (wines and wine types) and Wine law (wine law issues).
Picture left: By El Pantera - Own work, CC BY-SA 3.0, Link
Image right: Riedel glass, Sommeliers series
