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.
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