Flor

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)

Info Mine - Yeast

The Wine Mine Blagging Toolkit - 5 wine nuggets with which to impress your friends...

1. The job of yeasts in wine production is to metabolise carbohydrates - that's "turn sugar into alcohol" to you and me. So yeasts are the agents of fermentation.

2. Although the primary purpose of yeasts is fermentation, they can also impart certain flavours to the wine, especially as lees (see 3 below).

3. Once the fermentation gets to about 18% abv, the yeasts die, turning into lees - dead yeast cells. Lees can also be produced (i.e. yeast can die) due to lack of sugar and nutrients. Wines which remain in contact with their lees for a substantial amount of time have a richer, creamier texture and biscuity/bready flavours (they are sometimes called sur lie wines). This is an established technique used in champagne production. Other examples include some white burgundies and some Galician wines.

4. Following lees contact, wines will be cloudy with suspended particles. There are various ways of clarifying the wine, including filtering, racking (allowing the dead yeast cells to sink to the bottom of the tank, then gently pouring the clear wine off) and fining (adding substances which attract solid particles and proteins - traditionally egg whites and ox blood, but also bentonite and synthetic chemicals).

5. In new fermentation vats/wineries, yeasts may have to be added to the must (fermenting grape juice) to kick-start fermentation. In older set-ups there will be enough yeast in the atmosphere for this process to start of its own accord.

(For yeast in sherry making, see post on flor.)