Flor: the living film on Sherry

Biological ageing under flor is one of the genuinely distinctive things in winemaking, and it is almost entirely confined to a small area of Andalusia and a few other places, notably the Jura.

It is also the mechanism that determines whether a Sherry becomes a Fino or an Oloroso, which makes it the fork in the road for the whole category.

After fermentation, wines destined for biological ageing are fortified to around 15–15.5% alcohol. That is the window: flor cannot establish much above that, and much below it the wine is vulnerable to spoilage.

The yeast forms a film — the velo de flor — across the surface, several millimetres thick, in butts deliberately left part-empty.

The film consumes oxygen, ethanol, glycerol and acetic acid, and it produces acetaldehyde. Consuming glycerol is why Fino tastes so lean; producing acetaldehyde is why it smells the way it does.

Crucially, the film also acts as a physical barrier, so the wine beneath it does not oxidise. Biological ageing and oxidative ageing are opposite processes, not degrees of the same one.

Flor thickness varies through the year with temperature and humidity, and the Sanlúcar de Barrameda coastal climate sustains a thicker, more persistent film — which is why Manzanilla differs from Fino made a few miles inland.

If the film dies or is deliberately suppressed by further fortification, the wine begins to oxidise and becomes an Amontillado — which is exactly what an Amontillado is: a Fino that continued oxidatively.

It explains the entire Sherry style map, and it explains why Fino must be treated like white wine — refrigerated, and drunk within days of opening — while Oloroso keeps for weeks.