Technique
Flor
Living yeast veil of Saccharomyces strains that forms on fino and manzanilla and shields the wine from oxidation during biological ageing.
What flor is
Flor is a visible white to grey-white yeast veil that grows on the surface of a fino or manzanilla once the wine reaches 15 percent alcohol and sits in a partly filled butt. The layer comes from four main strains of Saccharomyces cerevisiae (beticus, cheresiensis, montuliensis, rouxii), which knit themselves into a resilient floating mat. Under a microscope the cells form long chains with tiny air pockets in between.
The yeast consumes oxygen, residual sugar, glycerol and some alcohol. In return it produces acetaldehyde, esters and glycoproteins. The wine stays pale because the yeast blocks oxygen, and it develops its signature notes of fresh bread, green almond and sea air. Without flor the same base wine would track toward oloroso.
How flor lives and dies
The yeast thrives at 15 to 16 degrees Celsius and around 70 percent humidity, conditions that hold year-round in Sanlúcar de Barrameda thanks to the Atlantic. Jerez and El Puerto run hotter and drier in summer, so flor thins in July and August and rebuilds in cooler months. Bodegas counter this with high ceilings, earthen floors that release moisture, and shaded courtyards.
Flor feeds on the wine but also receives help from the cellar. Butts are never filled past five sixths to keep an air gap above the surface, and younger wine is added each year to replenish sugar and nutrients. Above 15.5 percent alcohol growth halts. Cellar masters use that threshold deliberately when they want to kill the flor and tip a butt into amontillado or palo cortado.
Where flor is misread
The line that flor “just happens” if you leave a butt half full is not quite right. Outside Marco de Jerez, parts of Montilla, and the Jura in France, flor barely grows. The combination of regional climate, ambient yeast flora that has colonised the bodega walls for centuries, and the specific Palomino base creates a flor-friendly ecosystem. Producers elsewhere who have tried to copy the method usually end up with thin, patchy veils that fail to deliver consistent style.
Flor is a terroir element, not a recipe portable to any cellar.
In practice
On a bodega visit, when the cellar hand draws a venencia sample, you often catch the flor layer breaking on the surface. It is yeast, not mould, and the aroma straight out of the butt (bread dough, marine air, fresh almond) usually reads sharper than what reaches the bottle. The London sherry trade has pushed flor as a selling point since the en rama bottling format took off around 2010.