Pour a glass of fino. Look at the pale straw colour, smell the almond and the saline apple peel. What you taste is not wine alone, but the work of a living layer of yeast that has sat on the surface inside the cask for months. That is flor sherry: a white veil of yeast cells that gives fino and manzanilla their character, and protects the wine from oxidation as long as it stays alive.
What is flor? A living yeast layer on wine
Flor (Spanish for “flower”) is a biofilm. A thick, white to greyish-white layer of yeast cells that forms spontaneously on the surface of still wine inside 600-litre American oak butts. The Spanish call it velo de flor, the “veil of flor”. The cells stick together via cell-wall proteins and form a floating mat several millimetres thick.
That mat is not scum. It is an organised microbial colony that pulls oxygen from the air chamber above the wine and does metabolic work in the wine below. Without flor there is no biological aging, and without biological aging there is no fino, no manzanilla, and no fino phase of an amontillado.
In Marco de Jerez, the triangle between Jerez de la Frontera, Sanlúcar de Barrameda and El Puerto de Santa María, this process has shaped local production since the seventeenth century. What was long thought to be luck is in fact a precise microbiological balance.
Which yeasts? Saccharomyces beticus and family
The yeast that forms the veil is no longer a separate species in modern taxonomy. What was once described as Saccharomyces beticus turns out to be, after DNA analysis, a collection of races within Saccharomyces cerevisiae, the same species that ferments bread and beer.
Classical sherry microbiology recognises four flor races:
- S. cerevisiae race beticus, dominant in Jerez, forms thick veils
- S. cerevisiae race montuliensis, typical of Montilla-Moriles and certain Jerez bodegas
- S. cerevisiae race cheresiensis, smaller populations, often part of mixed flora
- S. cerevisiae race rouxii, formerly described as a separate species
Recent molecular work since 2015, by groups led by Legras, Fernández-Espinar and Eldarov among others, has used microsatellite analysis, MLST and whole-genome sequencing to show that flor populations are genetically far more diverse than classical culture studies suggested. Each bodega effectively carries its own mix of strains, and that signature seems to help explain why finos from different houses remain recognisable, even within the same DO.
How does flor survive? The feeding cycle in the cask
Flor does something unusual. The yeast does not eat sugar. By the time the veil establishes, the base wine has fully fermented to dryness and then been fortified to about 15 percent alcohol. Residual sugar is close to zero. Yet the yeast keeps growing. How?
Flor switches to aerobic metabolism and uses three main food sources that are still in the wine:
- Ethanol as a carbon source, oxidatively burnt off
- Glycerol, the sweet by-product of the first fermentation
- Acetaldehyde, partly produced and partly recycled in cycles
Glycerol levels in fino drop visibly during biological aging, sometimes from 6 to 8 grams per litre down to under 1 gram. That loss of glycerol is one reason fino tastes so dry and angular on the palate, while an oloroso, which ages without flor, retains its glycerol and stays fuller.
Meanwhile the yeast produces acetaldehyde from ethanol, and that compound defines the smell we recognise as classic fino.
Why 15% ABV? The alcohol limit of flor
Flor is a specialised survivor, not a miracle. The cells function optimally at around 15 to 15.5 percent alcohol. Above that, the cell membrane gets damaged. Above roughly 17 percent the yeast dies completely within weeks.
That single fact explains the entire logic of sherry fortification. Adding grape spirit after the first fermentation is not a flavour choice but a microbiological switch:
- Fortify to ~15% → flor survives → biological aging → fino, manzanilla
- Fortify to ~17% → flor dies immediately → oxidative aging → oloroso
An amontillado starts as fino under flor and is later topped up to 17 percent, killing the flor and letting oxidative aging take over. The result carries both signatures, first the acetaldehyde of flor, then the nuttiness and caramel of oxidation.
This threshold is also why home experiments to “grow flor” on a regular 12 percent white wine almost always fail: too little alcohol to suppress competing microbes, too much sugar and other food, so flor never becomes dominant.
Sanlúcar vs Jerez: why flor lives differently
The three sherry towns are close together as the crow flies, but the microclimate differs enough that the flor behaves differently. That is the whole difference between fino and manzanilla.
Sanlúcar de Barrameda sits at the mouth of the Guadalquivir, right on the Atlantic coast. The poniente sea wind keeps the bodegas cool and humid all year. There the veil grows thick and continuous, with no real seasonal cycle. Manzanilla ages under a constant, dense layer of yeast and develops a more pronounced saline-almond character, with the trademark “sea breeze” note that fans recognise instantly.
Jerez de la Frontera lies about 25 kilometres inland, with a more continental climate. There the flor goes through four seasons:
- Spring: thick, vigorous veil (warmth plus enough humidity)
- Summer: thinner, sometimes patchy (too hot and dry for optimal growth)
- Autumn: second growth peak
- Winter: stable, moderate veil
A fino from Jerez therefore allows just slightly more room for light oxidative hints during the summer months, while a Sanlúcar manzanilla spends the entire cycle under a dense veil. The same difference shows up in a careful tasting: line up a fino and a manzanilla side by side and the contrast does not come from the grape (both Palomino) or the fortification (both ~15%), but from how the flor has lived.
What does flor do to flavour? Acetaldehyde and salinity
The dominant aroma compound flor produces is acetaldehyde (ethanal). That compound smells consistently of green apple, apple peel, bitter almond and freshly cut wood. In normal wine, makers avoid acetaldehyde, because too much of it tastes oxidised. In fino it is the wanted signature.
Concentrations in fino climb to 300 to 500 milligrams per litre, ten to twenty times higher than in a regular white wine. Yet we do not perceive it as oxidative, because the rest of the flavour profile (dry, mineral, saline) is built around it and the flor actively shields the wine underneath.
Flor also builds other compounds:
- Sotolon (in low concentrations) → notes of nut, walnut and light spice
- Lactones from the oak interaction → coconut, vanilla
- Lowered glycerol → angular, dry mouthfeel
- Heightened salinity perception → mineral, “breeze”, iodine-like in manzanilla
The “saltiness” of a good manzanilla is not actually sodium chloride from sea air. It is a complex sensory effect produced by the combination of acetaldehyde, dry mouthfeel and low residual sugar, which together evoke a saline impression.
The puente: why butts are filled to only 5/6
A sherry cask is not a normal wine barrel. It is a 600-litre American oak bota, and it is deliberately not filled to the brim. The wine reaches about five-sixths of capacity, leaving roughly 100 litres of air space above. That space is called the puente (“bridge”) or sobretabla.
That gap is not a leftover. It is essential:
- Flor needs oxygen to respire aerobically, drawn from the air chamber
- The wine surface in a butt lying on its side is roughly 0.5 to 1 square metre, enough for a full biofilm
- The stave wood is porous enough to refresh the air chamber slowly, without letting the wine dry out
As long as the veil lives, it works as an antioxidant layer. The flor itself burns up oxygen in the air chamber and the top wine layer, so the wine below stays paradoxically reduced. A fino aged five years under flor tastes fresher than a white wine kept five years in a sealed tank.
Once the flor dies (through exhaustion, alcohol that is too high or temperatures that are too low), the system flips. The same air chamber now becomes an oxidation accelerator, and the wine shifts towards an amontillado or palo cortado profile. That is why bodega management is largely flor management: keep casks at the right temperature, refresh through the solera system on time, and watch the alcohol level.
The fact that fino looks so light in the glass and tastes so alive is no accident. It is the result of a biofilm that has lived on the wine year after year, burnt ethanol and consumed glycerol, in a cask that is intentionally not full. Understand flor, and you understand why sherry stands apart from every other wine.
Sources
- Martínez, P. et al. (1997). Velum formation by flor yeasts isolated from sherry wine. American Journal of Enology and Viticulture, 48(1), 55, 62.
- Esteve-Zarzoso, B. et al. (2001). Yeast population dynamics during the fermentation and biological aging of sherry wines. Applied and Environmental Microbiology, 67(5), 2056, 2061.
- Fernández-Espinar, M. T. et al. (2000). RFLP analysis of the ribosomal internal transcribed spacers and the 5.8S rRNA gene region of the genus Saccharomyces. Antonie van Leeuwenhoek, 78(1), 87, 97.
- Legras, J.-L. et al. (2014). Flor yeast diversity and dynamics in biologically aged wines. Frontiers in Microbiology, 5, 416.
- Eldarov, M. A. et al. (2018). Genomics and biochemistry of Saccharomyces cerevisiae wine yeast strains. Biochemistry (Moscow), 83(13), 1650, 1668.
- Moreno-Arribas, M. V. & Polo, M. C. (Eds.) (2009). Wine Chemistry and Biochemistry. Springer, chapter on biological aging of sherry.
- Consejo Regulador del Vino de Jerez. Reglamento DOP Jerez, Xérès, Sherry y Manzanilla, Sanlúcar de Barrameda. Accessed 2026.
- Jeffs, J. (2004). Sherry (5th ed.). Mitchell Beazley. Classic reference work on Marco de Jerez.