Published on September 15th, 2013 | by Ruben


Flor, the mystery of sherry

Flor is the veil or thin layer of indigenous yeast cells that forms on top of sherry wines. It is a kind of Ivory coloured, wrinkled, waxy foam, up to two centimeters thick, that protects the wine from air contact and that can only grow naturally in the specific climate of Southern Spain.

Flor basically divides all wines from the D.O. Jerez-Xérès-Sherry into two main categories: biologically aged sherry (which matures entirely under this layer of flor – Manzanilla and Fino) and oxidative sherry (which matures partially or entirely without flor – Amontillado, Oloroso or Pedro Ximénez). Recently quite a lot of research has been conducted to understand the microbiology of sherry winemaking and the specific nature of flor, especially since the popularity of biologically aged sherries has increased. We’ll try to give a short overview of what you should know.


Sherry winemaking process

The basic process for making biologically aged wines consists of two consecutive steps. The first step is fermenting the must obtained from pressing the grapes (nowadays in tanks, previously in wooden casks). This is done by adding cultured, non-flavour creating yeasts or sometimes a pie de cuba, an already fermenting must. The result is a “young wine” which will have reached a minimum of 13,5% alcohol. In Andalucia, flor will starts to develop on these base wines almost immediately after fermentation – the indigenous yeasts are present in the Andalusian air so it is simply the natural way of winemaking in this region.

In a second step, a qualitative selection is made and the wine is fortified (encabezado) with rectified wine alcohol. For biological sherries, made from the finest and most delicate base wines, this is between 15 and 15,4% of ethanol volume, the ideal strength for a healthy flor. For oxidative oloroso, the wine is fortified to 17% or more. In this case any existing flor will be killed as it cannot survive in this environment. The resulting wine, the sobretabla, is then ready to be poured into the last criadera of a solera.

Two traditional practices are then necessary to support the development of flor in biologically aged sherries. Firstly, barrels are only filled to four-fifths of their capacity (usually even less for a Manzanilla solera). Whereas direct air contact is avoided in almost all other wine regions, the air above the wine allows the biofilm of flor to develop on the wine’s surface. At the same time, the film isolates the wine from the air. Secondly, the Solera principle is essential, as the regular addition of new wine supplements the transfer of nutrients and keeps the flor thriving. In case the flor dies off (either naturally or intentionally), the sherry will have air contact and is then classified as an Amontillado; it will undergo an additional fortification to 17% or more and continue aging in an oxidative way.

The flor is not inert, it interacts constantly with the wine. The yeast cells consume certain compounds found in the wine, and they create others. In other words, the metabolic action of the flor constantly changes the composition of the wine and therefore its final aromas and flavours.

Read more about the sherry production process


flor yeast

Layer of flor at Bodegas Lustau


Climate conditions

Apart from the specific alcohol levels, flor needs a very particular climate to florish. Humidity is a fundamental factor, and the sherry casks are left open inside the bodega to promote flor growth. For the same reason the bodegas are not cellars but are instead at ground level. Moreover, most of them are designed to promote airflow, with high ceilings and specific ventilation windows.

The flor favours cooler climates and higher humidity, so the sherries produced in the coastal Sanlúcar de Barrameda and El Puerto de Santa María have a thicker cap of flor than those produced inland in Jerez. Also, sherry winemakers report a seasonal change in the colour and physical properties of the flor film: it will be thicker in Spring and Autumn.  It is also assumed that different strains of yeast dominate the flor depending on the seasonal changes in cellar temperature.

Note that similar biologically aged wines are produced in other parts of the world: France (Jura – Vin Jaune), Italy (Sardinia and Sicily), Hungary (Tokay), USA (California) and various South African and Australian regions. While these styles are similar to Spanish sherry, they use other yeast strains (whether natural or cultured) and different conditions.


A microbiological view on flor

A glass of Fino with florDuring biological aging of sherry, considerable microbial diversity occurs in the velum that develops on the wine. The four main races of flor yeasts are Saccharomyces Cerivisiae beticus, cheresiensis, montuliensis and (Zygo)Saccharomyces rouxii and they differ from typical fermentative wine yeasts which don’t form a yeast film. S. beticus is the most common strain, found in more than 75% of all biological soleras in the region, especially in the young criaderas. In older criaderas, S. montuliensis will become more prominent as it will develop more slowly and can resist higher levels of acetaldehyde. The other two races are relatively uncommon. Note that each bodega and each casks will have an individual personality and its own specific yeast population.

Although more than 95% of the film will consist of Saccharomyses Cerivisiae strains, other yeasts, fungi or bacteria may occur. Some of them (Debaryomyces, Zygosaccaromyces, Pichia…) are harmless or benificial to the process, increasing the complexity of the wine. Others (Dekkera, Brettanomyces) can lead to abnormal acidity and other unwanted effects.

There is a recent evolution which tries to genetically improve flor yeasts. New strains such as FLO11, SOD1 or MUC1 may facilitate the establishment of a more stable flor layer and shorten ageing times.


The effects of flor

Ageing under flor differs significantly from oxidative ageing. Let’s have a look at the most important effects.

  • Protection of oxygen: flor protects the wine from oxygen while also consuming most of it. This leaves biological sherries much paler and makes its aromatic profile more reductive.
  • Decrease of sugars: obviously Manzanilla and Fino are bone-dry sherries. Note that Oloroso is also naturally dry, but it will appear sweeter due to the presence of glycerol.
  • Decrease of the ethanol metabolism: the flor yeast (especially montuliensis and rouxii) will consume part of the alcohol, up to one degree of alcohol per year if it is not refreshed by the solera system. Dropping below 14 degrees will result in a bota desmayada (fainting cask) which is highly susceptible to bacteria and unwanted yeasts.
  • Decrease of glycerol: flor yeast will use it as a carbon source to support its growth. Therefore, biologically aged sherries will contain virtually no glycerol. As it usually adds weight and body to a wine, Manzanilla and Fino will appear more delicate even at similar strength.
  • Increase of acetaldehydes: up to 1000 milligrams per liter, twenty times more than a typical sobretabla. This will show as a yeasty or chalky aroma, nuts and notes of overripe (cider) apple. It is considered the best marker of biological aging, and it is inhibited mostly by montuliensis and rouxii. Apart from acetaldehyde, flor will also increase the content in other aroma compounds such as higher alcohols, lactones, and terpenes. They will impart other typical flavours like green almonds, salty notes and Mediterranean herbs.


Upon bottling, sherry will be filtered. The industry standard is to take out all individual yeast cells. However sherry that was bottled en rama is typically filtered with a larger filter. The biggest clots of yeast cells will be taken out, but it’s perfectly possible for individual cells to pass through. These kind of bottlings will contain some living flor, but given the small amounts, this is perfectly harmless for consumption. It’s even said to contain antioxidant compounds. In any case, the taste of en rama sherry will be richer and fuller.


We’ve tried to explain why the delicate flor is so important for a good Fino or Manzanilla and adds up to the unique character of sherry wines as a whole. Yet it doesn’t completely explain why it’s such a good match for food, and why it can display such a perfectly bright dryness that is unsurpassed by other types of wine. Perhaps some things are best left unearthed.

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About the Author

fell in love with sherry fifteen years ago, but switched to a higher gear in 2013 and started writing about it. Lived in Madrid for a couple of years, now back in Belgium. I also run a whisky blog over at

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  • Angeliki Tsioli

    Hi Ruben,
    Can you name the exact or approximate levels of humidity that are needed for the production of the flor? I can imagine that a minimum of 60% should be guaranteed and that there is no maximum. The critical level should be the lower one, since the more inland (for example Montilla-Moriles) the less the humidity, thus less appropriate for the flor. Then, if you can also explain the reason why the high humidity levels help the flor development, I would also appreciate. What exactly does humidity offer to these yeast? Or if the atmosphere is dry, what is their problem? I can suppose that evaporation is not a problem, since the veil protects from the contact with oxygen and the dry air…

    • Angeliki Tsioli

      I reply to myself immediately. Maybe evaporation from the casks is an issue, which makes the wine loose levels of alcohol, thus making the wine more vulnerable to bacteria and other yeast. But I am still interested in your opinion!

      • I must admit I’m no bio scientist. What I hear is that 65% relative humidity is the absolute minimum and above 70% is perfect for the growth of flor at an ambient temperature of around 20°C. There is not really an upper limit for humidity but above 30-35°C there is a high risk the flor would be killed. Maximum proliferation of flor is at 22-25°C though most bodegas will aim for 17-20°C to get a balanced character.

        What does water do? Well, isn’t water essential for all living organisms? (Yeast) cells simply dry out at a certain point.

        Evaporation (merma) is a different issue. Of course winemakers want to minimize it, but it will occur anyway, with or without flor. To minimize evaporation you need a humidity of around 60%. So they aim for 60% in the whole bodega and then place the Fino / Manzanilla casks in the most humid areas above 70%.

        It is also true that low alcohol levels are contraproductive for flor: it needs 14.8 – 15.3% alcohol as a nutrient. Sufficient alcohol also protects the wine from acetic acid bacteria.

        • Angeliki Tsioli

          Thank you! Nice book indeed!

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