Published on September 15th, 2013 | by Ruben Luyten1
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.
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), 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
During 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, Berttanomyces) 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.