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Sherry: Flor-ed by its Complexity
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If asked which wine style is the most fascinating in terms of its underlying science, I would without any hesitation, answer… Sherry. This wine style which originated around Jerez and Sanlucar in southern Spain (and still produces the pre-eminent examples of the style) is unique in that it undergoes two or three life changing phases from grape to finished wine - yeast fermentation, biological aging and depending on style, oxidative aging.
But let's start in the vineyard. Great dry sherry styles including fino are made from Palomino, perhaps the worlds dullest variety. Being pale in colour, low in tannins, and (most importantly) bland to the extreme, Palomino is ideal for the production of the dry base wine from which sherry is ultimately produced. The Palomino grapes are lightly crushed and the free run juice is immediately drained off to be fermented using a neutral yeast. The use of non-flavour creating yeasts together with juices low in coarse phenolic substances ensures that the base wine is neutral and above all else, delicate. The base wine is lightly fortified with a neutral high alcohol spirit to bring its alcohol content up to around 15.5%. Here is where the fun starts.
The wine is then pumped into older American oak barrels, but they are only filled to around 5/6th of their capacity. This practice would spell the death knell to any table wine as both oxidation and the growth of acetic bacteria would quickly cause it to spoil. Sherry is different. In most cases a thick film of yeast will quickly form on the surface of the wine. This ivory coloured veil has a wrinkly surface texture, is oily to the touch, and is a healthy 3-5mm thick. The film consists of billions of cells of the wine yeast Saccharomyces cereviseae. However these yeasts are a different race to that of the normal yeast involved in wine fermentation. Recent genetic research has shown that the film consists of five different races the most important being S. beticus, and S. montuliensis. These races have been shown to be genetically similar to each other in that they are all contain genes that make them resistant to both alcohol and to the compound responsible for the distinctly green apple and nutty character of Fino sherries called acetaldehyde. Most importantly, as a result of their higher lipid (fat) content compared with other wine yeasts, they all float - a property which is especially important if they are to contribute to a healthy surface film.
This film of yeast is called "flor" and it protects the wine from oxidation for a number of years that the wine is under it. The yeasts gain the energy needed for their survival by oxidising wine alcohols to a group of compounds called aldehydes. The aldehydes are the key ingredient necessary for the production of the delightful green almond, granny smith and nougat characters that characterise great fino sherry. The chemistry of flor goes a way beyond this, but needless to say, the flor film is primarily responsible for most of the complex characters seen in these wines. They wouldn't be remotely the same if they lacked its influence.
As a result of the depletion of nutrients in the wine together with a natural increase in its alcohol content over time, and/or adverse temperatures during summer and winter, the flor yeast film tends of weaken. The deceased flor yeast cells fall into the wine, rot down, and in doing so release further complex flavours. To sustain the flor film over the two or more years that are required, sherry winemakers regularly freshen up the wine by adding small amounts of younger base wine. Recent genetic research has shown that the different races of yeast also contribute to the sustained long term health of the flor. It appears that S. beticus is instrumental for the initial creation of the film but gives up its early dominance to the strong flavour producing S. monuliensis later on. However, in some wineries at least, the population dynamics of these yeasts appears to be less complicated with some races dominating the entire process. Sherry winemakers also often report a seasonal change in the colour and physical properties of the flor film. It has been long thought that different strains of yeast dominate the flor depending on the seasonal changes in cellar temperature. Indeed, those winemakers who inoculate with flor yeast cultures often select multiple strain to ensure a good consistent flor film year round. Clearly, together with differences in the base wine, the changes in the type and effectiveness of the flor account go a long way to account for the subtle differences found between different fino sherries.
To complete the winemaking, those wines which are deemed by the winemaker to have retained their delicacy throughout this process are again fortified with neutral alcohol to a desired alcohol level of around 16% and sold as a fino sherry. Those which have become a little too full and rich for the style are left to age in barrel for many more years, in which time they undergo a transformation to a different styles of sherry called amontillado. But all this does not explain why even the smell of a good chilled fino brings on an instant hunger, and after tasting it your mouth is left with a dryness unsurpassed by any other wine style. Perhaps some things are best left unearthed.