How is Sherry made?
There are two distinct ways of making this wonderfully versatile fortified wine, Biological or Oxidative. For the purposes of this article, we're using the dry styles to explain how sherry is made and the difference between ageing with yeast and with oxygen.
The beginning of all styles starts in a relatively similar way. The most commonly used grape is Palomino which is a white variety that has a very neutral aroma and flavour composition, therefore making rather bland still wines.
The soil is, however, a very important point. The temperatures of this area in the south of Spain, commonly known as the ‘sherry triangle,’ are exceedingly hot in summer often reaching over 35 degrees with no rainfall in sight. It's therefore vital that the soil protects the vines and retains some of the winters' rainwater in order to allow the vines to survive. The soil is known as albariza which in simple terms means 'is made from chalk'. The white colour reflects the suns' rays in a similar fashion to the white-washed houses in Santorini, Greece, therefore, having a slight cooling effect. The soil also forms ‘crusts’ preventing water evaporation in the heat of the day allowing the roots to acquire the full uptake of water.
During harvesting, the winemaker will make a decision as to which grapes are best suited to each style. This roughly works out to be the grapes grown on the best, lightest albariza soils heading towards being biologically aged. The wines undergo the usual alcoholic fermentation process converting the grapes' sugar into alcohol.
After the fermentation the wine is then fortified. This means that a neutral grape spirit is added to raise the ABV. The biologically aged wines are fortified to around 15% and those destined for oxygenic ageing are fortified higher to around 17%.
The wines then enter oak barrels, these are old and very large therefore there is no ‘oaked’ flavour imparted into the wines themselves, they instead act as the vessels for each process to occur.
In the simplest terms, Biological vs Oxidative means the same as yeasts vs oxygen. That is the best way to think about and understand these processes.
Starting with the wine that was fortified to 15%.
This has entered the barrels and filled to 5/6 of the barrels' volume allowing oxygen to be present. The oxygen is vital as it is a key component that the ‘flor’ requires. In order for biological ageing to occur a film of yeasts, known as flor, creates a protective layer over the top of the wine. This protective film stops the wine coming into contact with any oxygen, therefore, retaining fresh flavours and the pale colour. The flor feeds on alcohol (glycerol), oxygen and other nutrients in the wine in order to survive and slowly changes the flavours and aromas of the wine itself.
If the wine was instead fortified to 17.5% then this alcohol percentage is too high for the ‘flor’ to survive. The barrels are still filled 5/6 full but this time there is no protective layer over the sherry in the barrels. The oxygen is, therefore, free to interact with the wine slowly altering the flavours, aromas and colour of the wine. This can be imagined more easily by thinking about leaving a bottle of white wine open for weeks and allowing oxygen to ‘ruin’ the wine, giving flabby flavours and a vinegar note. This is what is actually occurring but due to the large volumes having a relatively small interaction with the air, the wine instead creates complex flavours that are characteristic to that style.
So far it's all relatively simple. The grapes are harvested, alcohol is produced, more alcohol is added and then either yeasts or oxygen interact with the wine.
Those who make this fortified wine have yet another trick up their sleeve-the Solera system. This is the most basic sense can be thought of as layers of barrels stacked on top of each other. The youngest wine enters the top level and each year a third of the wine is removed into the next row of barrels. The bottom layer, known as the Solera (the same name as the system itself) is where the wine for bottling is acquired from. The regions' rules only allow 1/3 of the wine to be bottled each time, therefore maintaining the style and consistency sherry lovers require.
The Solera system is basically a more complex way of blending, the younger wines mix with the older wines giving a more complex finished sherry. This process of blending is known as ‘running the scales’. The system is also vital for the wines that have been biologically aged as it replenishes the nutrients and alcohol that the flor requires to survive and thrive.
Want to know more? Read The Styles of Sherry.