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Sherry casks have been used to age Scotch whisky for well over 200 years. But what are distillers looking for when choosing casks, and how are they able to produce specific flavours? Does the type of wood also play a role? The Whisky Professor is here to help make sense of it all. 

Why Sherry?

The Scots loved Sherry. The first record of it being drunk in Edinburgh was in 1548, but it was in the 18th century when it became a fashionable drink. By then, Sherry and rum Punches were the preferred drinks of the roistering members of Scottish clubs, which were the trend-setters of the period.  There was also an element of patriotism involved, with many Scots either working for or owning Sherry houses: Arthur Gordon established Bodegas Gordon, then a distant relative William joined his uncle to found Duff-Gordon, while George Sandeman’s Port and Sherry interests were handled in Scotland by his relative Thomas from his base in Perth.  In those days, Sherry would have been shipped to Glasgow and Leith in casks. This coincided with a dramatic expansion of the whisky industry. As there were no domestic forests in Scotland to use for casks, distillers turned to those being landed on the docks. These included rum and wine, as well as Sherry, but it was the latter that became the preferred option.

What is Sherry?

Sherry starts as a dry white wine made from the Palomino grape. This is then fortified and aged. The styles fall into two camps. The first contains fino, its subset manzanilla and amontillado. In these, the wine is lightly fortified and then placed in butts of 500-700 litres and aged in a complex system known as a solera.  A thick, woolly blanket of yeast, known as flor, begins to grow on the surface of the liquid, protecting the wine from the air. As a result, the Sherry is pale in colour with a light crisp character, aromas of almond, chalk, green olive and, in manzanilla’s case, a distinctly salty tang. If this flor is allowed to die, then some oxidation takes place; the wine darkens in colour, and becomes an amontillado. The second camp contains oloroso and palo cortado. The same base wine is used, but this is fortified to a higher strength, thus killing any chance of flor growing (in the case of palo cortado, the yeast-killing process happens by chance, rather than by design). Ageing is in a solera and, because there is contact with air, the wines become darker and more nutty (walnuts), with some dried fruit elements. Pedro Ximénez (PX) – which is becoming increasingly used in Scotch maturation – is a separate style, made from the eponymous grape. After harvesting, the grapes are dried in the sun to concentrate the sugars and then fermented, fortified and aged. This naturally sweet wine with a huge raisined impact is used as a sweetening agent.  

What does a Sherry producer want from a cask?

The butts used in the solera system are not made from active wood (they tend to be old American oak casks), so the wood has little impact on the Sherry. The taste of fino is caused by flor, the nuttiness of oloroso by oxidation and the sweetness of PX by the ‘raisining’ effect.

What does a Scotch producer want from a cask?

A distiller wants a cask to contribute flavour to the maturing whisky: vanilla, coconut, spice and chocolate from American oak; tannin, resin, clove and dried fruits from European oak. So, in Sherry wine production the flavours are driven by oxidation rather than by oak, while in the whisky maturation process, it is the other way around. On one hand, Sherry producers want a cask which has little impact, while the Scots want the opposite. Sherry producers use American oak for their casks, Scotch producers insist on European oak. There seem to be two different stories.

What influence does the Sherry have on whisky?

All of this might seem as if it is the oak which is the main driver in terms of flavour in Sherried whisky. That’s not quite true either. What happens during the seasoning process is that the Sherry modifies the flavour compounds in the oak.

It is estimated that there is up to 10 litres of Sherry soaked into the wood of a Sherry butt. To see whether it was oak or Sherry that gave a ‘Sherried’ character to a Scotch, scientists simply added Sherry to Scotch to see whether the combination was the same as maturing in a Sherry butt. It wasn’t. Equally, ageing in an untreated European oak butt gave a different result.

The ‘Sherried’ character, therefore, is down to the Sherry itself, but also oxidation and the way in which the Sherry has interacted with and changed compounds in the oak. All of these then interact over time with the maturing spirit.

In very simple terms, there is more of an impact from the Sherry (or rather Sherry changed by oak and air) in the whisky’s youth, while in older examples you notice more of the oak (which is itself changed by the Sherry). This explains why a Sherry-finished whisky has more of a wine-driven character, when compared to a whisky matured exclusively in ex-Sherry casks.

What these flavours are will also depend on a number of factors. A first-fill bespoke European oak ex-Sherry cask will have maximum impact of clove, resin, dried fruit and tannin. The same cask filled for a second time will have less of those compounds available for the whisky. You can find ex-Sherry casks which have been refilled so many times that the ‘Sherry’ element is virtually invisible.

Some distillers – Dalmore is a notable example – prefer to use ex-solera casks, which will have low wood impact, but a higher contribution from the wine (and oxidation).

Horses, as they say, for courses.



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