Sherry butts and Whisky

A joint investigation by Paula MacLean and Chris Hoban of The Edinburgh Whisky Blog

The barrel (or cask, or butt or whatever it is called by whom) is absolutely fundamental to both the wine trade and the whisky trade, and has been for centuries. Where would Sherry be without its solera systems - or whisky without the used casks? Many great malt whiskies are matured in used Sherry butts: the Macallan, Glenfarclas and Aberlour for example, but there appears to be some confusion as to the precise species of oak and the precise liquid which was once contained therein.

Over the centuries casks have been made from many different woods, but oak has been proved to be the best for wines and spirits because of its strength, durability and breathing and flavouring characteristics. Indeed Scotch Whisky must be matured in oak. One problem, however, is that a barrel made from new oak imparts a lot of flavour and various polyphenolic elements (tannins for example) to its contents. This is fine for winemakers who want that toasty, buttery vanilla tang in their wines which are only in a new barrel for a couple of years or so, but it can ruin wines staying longer as well as spirits which may spend more than a decade in the barrel. The Scotch Whisky trade therefore very rarely use new oak and prefer to use seasoned barrels. The same applies to the Sherry trade.

A Sherry butt and a Bourbon barrel

The vast majority of casks used in Scotland are 2nd hand Bourbon casks, as in the US Bourbon must age in new casks which become redundant after one filling, but the rest consist mainly of used Sherry or other fortified wine casks. As far as Sherry casks - or more correctly "butt"s from Jerez - are concerned, these are almost all made from American White oak (Quercus Alba), and are rarely if ever sold, as the last thing Sherry producers want is their elegant wines tasting oaky from new butts. Furthermore, and especially with Finos, the gradual build up of dead yeast cells at the bottom of the butt adds subtle flavours to the wine which would be lost if the butt was sold and/or replaced. Sherry producers use their butts for as long as is practical, in reality till they disintegrate or start to show signs of rot.

When a bodega needs a new butt it will be "envinado" or filled with fermenting must or inexpensive and plentiful Sherry till that wine has absorbed the worst of the harshness of the new oak, before the butt can be used. It is exactly this process which is generally used now for Scotch Whisky Sherry casks. Until the 1970's Sherry was exported in 500 litre "export" butts and bottled at its destination, so there was an abundance of  butts, but now that Sherry is bottled in Spain, they have to be sourced in different ways. Usually the whisky company buys Spanish oak which is cheaper than importing American, has it coopered in Jerez, filled with Sherry (usually Oloroso) for a couple of years to impart Sherry flavour and colour and extract the worst of the tannins. The wine so used will only be suitable for cheap blends or distillation, but the cask will be ready for whisky and may be used 2 or 3 times.

Normal 600 litre Sherry bodega butts are coopered in Jerez from American Quercus Alba (White Oak). Various alternatives have been tried, but this has proved best due to the wood's strength, water and rot proof characteristics. Spanish oak is mainly Quercus Robur which grows in the Atlantic North of Spain and the North West and is good for wine ageing, but as few forests are managed the trees tend to sprawl rather than grow tall and straight, making coopering difficult and yields low. And there are not so many forests, so there would never be enough Spanish oak to supply the wine trade should they want to use it. Worse still, forests without either protection or economic purpose could gradually disappear, so we should be grateful to the whisky trade who now manage some forest sustainably.

The Sherry butts used to age whisky, then, are no longer quite the same as they were, but fulfil the same purpose, ie to age the whisky and imbue it with some Sherry flavour. The two can go fantastically well together, but too strong a Sherry influence can spoil a whisky despite the large capacity of a butt. (The bigger the barrel, the lesser the flavour influence and the longer time needed to age). Although bodega butts are of 600 litre capacity, the Scotch Whisky trade uses butts of 500 litres, and the Sherry used to season the butts could be any type, but it is usually dry oloroso or sometimes sweet Pedro Ximenez.

Chris and I, who have been debating these matters for ages, were fortunate enough to attend a recent event in Edinburgh where we met a number of leading whisky blenders and were able to discuss these matters with them. They confirmed the above. Many also use the Sherry butts for "finishing" the whisky, that is ageing it for say 10 years in bourbon casks and then transferring the whisky to a Sherry butt for the last year or two of its ageing. This way the Sherry gives an extra dimension of flavour but can't dominate the whisky, and the butt can be reused. So hurray for whisky and hurray for Sherry!