What has the cask previously held?
The prior use of a cask can have a huge impact on the character of the spirit eventually filled into it.
Bourbon & American Whiskey: America’s distillers are required by law to use virgin oak, when combined – as many frequently are – with hot climate maturation, this creates an extremely powerful and assertive style of whiskey. One dominated by the influence of the oak and highly concentrated in style. Often displaying intense flavours of spice, liquorice, dried fruits and vanilla. This residual character contributes to the signature effect of 1st fill ex-bourbon barrels on Scotch Whisky. The slightly less active oak along with a different type of distillate and the cooler climate maturation conditions of Scotland combine to produce
whiskies which are often filled with the natural sweetness of barley but bolstered by the creamy vanilla, coconut and soft spice tones of the casks and their residual American spirit.
Sherry: The British have long been the world’s leading consumers of sherry. This has had the fortunate historic side-effect of making sherry casks abundant throughout these isles and of ready availability to the fledgling whisky industry of the 19th century. Casks – historically often made of American oak but also Spanish – would be used to mature and also to transport sherry from Spain. These casks, once disgorged here in the UK, often in merchant ports like Bristol, would then be transported to Scotland and used for the maturation of Whisky.
Today many of these ‘old style’ sherry casks are extremely hard to come by as sherry is no longer legally allowed to be bottled outside Spain and UK consumption levels have dropped significantly. So many producers commission specially seasoned casks in Spain or operate their own seasoning bodegas. Here at Bruichladdich we still source parcels of traditional sherry casks which have been used for many years to mature top quality drinking sherries. Sweeter styles of long-aged sherry such as Pedero Ximenez or Cream Sherry will impart dense, dark fruits and syrupy sweetness. Whereas drier Oloroso or Amontillado can give a more leathery, earthy and rancio (musty) character to a whisky. Casks which have contained paler, dry sherries, such as Manzanilla or Fino, commonly impart lighter colours and nutty, salty, and exotic fruit flavours.
Wines: A large number of ex-wine casks are deployed in contemporary whisky maturation. The primary factors of influence here tend to be sweet or dry, and red grapes or white. Sweeter white wines such as Sauternes can add complex layers of orchid fruits, honey and elegant sweetness. Wheres a drier red grape wine cask – like an ex-Bordeaux or Burgundy – will often add punchier notes of spice, red fruit and tannin. Then again, sweet red wine casks like Port “pipes” can add concentrated, sweet, jammy red fruit characteristics.
These types of cask often have residue of the previous contents absorbed into the wood, which can be dissolved by the stronger spirit and imbue the developing whiskies with additional flavours and colours.
The activity level of a wine cask is also greatly dependent on its fill number. A fresh, first fill ex-red wine barrique can be a swift acting and powerful cask, whereas that same cask on its second fill could be a perfect vessel for a longer, slower full- term maturation. We use a wide variety of these such casks at Bruichladdich to broaden the range of options and flavours for our whisky recipes.
Almost all casks are subject to varying degrees of heat treatment at cooperages. This consists of charring, toasting or usually a subtle combination of the two. These processes are more associated with ex-Bourbon casks than with historic sherry casks. Although, nowadays distillers often use heat treated French oak casks from coopers and wineries across Europe, or specially commissioned European oak sherry casks which have received varying degrees of heat treatment.
Charring: This is the exposure of the inner surface of the staves to a direct flame. It creates a layer of charcoal which acts as a carbon filter. This is essential during the initial years of maturation as it effectively purifies a distillate, drawing out sharper or less pleasant sulphur compounds and congeners which might otherwise develop into off-notes. It’s part of the ‘extractive’ process during maturation and one of the key reasons why oak is such an integral material in maturing Scotch Whisky. There are varying levels of charring, usually graded from 1 – 5. The lowest being very light is achieved with around 15 seconds of direct flame, whereas a 5 would need above 55 seconds. Charring also caramelises wood sugars, chiefly hemicellulose, which when dissolved into spirit add colour and many of the sweeter flavours associated with more active oak. This type of charring is more common with American oak ex-bourbon barrels and the sugars it makes available to the whisky are part of the ‘additive’ process of maturation.
Toasting: Different to charring, toasting is more about deeper heat treatment of wood – although some incidental toasting occurs when wood is charred too. This is again about caramelising sugars and opening up the pores to make deeper parts of the oak staves accessible to the spirit. It’s often done in combination with charring to varying degrees and tends to accentuate spiciness, sweetness and colour in a maturing whisky.