Contributing up to 70 per cent of a malt’s flavour, the cask is a vital factor, but in the course of its life-time a cask offers varying maturation influences and has, of course, already led an active life before reaching Scotland.The vast majority of casks used to mature Scotch malt and grain whisky are American oak, typically fashioned from 40-100 year-old trees harvested in Missouri, Kentucky and Tennessee.The type of soil in which the oak is grown and the manner in which it is dried (kiln drying versus air-drying) are also influential, and provide distillers with further criteria. Glenmorangie, for example, uses oak from the Ozark Mountains in Missouri, where a combination of low rainfall and poor soil fosters slow growth, which in turn promotes the level of vanillin.Air-drying for two years breaks down the oak tannins reducing astringency, and enabling cask-driven oxidation to occur more effectively. This in turn heightens the maturation potential of what are referred to as ‘designer’ casks, yielding more vanilla and other aromatics.Charring bourbon barrels on the inside is standard practice, but the terminology used to specify the degree of charring isn’t, with terms such as ‘light’ and ‘heavy’ alternating with a scale of one to four.Moreover, the same term may not have exactly the same meaning at different cooperages.A number one char is typically referred to as burnt toast, while a number four resembles a certain predator’s hide, which explains why it’s known as an ‘alligator char.’Lighter charring tends to promote greater sweetness, honey and body, while heavier charring typically gives more vanilla, crème caramel, toastiness and a richer colour.A surface level typically 2-4 mm deep essentially becomes pure carbon, with wood sugars partially caramelised in the underlying 2-3 mm, releasing flavour compounds such as vanilla (the majority of the stave, around 20 mm, remains unaffected).Charring also opens up the surface, enabling the spirit to penetrate the oak more readily, and begin extracting various flavour compounds, particularly vanilla, while tannins help promote body, balance and structure. A layer of char also acts as a ‘filter,’ tackling immature elements
within the spirit. As bourbon barrels can only be used once, with a minimum two year ageing period, the Scotch whisky industry is a core consumer group.Placing an order with a large cooperage, which acts as a broker, usually means barrels from different distilleries with various charring levels (the filling date is usually stamped on the barrel).Ordering direct from a distillery sees barrels shipped in batches after being emptied to bottle a specific style, and will (theoretically) comprise the same age and charring level.The traditional practise of dismantling and shipping barrels as ‘shooks’ (staves) for reassembly in Scotland, has declined enormously over the past 10-15 years.This saw standard 190 litre barrels enlarged to 250 litre casks. Every fifth barrel was effectively cannibalised to enlarge four others, which subsequently received the prefix ‘dump’ or ‘remake,’ to indicate their new dimensions. As this entails additional costs, including new hoops, a new head and labour, it’s an expensive way of sourcing a hogshead.Sherry casks account for a fraction of the industry’s cask inventory, and as The Macallan only uses sherry barrels, the distillery accounts for around 65 per cent of sherry casks arriving in Scotland, with around 20,000 casks continually being matured on behalf of the company in Spain.The Macallan’s casks are fashioned from 100-150 year old Galician trees, quarter sawn against the grain to make the oak less porous (a standard practice), with air-drying reducing the moisture level to around 12 per cent.A slower process than using a kiln, this entails some bacterial presence which in turn promotes effective and balanced tannin potential within the oak.At the cooperage in Andalucia casks are toasted (not charred), penetrating the surface of the oak to a depth of around 1 mm. Less intrusive than charring, this is still sufficient to caramelise wood sugars. Casks are seasoned by filling them with must, the juice pressed from locally cultivated palomino grapes, from which sherry is produced. There is no need to add yeast, which is present in sufficient quantities within the grape skins to effect fermentation, over 10-15 days or longer, yielding a wine of around 11% abv. Fermentation is an effective method of flushing out the most tannic elements, which would otherwise dominate the new make spirit with a gingery spice character.Drained after a ‘season,’ which typically means September-April, casks are subsequently filled for around 3 further years with 2-3 year old dry oloroso sherry, at a strength of 18% abv. Wines utilised during the seasoning process remain the property of Gonzalez Byass, with the casks owned by The Macallan.Prior to being shipped, each cask receives ‘one for the road’ in the form of 5 litres of wine, helping to maintain freshness during a 4-6 week journey to Scotland. (This is of course emptied prior to filling with spirit in Scotland).As a life-cycle of a bourbon or sherry cask can be up to three or four fills, this could mean up to 50 years active service. Each fill is not simply a case of delivering the same influence in a reduced format, as the way in which a cask matures a malt (and how it influences the original distillery character) changes with each fill.A first fill bourbon barrel for example can contribute a higher level of vanilla, sweetness and colour, compared to a second fill which also sees the effects of oxidation come through more.How a cask’s active life-span is divided up between different fills obviously varies among distilleries.Similarly, a malt may comprise a recipe of various fills of bourbon and sherry casks, or a specific fill of either type of cask, depending on the house style. Glenmorangie for example uses a combination of ‘designer,’ first and second fill bourbon casks, while The Macallan uses first and second fill sherry casks.However, first fill casks not delivering The Macallan’s required range can be withdrawn (and used for blends), while star-performing second fills may go on to provide a third fill.When no longer appropriate for ageing malt, certain second fill Macallan casks are utilised as marrying vessels (being essentially inert with a minimal wood extractive influence).Rather than retiring casks from the ageing process, another option is to extend a cask’s life-span by recharring. Specialists such as the Speyside Cooperage offer the choice of a ‘dechar, rechar,’ while a ‘dechar, rechar, dechar’ service sees the second dechar recreating a ‘toasted’ interior.It only takes a few minutes to dechar, with one option being a large wire brush fitted to a motor that go’s up and down the length of a rotating barrel.Another method is a wire flail, which means stainless steel ropes comprising 3-4 strands, rotating along the length of the barrel. Either method reveals a smooth ‘singed’ surface, without actually exposing any new oak.Recharring can take around three to four minutes using gas burners (which also of course partially caramelises wood sugars in the layer underlying the char). Recharred casks can either be filled with spirit, or undergo additional ‘re-seasoning.’ European oak casks, for example, may be filled with sherry to help ‘recreate’ the original influences, prior to filling with spirit.The essential question is to what extent a trip to the cooperage can rejuvenate a cask? While maturation potential is certainly extended, the best result rejuvenation can achieve is a ‘semi’ first fill status.A recharred bourbon cask, for example, could contribute less vanilla, coconut and sweetness, though a similar oakiness and colour compared to a first fill.Rejuvenated casks may hold up to two further fillings, after which there is the option of additional rejuvenation, with two rejuvenation treatments usually the limit.“American oak probably lends itself better to dechar, rechar, dechar treatment, as Spanish oak has ten times more tannin, so this treatment brings out the very spicy impact, whereas with bourbon barrels it brings out more vanilla,” says The Macallan’s David Robertson.“There don’t seem to be any shortcuts in rejuvenating Spanish oak casks. We’re better off buying fresh wood stocks from Spain, rather than sending an empty cask to Spain for dechar, rechar, dechar and putting it into a bodega system for three years.“The key for us is the unique balance of wood extractives (predominantly tannin) that drives the aroma, flavour and natural colour we require.”This also raises the issue of what role recharred casks can play in the overall inventory. As the practise of recharring is only about 15 years old, research into the influence of recharred casks is still on-going, with the current focus being to age whisky for blends.Whether rejuvenated casks play a more prominent role remains to be seen.