Charring entails igniting the interior of a barrel using a flame, with the barrel allowed to burn for a specified time, depending on the level of charring required. The options are one to four. A number one char requires about 20 seconds flaming, a number two is 25-30 seconds, a number three is 35-40 seconds and a number four takes 40 seconds to a minute. This highlights the absence of standard timings, as each cooperage has its own regime. But the principle is the same. Longer charring creates a deeper layer of char: around 2mms for a number one, up to 4mms for a number four char.
The intense heat of charring also creates a layer of toasted oak beyond the char layer. Longer charring results in a progressively deeper toasted layer, extending up to 12mms deep (a stave totals 25mms). Flavour compounds within this toasted layer are caramelised and broken down into smaller groups that renders them ‘flavour active’.
The usual charring levels for Bourbon barrels are a number three and four. Longer charring breaks up the char layer more thoroughly, making it easier for the spirit to transit and reach the toasted layer. This is where flavour compounds dissolve, when in contact with the spirit, and then migrate to join the ‘bulk’ of the maturing spirit.
The char and toasted layer have separate, albeit complementary roles. The toasted layer adds flavour, whereas the char ‘removes’ certain compounds from the spirit by absorbing them. This is how it works when ageing Bourbon.
“The char is essentially a filter absorbing acids and oils, some of which are in the spirit derived from corn, while others are released from the toasted layer. The result is a cleaner, crisper character, with enhanced mouthfeel,” says Chris Morris, vice president, master distiller – whiskey innovation for Woodford Reserve.
The char also absorbs sulphur compounds, which stem from malted barley. This isn’t a big deal for Bourbon as malted barley’s presence in the mash bill is minor. However, as malt whisky is distilled entirely from malted barley the level of sulphur compounds in new make spirit is higher. Sulphur conveys a pungent range: vegetal, meaty, sweaty socks and burnt matches, which mask lighter notes. Lowering sulphur ‘reveals’ fruit and sweetness.
An ideal case study of all the charred cask permutations is Glenmorangie. In addition to filling ex-Bourbon barrels for full maturation, mature malt is also ‘finished’ (undergoes subsequent ageing) in virgin charred casks, an example being Glenmorangie 15 Years Old. Glenmorangie also fills virgin charred casks with new make spirit for full maturation and this is part of the recipe for Glenmorangie The Tarlogan.
“The difference between the influence of first fill Bourbon barrels and virgin charred casks is the intensity, rather than the range of flavours they contribute. Virgin charred casks are super-charged with flavour: more vanilla creaminess, honey, a lot of coconut and the clove notes are super-pronounced,” says Brendan McCarron, Glenmorangie’s head of maturing whisky stocks.
Cask influence should of course be considered in conjunction with the new make spirit profile. Glenmorangie is elegant and unpeated, while Ardbeg is peated and fuller-bodied.
“Glenmorangie really invites the influence of virgin charred casks, and within a few years you see a lot of vanilla and coconut. Ardbeg new make spirit takes longer to show the influence, after six to seven years the result is a smokey panna cotta. If we want to increase the sweetness in Ardbeg we usually add more sherry-matured malt, but this also seems to mellow the smoke. Using virgin charred oak gives a frisson of roundness and smoothness, and elevates the smoke rather than hiding it. Virgin casks are another string to your bow, they give a really punchy flavour and a little goes a long way,” says McCarron.
From Bourbon to malt
As Bourbon distillers can only use barrels once, there’s a constant supply of ex-Bourbon barrels, which are used to age various styles of whisky and account for the majority of casks in Scotland.
“Barrels are typically used for four to six years to mature Kentucky Straight Bourbon, and it’s primarily a case of Bourbon extracting from the barrel, with around 85 per cent of the extractable compounds removed during that time. When Bourbon barrels are refilled with another style of whisky there is primarily less vanilla and caramel available,” says Morris.
So, does Bourbon only take away from the barrel, or can it add anything which subsequent occupants can access during maturation?
“There could be certain compounds penetrating the oak that would not be there without the Bourbon, but it depends how long it takes the barrels to reach Scotland. The fresher the cask the greater the potential for residual liquid to remain absorbed within the staves, rather than drying out,” adds Morris.