Whisky Magazine Issue 88
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Ian Wisniewski asks what is the effect of toasting or charring casks, and how does this influence the character of malt whisky?
Malt whisky is usually aged in casks previously used to mature bourbon or sherry, with bourbon barrels having been charred on the inside, while sherry casks are toasted. This is done by applying a flame, with the heat ‘releasing' various flavour compounds bound up in the oak.
A certain level is extracted when the casks are used to age bourbon and sherry, but a significant level remains to be extracted by malt whisky. Consequently, if casks weren't charred or toasted they would contribute significantly less flavour (and excessive oakiness) to malt whisky.
Exactly what instigated the tradition of charring and toasting is uncertain. The essential difference between them is that bourbon barrels are exposed to more intense heat.
“You're looking to flame the interior of bourbon barrels, but not to ignite the interior of a Sherry cask. All cooperages have their own individual regimes to achieve this, that's why bourbon distillers tend to stick with one cooperage to promote consistency,” says Dr James Swan, a consultant renowned for his research into the influence of oak.
Bourbon barrels are charred to varying degrees. A number one char resembles burnt toast, with the heaviest being a number four, known as an ‘alligator char' as it looks like alligator hide. Most barrels receive a number three, with fewer number fours, and far fewer number one or two chars.
“At the Brown-Forman cooperage barrels (without the ends) are charred on a ‘production line' fitted with separa...