Aging whisky in barrels isn’t new. Wood has long been an integral part of how whisky is made. Time spent resting in barrel allows the spirit to reduce its aggressive tones and acquire flavours from the barrel.
If only making great whisky was as simple as making a distillate from some malt and letting it sit in a cask for a number years. There’s a lot more to aging in oak than simply colour and flavour transference. And what are those flavours? Your favourite Single Malt Scotch whisky is a complex blend of esters aldehydes, polyphenolic compounds, lactones, cellulose and alcohol, just to name a few.
Even the sweet smell of vanilla isn’t simply a matter of adding a dash of vanilla seasoning, it’s a polyphenolic aldehyde developed from lignin degradation. Lignin breakdown is enhanced when a barrel has been charred, so not surprisingly, the light vanilla quality we associate with Single Malt Scotch whiskies is likely encouraged when the spirit has rested for a prolonged period of time in ex-Bourbon casks. Bourbon casks are heavily charred and only used once before being sold off to age other spirits.