Bourbon distillers, long bound to the traditional barrels of American oak, are beginning to experiment:
[Distiller Chris] Morris put standard six- to seven-year-old Woodford Reserve in a maple wood barrel as well as former sweet wine casks to lend more chocolate, nutty and dark cherry flavors not usually found in bourbon. Much like the original Woodford Reserve mingled with the new charred American oak barrel, the “Four Wood” chemically reacted with its barrel wood to produce a particular set of flavors. The former fortified wine barrels had wine soaked into the wood and are larger than standard whiskey barrels, giving the Woodford Reserve a larger surface-to-whiskey ratio as well as the small-scale fruity flavors that remained from the barrel’s former alcohol. …
Aging has even gone beyond stationary warehouses.
For its Ocean-Aged Bourbon, Jefferson’s Reserve placed several barrels on a 126-foot ship and let the casks cruise at sea for nearly four years. The increased oceanic air pressure (compared with its warehouse), along with the Panama Canal’s extreme heat pushed the whiskey deeper inside the wood, causing the wood sugars to caramelize and add a rumlike black hue. The whiskey breathed a little easier, too, says Trey Zoeller, who co-founded Jefferson’s Reserve. “The porous nature of the barrel not only allows for evaporation of bourbon out of the barrel, but also [for] the barrel to breathe in the salt air, giving it a briny taste,” Zoeller notes.
Update from a reader:
Larger barrels will have a smaller surface-to-volume ratio than smaller barrels. This applies to other objects as well – it’s why small animals lose heat faster than large animals in cold environments.