Master distiller Harlen Wheatley of Buffalo Trace Distillery draws a bourbon whiskey sample out of the barrel and pours it into a brandy snifter glass. Wheatley raises it into the light; the bourbon illuminates with rich colors of caramel, gold, straw yellow and light brown. He tastes the seven-year-old drink known as W. L. Weller and says, "That's really coming along."
As Wheatley moves onto the next barrel, the glass sits in the light, the bourbon shining brightly and illustrating the chemical change wrought by the barrel. After being poured into the barrel, the colorless spirit sat there or "aged" for seven years. The liquid mingled with the wood, giving the bourbon it's color, taste and smell.
A new generation of distillers have begun to break time-honored tradition and tinker with the barrels, relying on science and experimentation to bring new flavors into the spirits. For the bourbon whiskey business, the barrel is everything.
Bourbon barrel science
All bourbon is whiskey, but not all whiskey is bourbon. Congress declared bourbon "America's Spirit" in 1964 and, according to the Federal Alcohol Administration Act, bourbon whiskey must be produced from a fermented mash of at least 51 percent corn, although malted barely, rye or wheat can be added to the mash. Once distilled, the clear spirit must be stored at not more than 125 proof in charred new oak containers. Coopers have long built bourbon barrels out of American white oak, specifically the species Quercus alba, instead of the popular-for-wine French white oak or other common oaks.
American white oak's durability as well as its ability to hold water and oxygenate it make it favorable for bourbon barrels. American oak is denser and harder than French oak, making barrels less prone leakage. Q. alba also yields different flavor profiles appealing to the U.S. whiskey market, says Brad Boswell, whose Independent Stave Company makes the majority of U.S. whiskey barrels.
"American oak has lactone levels certainly not found in French oak," Boswell says. Lactones, a molecule in oak wood that imparts taste, yield a coconut flavor that can be controlled by cooking, toasting and seasoning the wood. Before a barrel is assembled, barrel pieces called staves air dry either indoors or outdoors. This process slowly degrades the wood "because of the microbial activity that grows and feeds off the wood. The rainwater, snow and the natural elements leach the [bitter] tannins out of the wood," Boswell says.
Once air-dried, the staves form a 53-gallon barrel that is later charred to filter out organosulfur compounds that are not eliminated in the distilling process. Distillers pour fresh distillate into the new, charred oak barrel, and the change begins.
Pressed into the wood
At the chemical level, the wood's lactones, sugars, tannins, cellulose, hemicellulose and lignins interact with the esters, aldehydes, butanols and two-methyl butanols to give the whiskey flavors of vanilla, caramel, spice, toast, smoke, coconut, coffee and mocha. Vapor and barometric pressure push the whiskey deep inside the wood, bringing out more intense flavor notes. Wheatley says the higher a barrel is in a several-story warehouse, the more pressure the barrel exerts on the liquid, pushing it deeper inside the wood. "When it's real hot outside with high pressure you hit the side of the barrel and the bung [stopper] will shoot up four or five feet," Wheatley says. "If it's cold outside with low pressure, it's hard to get the barrel bung out. It will just sit there."
Such observations are valuable for Wheatley and others who have dedicated their careers to studying the complexities of bourbon whiskey. For 200 years distillers did not have the science or perhaps the desire to test the limits. They just followed the procedures and practices from distillers before them: Make spirit from fermented mash, age in new charred oak barrel and bottle.