If you’ve explored many single barrel offerings, you already know that each of these barrels has it’s own unique fingerprint that imparts similar, yet infinitely varied profiles to the Bourbons they contain.
Anyone into Whiskey knows that Bourbon flows in and out of the wood due to changes in temperature with the seasons, and most are familiar with the “red line” showing the deepest penetration into the wood, but what exactly is happening inside the barrels as they sit in rickhouses season after season? And what is it about oak that produces such a magnificent flavor in Bourbon?
Why Oak?Here in the U.S., you might wonder why oak became the standard for barrel construction. If you’ve been to a hardware store recently, you know pine is widely available and relatively cheap, so why not use it?
Those familiar with Christmas trees, or have ever been in a pine forest, know part of the answer. That characteristic resinous scent would be imparted to whatever was placed inside the barrel. Also, the cell structure of pine is too “loose” and would leak and degrade over time with direct contact to liquid inside.
The wood of the oak tree has a much tighter cell structure allowing for the barrel to hold up over time without leakage and allow for a slower, more gentle interaction between the Bourbon and the wood.
The American Oak (Quercus alba) most often used in Bourbon barrels has an additional cellular structure called the tylose. Normally serving to help prevent fluid loss in times of damage or stress for the living tree, tyloses also help make this species particularly resistant to rot and leakage with prolonged contact with Bourbon.
Importantly, small amounts of oxygen can still permeate the structure and play their part in maturation of the distillate. The ideal oak trees for producing barrels are often 100-200 years old, so the next time you take a sip of your favorite Bourbon take a minute to think about what the world was like when that tree started growing. There’s a lot of history in that glass you’re holding.
Why do they char and toast the wood?For making Bourbon barrels, the wood has to be thoroughly dried for ease of construction as well as making it ready for long term liquid storage. Different cooperages do this different ways, but most have a long air drying period for the source wood.
The next steps of toasting and charring are different based on the preferences of the distillery, but to be called Bourbon, the barrel must be at least somewhat charred.
Toasting refers to a more gentle heating that results in less caramelization and is less common than charring alone. For char, think of wood in a campfire. The wood is burned until black producing more profound effects on the structure of the wood.
As an example, Wild Turkey prefers a longer firing number 4 char for most of their products, sometimes called an ‘alligator char’ due to the look of alligator skin.
The addition of heat causes chemical reactions to take place in the wood which are important to developing flavor compounds that will eventually be imparted to the Bourbon inside (more on these below). The charring also creates a layer that can help filter out sulfur and other compounds that would create off flavors in the wood as the Bourbon passes through them over time.
Where does the barrel flavor come from?Flavor in Bourbon is a complex interaction of the grains used, the distilling process, the storage conditions, and the barrel. Herein we’re just going to talk about the barrel.
In terms of structure and flavor components, oak wood can be oversimplified into 4 components: cellulose, hemicellulose, lignin, and wood extracts.
CelluloseCellulose had largely been believed to be a fairly unimportant participant in the maturation process, but a study by Harlen Wheatley of Buffalo Trace in collaboration with researchers at the University of Kentucky is delving into its importance.
Though the long, linked chains of glucose that make up cellulose tend to survive the barrel charring process, they breakdown with extended contact with Bourbon. They estimate between 3-5 pounds of sugar are extracted over a 6 year maturation period, adding to the mouthfeel and flavor of the Bourbon.
HemicelluloseHemicellulose is composed of various somewhat smaller sugars like glucose, xylose, and galactose. These breakdown with the addition of heat, such as toasting or char, to form aromatic compounds. Some you may recognize from your glass are cyclotene (maple, caramel, or licorice), furfural (almond, walnut, or grainy) and hydroxymethylfurfural (buttery, waxy, or musty).
LigninsLignins are components of the oak cell wall that are composed of various sugars, acids, and aldehydes that can also be broken down when heated.
These continue to degrade during the maturation process, in part due to interactions with alcohol in the Bourbon. The most notable of these products are vanillin and vanillic acid, which as you might guess, are what we associate with the flavors of vanilla beans and vanilla extract.
Wood ExtractsWood extracts are the smallest by percentage in wood, but are by far the most varied and influential to the flavor of Bourbon. The cis-lactones are often considered to be what produces the characteristic Bourbon aromas and flavors. The most recognizable of these are often described as coconut, vanilla, and sweet.
But these compounds can produce all sorts of notes, anything from clove to sawdust to grass to peach and so on. These compounds evolve throughout the aging process and each barrel will develop it’s own unique signature that can only be discovered by stealing a quick taste.
Also included in the wood extracts are tannins. Most of the time you think about tannins in reference to a bold red wine and are derived from the skins of the grapes, but Bourbon gets its share from the barrel. Not only do these compounds add to the subtle astringent flavor, but they play important parts in the oxidative reactions breaking down lignin, removing some sulphur off notes, and producing what some have described as “ethereal top notes” through reactions with the alcohol.
Why is ageing so important?We’ve all experienced the flavor of Bourbon changing drastically over different periods of ageing. And if you’ve been lucky enough to try some very old expressions, you may have noticed that sometimes extended ageing doesn’t always create the best flavors.
Many of the compounds discussed above evolve with more trips into and out of the wood, chemical reactions over time, and concentration with evaporation. And while most of the time these create favorable flavors, this isn’t universally true.
Work by Tom Collins (one of the best names in alcohol research) has shown that throughout ageing fatty acids lengthen. In general, the longer chain fatty acids are associated with more pleasing flavors.
They are also more soluble in higher alcohol expressions of Bourbon, making ageing essential to a good barrel proof Bourbon. It takes experience, planning, and a lot of tasting to make sure these processes result in the overall flavor the master distiller is looking for. And when they don’t, that’s where expert blending comes in.
Why does the barrel have to be new (first use)?The shortest answer to this is that it is required by U.S. Federal regulations. Bourbon must be aged in “new, charred oak containers” in order to meet the legal definition of the spirit.
It is likely that this addition was made to maintain the character and integrity of the flavor profile of Bourbon Whiskey. Repeated use of barrels for Whiskey, or other liquids, would result in dampened and diluted versions of all the flavor contributing factors discussed above.
I don’t know about you, but after all of this deconstruction of flavor, I’m about ready to sit down and just appreciate the experience of a good glass of Bourbon. Off I go, cheers!
Cover image: Russell's Reserve (Facebook: @officialrussellsreserve)