By: Sarah E. Coleman

More than just a transportation vessel, the barrel in which bourbon is stored plays a key role in shaping the final product.

While it might seem that a bourbon barrel is nothing more than a unique way to store America’s signature spirit, nothing could be further from the truth. Master distillers, and others involved in the bourbon-making process spend as much time agonizing over the sourcing and creation of the barrels as they do over the recipe for the bourbon stored within it.

All barrels made in the United States that hold bourbon are required, by federal law, to be made of charred oak and to be used only once. While this may seem wasteful, there is actually a healthy market for used bourbon barrels. Some barrels are reused to hold wine, rum, scotch and beer. A more-recent phenomenon, driven by the explosion of interest in bourbon, is to repurpose used barrels for everything from garden planters, furniture and wine racks, to unique works of art.

Why White Oak?

Most distillers in the Unites States use white oak (Quercus Alba) to create their bourbon barrels. Though history has shown that this is the wood of choice for storing bourbon, some distillers have used French Oak and other types of oak to store their aging bourbon.

So, why is oak the wood of choice for bourbon barrels? For one significant reason: White oak wood contains tyloses, which essentially act as “plugs” for the pipes that allow for the flow of water inside a tree when it is living. Activated when the tree is stressed, tyloses dam up the vascular tissue of a tree to prevent further damage.

In barrel making, these tyloses create a leak-proof seal that is ideal for storing liquids with little air exchange. Additionally, white oak is abundant in North American forests and the trees can grow quite large. Many distilleries in Kentucky source their wood from Ohio, Kentucky, Tennessee, Missouri and Minnesota.

A Complex Container

Bourbon has hundreds of compounds that contribute to its unique flavor and many of these compounds are naturally found in the oak wood from which bourbon barrels are made; the wood imparts its color and flavor into the finished product. Wood is highly complex; its chemistry varies by individual tree and depends on where it grew, what elements it experienced as it grew, and what stressed the tree during its growing season.

Because of this, some distilleries are now producing test batches of bourbon using barrels made from trees in specific locations to see what characteristics might be imparted into the bourbon’s flavor. 

Oak barrels add taste and aroma to the whiskey. Some of these include vanillin, toastiness and wood sugars. Oak also removes undesirable flavors from the spirit, including sulphur compounds.

Anatomy of a Bourbon Barrel

Barrels are made in a cooperage, and those who make them are called coopers. “Coopering,” is a skill that has been used since ancient times; barrel making has been depicted in ancient Egyptian tombs as well as been mentioned by Greek writers.

Kentucky, home of the Bourbon Trail, boasts multiple cooperages located throughout the state. Each bourbon maker has different barrel requirements in order to set their spirit apart from others, but some barrel characteristics are the same no matter what company is producing them. 

Every barrel used to hold bourbon is made of between 33 and 36 staves, or pieces of wood. Irregularly shaped, these pieces are not held together with glue or nails; they are held together solely by the pressure of the wood on the metal rings. Each barrel has six hoops and 12 rivets that hold the steel together, and can hold up to 53 gallons of bourbon.

Though nowadays the hoops holding the wood together are generally made of iron, years ago these rings were made from flexible bits of wood. These hoops go over the “bilge,” or the “bulging center” of the barrels’ shape.

The liquid is placed inside a barrel using the bung hole, a hole within a wider stave in the barrel. A “bung” or plug is used to seal the hole in the bourbon barrel.

An Intense Process

Barrel making, like most parts of the bourbon-making process, is incredibly hands-on. Though White Oak is readily available, barrels are not easy to make. The wood must be harvested and sawn carefully to avoid mold and decomposition. The wood then must be resawn into staves and made into barrels. This entire process can take up to three years.

How long the wood is dried before it is made into barrels has a direct impact on the taste of the bourbon it produces. Though both air and kiln drying are acceptable means of drying the wood used to make bourbon barrels, most wood used to make barrels has been kiln dried for a few weeks before it is pressed into use.  Drying the wood too quickly can create cracks and splits that can cause a barrel to leak.

When drying the wood outdoors, exposure to the elements helps impart sweet, spicy and fruity flavors into the final whiskey. Green wood is high in tannic acids, which can taste harsh; exposure to rain and sun can tame tannic acid levels while unlocking the vanilla flavors in the wood. Exposure to high heat (like that of a kiln) helps finish the drying process.

These piece of wood are then cut into staves to create the pieces of wood that will be used to form the barrel.

The Charring Process

In many cooperages today, wooden staves are steamed prior to being shaped into a barrel to allow them to be bent into place before the hoops are added. They are then charred with an open flame to caramelize the wood and release desirable flavors to be imparted to the bourbon ages inside. There are several theories around when barrels were first charred intentionally to impart flavor to aging bourbon. Some accounts depict that the first charring occurred by accident. Prior to the Industrial Revolution, when steam was harder to source, coopers would assemble the staves, then use a strong fire to heat the inside of the barrel in order to make the wood flexible enough to be bent into barrel shape—this charring was a necessity of the manufacturing process. Only after steam was more readily available did it became clear that this charring was unexpectedly adding flavor to the bourbon.

Though charring is no longer necessary to shape the barrel, it is now a key component in what creates the unmistakable flavor of American Bourbon whiskey. The caramelization of the oak allows a myriad of flavors to be imparted on aging bourbon, including high levels of vanillin, caramel flavors, tannins, spices, and cloves.

The use of fire on the inside of the bourbon barrels breaks the chemical bond of lignin and cellulose, and creates smaller molecules that add flavor to the liquid stored inside it. Each distillery has preferred specifications for how they want their barrels created, including how they are dried and toasted, as well as the level and length of charring they prefer.

Bourbon barrels are typically charred for 40 seconds to 1 minute, but some distilleries have experimented with longer charring periods to see how it affects the finished product. Charring is the most dramatic parts of the barrel-making process. Open-ended barrels are placed over burners and exposed to heat as high as 1,500 degrees F. The amount of time the barrel is exposed to the flame will determine how deep into the wood the char goes.

Different coopers define their own char levels, typically on a numeric scale. For example, Independent Stave Company defines a #1 char as 20 seconds of open flame, while a #7 char is an entire 90 seconds of charring.

The Aging Process

Once charred, the heads of the barrel, also charred through open flame, are attached to each end and the bung hole is drilled into one of the staves on the middle bulge area. The barrel is then filled with water to check for leaks, and if none are found, it is shipped off to a distiller to fill with bourbon for aging. Most distillers also conduct their own inspection of barrels before filling them with bourbon. A leak or imperfection in the barrel could cause the precious bourbon inside it to go bad–a significant loss as a product that ages for many years cannot be replaced easily.

Distillers fill the barrel with whiskey and move it to an aging warehouse, where it will be stored. “Bourbon” has no minimum age requirement–it could be aged for one day and still be called “bourbon.” However, if the product is labeled as “straight bourbon,” it must be aged for a minimum of 2 years.

Barrel houses are buildings where barrels are stored, but they can vary tremendously. The environment in which a barrel ages is a critical part of the resulting bourbon flavor, and different distillers have their own traditions and preferences for how they age their product. Some are heated, some are unheated; some are constructed from brick and some from metal. Some buildings have windows, and some do not. Some utilize a shelf-like system to store the barrels (known as ricks, hence the term “Rick House”), and some store barrels on pallets. Distillers will rotate their barrels periodically to expose them to multiple temperature variations, which helps create consistency in flavor among different barrels aging in one warehouse. The barrel also “breathes,” as the alcohol passes in and out of the charring, creating a smoother bourbon as impurities are filtered out.

A Perfect Product

While each distillery has their own recipe for not only the grain bill used in their bourbon, they also have a finely tuned recipe for the creation of their barrels, involving everything from how long the oak wood they use is aged to how many seconds a barrel is charred for. This attention to detail allows each bourbon to have its own distinct flavor, creating vast differences between bourbons and even between different bourbon products from the same company.

The Angels’ Share

By federal law, “straight bourbon” must be aged for a minimum of 2 years. Though the barrel may be filled completely full before being placed in the barrel house, when it is removed and then opened for tasting, some of the liquid is missing. Caused by evaporation, this phenomenon is known as the “angel’s share,” and can be 10 percent of the barrel’s volume the first year the barrel is aged and 2 percent every year thereafter. Legend has it that guardian angels watch over the product as it ages, hence the unique moniker.