Although the oak trees used to make barrels can grow for more than 100 years before they’re felled and sawed into planks, the wood is still not ready to hold spirit. Staves need more time and various applications of heat before they’re ready to be bent into shape.
Specifically, before it can become a barrel, wood will be seasoned, toasted, charred, or some mix of those treatments. And while there are plenty of confident claims from coopers and distillers about those methods, the scientific literature about these subjects is far behind. The research is catching up, however, and there is plenty of fascinating information becoming available to distillers today.
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Seasoning
Like many aspects of modern cooperage, seasoning began as a production necessity, not as an intentional flavor decision. Fresh-cut wood is too wet to use for barrels, so it must first be dried for the final cask to be structurally sound. Historically, this meant simply stacking planks after milling, leaving the wood to dry in the sun until the moisture content was low enough to turn into staves. Depending on location, climate, and varying weather conditions, this process traditionally took from 18 to 36 months.
Besides making the wood usable for coopers, seasoning includes several natural processes that both affect the wood and have a dramatic impact on the flavor of the spirit or wine that will be aged in it. However, the precise mechanisms for the changes that take place in the wood during seasoning are not well understood. Certainly, heat from the sun and movement of air among the stacked planks does dry the wood. However, the effect on the chemical composition of the wood during this process is difficult to assess.
Many barrel makers believe the freeze-and-thaw cycle during the winter helps to break down the wood’s structure, making certain extractives more available to the spirit. Some scientists believe that rainfall may play a huge role in the benefits of outdoor seasoning. Rain soaks into the wood, eventually, extracting phenolics in much the same way that spirit does. Those phenolics include bitter tannins that would otherwise end up in spirit or wine later.
Another piece of the seasoning puzzle is a living creature: fungus. Hundreds of species of fungus have been found to live in oak as it dries in the lumber yard. Many of these fungi break down tannins and wood cellulose by means of enzymes. Some of these enzymes persist in wood long after it’s been toasted, charred, and filled with liquid. The enzymes remain active and can continue to decrease spirit bitterness for years.
Whatever the exact causes of these changes, seasoning clearly has a profound impact on the flavor compounds present in wood. Broadly speaking, seasoning helps remove wood tannins, including those that contribute to bitterness. Perhaps more importantly, many compounds are enhanced by seasoning, including those that provide characteristic notes to whiskey and other spirits. Wood that has been seasoned has an increased amount of extractable syringaldehyde, which provides spice and wood notes; furfural, which lends almond or grain notes; and vanillin.
However, while some compounds increase or decrease in a relatively linear way, others do not. For instance, some experiments have shown that outdoor seasoning does increase the availability of vanillin, but only up to 18 months. After another year and a half, the compound begins to decrease.
To make things more complicated, wood can be dried by kilning instead of outdoor seasoning, or a cooper can use some combination of the two methods. Kilning doesn’t seem to increase the levels of extractable wood compounds as intensely as outdoor seasoning, but there are plenty of exceptions to that rule. According to Gregory H. Miller’s Whisky Science: A Condensed Distillation, “eugenol is enhanced by kilning of Quercus alba, very slightly enhanced by natural seasoning, but lowered by the mixed approach.” Conversely, some studies have shown that levels of whiskey lactones in oak—important for the coconut and wood notes they impart to spirits—actually decrease with kilning.
Seasoning is often seen as superior to kilning and longer seasoning as preferable to shorter seasoning. You may even see blogs or certain barrel makers claiming as much. After all, the cost of barrels rises with the length of seasoning, so shouldn’t that price correlate with a better product?
However, because of the large number of variables involved, the science doesn’t back up this simplistic understanding. Still, seasoning is clearly beneficial, and knowledgeable coopers should be able to help determine what length of seasoning, kilning, or combination of the two would best match the distiller’s targeted flavor profile.
Toasting
Toasting involves heating wood while keeping it below the temperature at which it will combust. This has been standard practice for wine casks for centuries, but historically it hasn’t been used as much for whiskey casks. However, toasting is increasing in popularity, and many coopers who cater to the whiskey world are now offering it. In fact, one of the largest barrel companies, the Kentucky-based Independent Stave Company (ISC), includes toasting for their entry-level barrel.
A wide variety of toasting methods are used by different barrel makers, ranging from a simple fire inside a barrel to patented high-tech methods. Oregon Barrel Works in McMinnville, Oregon, opts for a simple open flame fueled by scrap wood. ISC uses a similar open flame with a technological twist: heat sensors connected to a computer let the coopers track the toasting process and adjust temperatures accordingly. Tonelería Nacional, based in Chile, uses a convection-style system that can be dialed in exactly, avoiding the caramelization that can take place to some degree with an open flame. West Virginia Great Barrel Company, meanwhile, provides infrared toasting.
Aside from the wide variety of toasting methods, there’s also huge variation in time and intensity of heat. Most barrel makers offer some version of light, medium, and heavy toasts—however, these categories can be quite different among facilities. In addition, some coopers offer even more levels of custom toasting. Usually this involves applying multiple temperatures for different lengths of time, meant to enhance specific compounds. For instance, a cooper might offer a custom toast profile meant to increase notes of cinnamon or almond in a spirit. Most companies keep these profiles secret.
However, despite the staggering number of methods and the variation, studies show that toasting generally results in higher levels of dozens of compounds that have a positive impact on whiskey, including furfural, guaiacol, and maltol. The latter is especially important in bourbon, where it’s associated with a sweet, burnt-caramel aroma.
Conversely, casks that haven’t been toasted or seasoned—or have received inadequate amounts of either process—will impart off-notes such as sawdust, green walnut, unpleasant earthiness, or even a rancid note. The sawdust note is especially egregious and is often due to (E)-2-nonenal, a compound that brewers may recognize; it’s associated with the stale, cardboard-like notes in oxidized beer.
Charring
Compared to toasting and seasoning, charring is a relatively straightforward process: Fire!
Direct flame applied to wood leads to combustion, blistering, and the breaking down of wood sugars. The origins of the practice are obscure—despite the tall tales that float around as origin stories in the bourbon world—but by the late 18th and early 19th century, the benefits of barrel charring were widely known. In fact, an 1896 article in a magazine called The Woodworker describes charring as a unique and prevalent feature of bourbon barrels.
Unlike toasting, charring is a straightforward process that differs little among barrel makers. The char level, usually rated one to four, simply correlates with how long direct flame is applied to the wood—from about 15 seconds up to one minute.
Likewise, the science of charring is thankfully straightforward. Hemicellulose, a major component of wood that is full of pentose sugars, breaks down when exposed to high heat, resulting in an abundance of those wood sugars becoming available for spirit to soak up. This is why bourbon and any other spirit aged in newly charred barrels are much sweeter than spirits aged in a used or uncharred cask.
In addition, furfural—already made partly available by seasoning and toasting—now grows by an order of magnitude at the higher temperatures of charring. Similarly, lignin, which is a major component of oak, degrades under the high heat of an open flame, releasing more of the important flavor compounds such as syringol (sweet, smoky), guaiacol (woody, spicy), and more vanillin. Lastly, Maillard reactions that take place at these high temperatures produce more maltol as well as cyclotene, an important compound that gives notes of caramel.
Aside from these compound “additions” to spirits, charring also plays an important subtractive role. A study from 2008 proved what many distillers already suspected: Charred wood can help remove unwanted congeners, such as fusel oil and sulfur compounds.
Knowing What You Get from Your Oak
Like many aspects of spirit production, the barrels we use to age our products are immensely complex. The wood itself is an almost ancient tapestry of compounds ready to fill whiskey and rum with flavor and nuance.
Beyond that, we have an even greater capacity to alter that tapestry through seasoning, kilning, toasting, and charring. And while we all might wish that these methods of barrel preparation resulted in simple changes that fit neatly into a catalog-like format—just tick the boxes of the flavors you want—the truth is more complicated.
However, there’s a growing body of research that coopers and distillers can access in the quest to optimize flavor profile and produce the best spirits we can.
