The New Science of Old Whiskey

Bourbon makers attempt to perfect a 2,000-year-old invention.
Buffalo Trace Distillery

In April 2006, a tornado struck Warehouse C at the Buffalo Trace Distillery in Frankfort, Kentucky. In the aftermath, the building looked like a diorama: part of the roof and one wall had been artfully removed to reveal the 25,000 barrels stacked inside. Miraculously, not a single one of those barrels was damaged—proof, perhaps, of the Major League manager Leo Durocher’s maxim: God watches over drunks and third basemen.

Repairing the warehouse took several months, and during that time the barrels on the upper floors were exposed to rain, heat, and sun. Mark Brown, Buffalo Trace’s president and CEO, joked at the time that the distillery should sell the whiskey as “tornado-surviving bourbon.”

It turned out to be no joke. The barrels were opened about five years later (the liquor inside had then aged for nine to 11 years) and, says Brown, “the darnedest thing is, when we went to taste the whiskey, it was really good. I mean really good.” The company decided to label the bourbon “tornado surviving,” and aficionados—who also found it superior to the usual product—quickly snapped it up. One went so far as to write Buffalo Trace and ask whether it planned to make more. “Not deliberately,” Brown replied.

Yet the tornado bourbon got the distillers wondering: What are the perfect conditions for storing the barrels in which bourbon ages? It’s a question that no one had really asked before, despite the oft-noticed phenomenon that barrels situated near the windows in warehouses have a tendency to become what managers call “honey barrels”—that is, ones that produce above-average whiskey. Moreover, the storage question was a logical follow-up to one that Buffalo Trace had already been pondering: How do you make a perfect barrel?

Both questions are critical for any maker of bourbon. It’s well known that a good portion of a whiskey’s flavor comes from the barrel it ages in, but less well known is just how large a portion we’re talking about. I recently asked a dozen or so people involved in the bourbon industry how much of the flavor comes from the barrel, and how much comes from other elements, such as the grains used or the distillation method. Most said that 60 or 70 percent of the flavor comes from the barrel, and one went as high as 80 percent. No one I spoke with estimated the proportion at less than 50 percent—meaning one of the trendiest liquors on the contemporary cocktail scene owes most of its flavor to a technology that’s thousands of years old.

Liquor barrels are essentially Dickensian nano-factories—dark, sooty, mysterious places from which marvelous things emerge. But they came to that role only after long service as simple containers for shipping and storage.

“The barrel, like the wheel, is one of the outstanding basic inventions of mankind,” wrote the historian William B. Sprague in a 1938 essay. Wooden-stave barrels first appeared millennia ago. In the first century A.D., Pliny the Elder noted their widespread use in the foothills of the Alps.

Used for centuries to transport everything from whale oil to pickles to nails, the barrel is far more ingenious than it appears at first glance. It bows out slightly in the middle (called the bilge), so each individual stave forms a longitudinal arch linking a barrel’s top and bottom. Each stave also abuts two adjoining staves, so they form, collectively, another arch, this one latitudinal around the circumference of the barrel. Barrels are thus remarkably stout and durable: if one tumbled off a ship’s gangway onto a wharf during loading, the impact of the fall was shared by all the staves, reducing the risk of breakage.

Barrel design also evolved such that a single stevedore could move one weighing hundreds of pounds. When standing upright, a barrel can be tilted and rolled on its edge; on its side, it’s even more maneuverable—only a tiny portion of the barrel touches the ground, so it can be spun in any direction, and a light push will start it moving. A skilled worker can bring it upright by rocking it a few times and popping it into position.

It took a long while, but in the past few hundred years, discerning drinkers of spirits noticed that something interesting took place inside containers made of white oak, which were often used for liquids because oak is tightly grained, preventing leakage. Rum shipped from the West Indies to Boston and whiskey shipped from Kentucky to New Orleans were always better when they arrived than when they left.

Today, barrels are central to the making of all manner of alcoholic beverages. Most winemakers use oak barrels that are toasted—that is, lightly browned on the inside through indirect heat. Liquor manufacturers more often use charred oak barrels, which are blackened with direct flame. Every spirit has its own barrel culture. Cognac tends to favor French oak (which is higher in tannins than American oak). A few rum makers, such as Zacapa Rum, from Guatemala, employ used sherry casks for finishing their product after it’s been aged in oak, adding flirtatious complications to the end result. Some American craft distillers, seeking to distinguish themselves from the major producers, have been experimenting with barrels made of alternative woods, including hickory and maple. Even some microbrewers are getting into the game, finishing beer in used bourbon casks.

But bourbon has long been the main driver of the spirits-barrel industry, for one simple reason: to be labeled “bourbon,” a whiskey is required by law to have been aged in new, charred oak barrels, which impart far more flavor than barrels that have been used previously. (After aging bourbon, barrels are typically sold to makers of rum, scotch, or tequila for reuse.) Bourbon producers thus tend to focus more intently on barrels than other spirit makers do. Brown-Forman, which makes Woodford Reserve, Old Forester, and Jack Daniels, has operated its own cooperage since 1945 to ensure a steady stream of quality barrels. “Why would we entrust all our color and 50 to 60 percent of our flavor to an outside vendor who’s also supplying our competitor?” asks Chris Morris, the master distiller of Woodford Reserve.

This barrel flavoring takes place in part because alcohol is a solvent that gradually breaks down elements in the wood over time. White oak in particular has an abundance of appealing flavors, including vanilla, nuts, and coconut, as well as butterscotch notes from sugars in the wood, which are caramelized during charring.

During the summer, heat increases pressure inside a barrel, and some liquor pushes itself through the char in the barrel’s wooden pores, enabling the carbon to filter out impurities. During the winter, the liquor moves in the reverse direction. The process is repeated with less vigor during the heating and cooling cycles of day and night. So merely by sitting in a barrel, whiskey is slowly being filtered through the barrel.

One bourbon maker lashed five barrels on a trawler, where they were subjected to salt air, sun, and constant rocking.

The rise of premium liquors in the 1980s, and a growing consumer willingness to pay more for complex aged spirits, brought new attention to the science of barrel aging. Three well-regarded players in the spirits industry—Brown-Forman, Independent Stave (one of the world’s largest barrel companies), and Buffalo Trace (which makes the super-premium aged bourbon Pappy Van Winkle, as well as Blanton’s, W. L. Weller, and others)—each launched programs to minutely analyze the chemistry of barrels, using new technologies that let them parse ingredients on the parts-per-billion level.

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Wayne Curtis is an Atlantic contributing editor.

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