The short answer: Whiskey comes off the still clear. Its color, and a large share of its flavor, comes from the oak barrel. The type of oak, how long the wood dried outdoors, and how hard the barrel was toasted and charred all shape the vanilla, caramel, coconut, spice and grip you taste.
How much comes from the barrel?
All of the color in a straight bourbon comes from the barrel, since new-make spirit is clear. Flavor is harder to measure. Distillers and coopers often credit the barrel with well over half of it. A 2010 trade write-up of a Woodford Reserve release put the figure near 70 percent of flavor and aroma. No one has published a single agreed number, so read any percentage as an informed estimate.
Few companies have studied the barrel as closely as the one that makes many of Kentucky's.
Independent Stave Company: a century of barrels
Independent Stave Company, whose barrel business goes by ISC Barrels, began in 1912 when T.W. Boswell, who grew up in the white oak country of the Missouri Ozarks, started a stave mill under his own name. During Prohibition the business kept going by making barrels for fruit and salted meat.
His son J.E. Boswell renamed it Independent Stave Company in 1936. The name made a point: it was a mill not owned by any distillery, brewery or winery, free to supply anyone. In the 1950s the family opened its first cooperage in Lebanon, Missouri. A second, in Lebanon, Kentucky, followed in 1983. The third generation, John Boswell, expanded overseas, including a stave mill in France's Vosges region, and founded the company's research and development division. The fourth generation of Boswells runs it today.
That research arm is what sets ISC apart. Its research center in Lebanon, Kentucky, keeps an experimental whiskey library of barrels that vary one thing at a time: oak species, grain, seasoning, toast and char. Andrew Wiehebrink, ISC's director of spirit research and innovation, publishes the results as numbered trials. Many of the findings below come from that work.
American white oak versus European oak
Bourbon barrels are almost always American white oak, Quercus alba. Scotch and wine producers also use European oaks such as Quercus robur.
- American white oak is known for higher levels of oak lactones, the compounds behind coconut and sweet wood notes. In ISC's Trial 4, bourbon aged in American oak had the highest lactone levels of the three oaks tested.
- European oak tends to carry more tannin. Oxford research on Quercus robur describes it as high in tannins but low in oak lactones.
ISC also found the differences take time to show. At three years, its tasting panel did not pick out a clear difference tied to lactones. By four years, the American oak whiskey was nearing the level where tasters reliably notice them.
Why bourbon needs a new charred barrel
Federal rules (27 CFR 5.143) require bourbon and rye whiskey to be stored in charred new oak. Corn whiskey is the odd one out: if it is aged, it goes into used or uncharred new oak. You can see how these rules show up on labels in how to read a liquor label.
The new-barrel rule creates a steady supply of used barrels. Many go to Scotland, where ex-bourbon casks are a staple. Scotch must be aged at least three years in oak casks of no more than 700 litres, and used casks are allowed. Others go into aged tequila, rum and barrel-aged beer.
Seasoning: letting the staves sit outside
Before a stave becomes a barrel, it dries in the open air. ISC seasons staves and heads on its yard for at least three to six months, with 18 and 24 month options.
That time does more than dry the wood. It leaches out some of the harsh tannins. Fungi on the wood help break it down chemically, as writer Charles K. Cowdery has reported. Most important, hemicellulose and lignin, which make up about half of a stave, begin to break into pieces that later turn into flavor.
ISC measured the payoff. Bourbon aged one year in barrels made from 18-month seasoned wood showed a 32 percent boost in overall flavor concentration compared with 6-month wood. Wiehebrink summed it up: "More seasoning = more flavor."
Toasting versus charring
Coopers use fire in two ways.
- Toasting is slow, gentle heat. At ISC's Missouri cooperage, a computer tracks each barrel's heating against a target curve, and profiles run from about 7 to 45 minutes. Toasting builds a thick band of cooked wood that releases vanilla, caramel, coffee and spice.
- Charring is a short, intense burn that blackens the inside surface. Spirit barrels are usually toasted first, then charred.
Char is graded by level, set mostly by burn time. Whisky Advocate gives these approximate ranges:
- #1: up to about 15 seconds, sometimes called a flash char.
- #2: about 20 to 30 seconds.
- #3: about 30 to 35 seconds, the most popular level.
- #4: up to about a minute. This is the "alligator char," named for the cracked, scaly surface it leaves.
Each distillery sets its own spec. Woodford Reserve's standard char is about 25 seconds, and the second barrel for Woodford Double Oaked gets about 5. The numbered system is fairly new. ISC's records show it wasn't used until the 1950s. Before that, buyers named a thickness. ISC holds an 1826 letter asking for barrels charred to a sixteenth of an inch.
The char filter and the red layer
Charring leaves two zones in the stave.
- The char layer is mostly carbon. It holds little flavor itself, but it acts as a filter, taking out some rough compounds as the whiskey moves in and out.
- The red layer sits just beneath it. Heat has broken the wood's sugars and lignin into caramel-like notes and vanillin. This is where most of the good flavor comes from.
Here ISC's research turned up something you might not expect. A lighter char gave more flavor, not less. In Trial 1, bourbon aged about four and a half years in Char #1 barrels had the highest total extractives and the most vanillin, with concentration falling as char got heavier. In Trial 2, Char #1 barrels gained 30.31 mg/L of extractives between their first and second years, against 22.55 for Char #4. ISC's explanation is that a thinner char gives the spirit easier access to the red layer. The overall flavor profile stayed similar across levels.
The compounds you taste
- Vanillin: vanilla. It forms when heat breaks down lignin.
- Oak lactones: the cis form smells of coconut. The trans form leans grassy, almost like celery. Very high levels read as woody.
- Caramelized wood sugars: caramel, toffee and brown sugar, from heat acting on hemicellulose.
- Tannins: oak ellagitannins bring dryness and grip on the palate. Seasoning and heat soften them.
- Eugenol: clove and spice.
To pick these out in your own glass, try our guide on how to taste whiskey.
Barrel size
The standard American whiskey barrel holds 53 gallons. The usual account is that barrels held about 48 gallons before World War II, and distillers went up to 53 to stretch scarce wood, since that was as large as they could go and still fit existing warehouse racks. Distilleries repeat this widely, though we have not seen a wartime record confirming it.
Size changes speed. A small barrel has more wood surface for each gallon, so it extracts faster. Balcones, working with ISC, saw the difference between 5-gallon and 53-gallon barrels right away, and found that larger barrels took on oak and tannin more slowly still. Balcones has since filled 500-litre barrels to slow things down further, betting that a gentler pull from the wood gives the whiskey time to develop more depth. Faster is not the same as older.
Finishing casks
A finish moves mature whiskey into a second barrel for a short stretch, often a cask that held wine, port, sherry or rum, or a fresh new oak barrel. Most of the effect arrives early. In ISC's Trial 8, two-year-old bourbon moved into a new oak barrel picked up most of its extra extraction within the first few months. Maker's Mark's 2022 BRT releases each spent eight extra weeks with added toasted oak staves.
For how climate and warehouse position shape the same barrel, read why climate and warehouse location change how whiskey ages.
FAQ
Why does bourbon have to use a new barrel?
Federal standards of identity require bourbon and rye to be stored in charred new oak containers. A fresh charred barrel gives the most color and wood flavor, which is a big part of bourbon's style.
What happens to bourbon barrels after they're emptied?
Many go to Scotland for Scotch. Others age tequila, rum and beer.
Is a heavier char better?
Not necessarily. ISC's trials found that lighter char gave higher total flavor concentration and more vanillin, while the overall flavor profile stayed similar. Heavier char filters more and suits longer aging for many distillers.
What is alligator char?
It is the common name for a #4 char, a burn of up to about a minute that leaves the inside of the barrel cracked and scaly, like alligator skin.
Do small barrels make whiskey age faster?
They extract oak faster because there is more wood surface per gallon. That is not the same as the slow changes that come with years in a full-size barrel.