How does bourbon get its flavor?

Source: Modern Thirst

My friendly neighborhood liquor store has noticed that I'm a pretty frequent flyer when it comes to bourbon, and they'll sometimes tip me off when good stuff comes in off the truck. Just before the holidays, the manager rang up a bottle of Old Rip Van Winkle 10-year for me. A few weeks later, it was some Weller's Antique 107 (not so hard to find if you don't mind a 30% markup, but I mind). Last weekend, when I asked if they had any Blanton's squirreled away, I was offered a bottle of their barrel pick.

I'm not A-list or anything (I think there are a fair number of people who need to die or move away before they'll offer me any Pappy 20-year), but it's a very nice gesture, and I'm grateful to them for it.

What this means is between a few good finds from a trip to Louisville last summer and the fruits of my budding romance with the local beverage peddler, I've got the beginnings of a bourbon collection.

Here they are, minus the Blanton's I picked up over the weekend, and the Angel's Envy and Eagle Rare I usually also have on hand:

If you know anything about bourbon, you're probably drawing some conclusions about me at this point.

Maybe you're thinking that I should spend less money on bourbon and more money on making my basement stairwell presentable. If that is the case, I would like to introduce you to my spouse, as you two would really hit it off.

If you're concerned that no one person needs this much bourbon in their home and I'm at risk of becoming a hoarder, then you probably found this blog by mistake and should head back to your favorite search engine to find the thing you were actually looking for.

Or, perhaps you've gotten the impression that I have a major sweet tooth. If that is the case, you would be right! With a few exceptions, the bourbons I favor are either wheated, or they're finished in barrels that previously held a sweeter substance.

What is it that makes bourbon taste sweet? Why do some bourbons taste sweeter than others? And how do different bourbons acquire different flavors?

In Good Taste

Bourbons can taste hot and spicy, mellow and floral, herby and sweet, or anywhere in between. The extent to which these flavors are imparted depend on a few factors: 1) the raw materials, 2) the barrels the bourbon is aged in, 3) the proof, 4) how long the bourbon is aged and its environment while that's happening, and 5) any finishing steps taken before the bourbon is bottled.

  • Hot, peppery: typically comes from having more rye in the mash bill, as well as higher alcohol content in the finished product
  • Sweet: high corn content in the mash bill, lower rye content (in favor of wheat), hemicellulose and lactones in the wood of the barrel
  • Floral or herbal notes: typically from the yeast
  • Cereal or grain flavor: usually present in younger bourbons, where the flavors of the mash aren't yet replaced by the flavors from the barrel
  • Vanilla or marzipan: lagnins in the wood of the barrel
  • Wood, custard, coconut: lactones in the wood
  • Caramel, toffee, malt, chocolate: from the barley and/or hemicellulose in the wood
  • Dry, spicy, tannic: from the tannins in the wood
For a longer, more thorough explanation, read on.

The Raw Materials

Photo: Wyoming Whiskey

You reap what you sow, or so they say. The quality and unique characteristics of the grains, yeast, and water used in making bourbon affect the quality of the final product, as you might imagine.

Of the raw materials, bourbon drinkers seem to focus mostly on the grains – specifically, the proportion of the grains relative to one another, which is called the mash bill.  In order to call it bourbon, the mash bill must be at least 51% corn, which means that distillers have some constraints in terms of how much they can leverage rye, wheat, and barley to alter the flavor.

Many, including the American Bourbon Association, claim that bourbon's corn-heavy mash bill is why bourbon tastes sweet. I'm sure this is partly true – avid fans of Scotch or American/Irish/Canadian whiskey can probably point out specific labels that are sweeter than others, but bourbon seems to be the only type of whiskey that is consistently so. That said, corn whiskey doesn't exactly tickle my sweet tooth the way finished bourbon does, so I'll highlight some other contributing factors in later sections.

First, let's talk about the other grains – rye, barley, and wheat – and how they impact a bourbon's flavor.

Rye-forward bourbons like Bulleit and Four Roses Single Barrel are hot and spicy, while "wheated" bourbons, like W.L. Weller and Pappy are considered smoother and sweeter – my sense is this is more about the absence of rye than the presence of wheat, but I could be wrong. Barley, meanwhile, is apparently used in bourbon more for its importance to the fermentation process than its impact on flavor, though it does add some toffee and chocolate notes in the end.

The yeast strains used by various distilleries may not get the spotlight like mash bills do, which is surprising given how often yeast comes up when people talk about craft beer. This Whisky Advocate video with Four Roses' Al Young explains just how important yeast can be to a bourbon's taste and aroma. Distillers may only have four grain options and just 49% of the mash left to play with, but the combination of the mash bill and the yeast profile opens up lots of room for creativity, adding fruity, herbal, spicy, and/or floral layers to the final product.

Finally, some distillers and bourbon industry cheerleaders claim the water makes a difference. Kentucky's limestone water, or "hard" water rich in mineral content, is said to be one of the reasons distilleries in the state have an edge in producing bourbon – the alkaline properties are said to be more conducive to the fermentation process, and the limestone does some filtering as well.

The Barrels

Angel's Envy Tasting Room (Photo Copyright The Rookie Barkeep)

In addition to the corn-heavy mash bill, the whiskey must be aged in new oak barrels in order to be bourbon (an ancient lobbying victory of the cooperage industry, the fine folks at Koval shared during a distillery tour), and the insides of those barrels must be charred.

That oak, as well as the extent to which it's charred, is where bourbon gets most of its flavor. As this article explains, there are four important compounds in the wood itself – lignin (contributes vanillin), hemicellulose (wood sugar), lactones (provides woody or coconut flavors), and tannins, which add spice and dryness.

Tannins can be troublesome long-term – the longer a bourbon ages, the more the tannins in the wood begin to overpower the other flavors. However, as the folks at Angel's Envy explain, higher char levels can reduce the interactions between tannins and bourbon. Charring of any level also catalyzes the more delicious reactions involving hemicellulose, lignin, and lactones.

The Proof

Bourbon can be distilled to a maximum 160 proof, must be casked at a maximum of 125 proof (62.5% alcohol), and not enter the bottle at less than 80 proof.

Generally, more alcohol means more heat and spice, but the alcohol content also plays an important role in the aging process – the higher the concentration of alcohol in the liquid going into the barrel, the more sugar and flavor it will leach from the charred wood, usually leading to more sweetness and stronger vanilla and caramel notes.

Ultimately, the final flavor profile depends not just on the proof at which the bourbon went into the barrel, but also how much water was added to the bourbon prior to being bottled (see "Finishing" below).

The Aging Process

Photo: Jefferson's 

The length of time a whisk(e)y ages is a big selling point, but why?

The answer is that the longer bourbon sits in a barrel, the longer it has been interacting with the wood.

Changes in temperature and humidity cause wood to expand and contract, and in a barrel, that results in the wood pulling liquid in and expelling it. The more frequent and substantial the fluctuations, the more interaction you get between wood and spirit, and the more flavor transfers from the wood into the spirit. The temperature and humidity levels around the barrel also affect proof and evaporation, which is why distillers put so much thought into the locations, materials and configurations of their rackhouses.

Jefferson's Ocean, a wheated bourbon produced by Jefferson's Bourbon, sits in barrels on ocean liners that track Great White Sharks. (Yes, for real – you can learn more about it here.) As the website states, "each voyage of Jefferson's Ocean typically crosses the equator four times, visits five continents and over 30 ports on an average sailing." Not only do the barrels experience more airflow and bigger fluctuations in temperature and humidity than rackhouse-aged barrels, but the fact that the ship is constantly in motion also means the liquid inside the barrels is never still – all that sloshing around means the spirit interacts differently with the charred wood on the inside. When I tried some for myself, it could have been my imagination, but I'm pretty sure the taste picked up some salty sea breeze flavor.

It's not just the compounds in the charred wood adding flavor as the bourbon ages, it's also the fact that the filtering process has been extended. The char imparts flavors of its own, but the charcoal that's produced by charring the barrel also serves to filter the whiskey, removing impurities and flavors that make the difference between moonshine and bourbon.

The Finishing Process

Angels Envy finishing in port casks (Photo copyright The Rookie Barkeep)

After at least two years (but typically longer), the distiller can bottle the spirit at cask strength, they can decrease the proof to as low as 80 by adding water (called "proofing") and then bottle it, or they can take a more creative route first.

Maker's Mark's Private Select program allows corporate and individual customers to personalize their barrel pick by adding their choice of 10 staves. These staves consist of different types of American or French oak charred at different levels, and they are added to bourbon still in their barrels at the very end of the aging process to impart additional layers of flavor.

Other distilleries finish their bourbon in used barrels to impart distinctive flavors from whatever it previously contained. Angel's Envy finishes their bourbon in port barrels (their rye is finished in rum casks), Wyoming Whiskey finishes their Double Cask variant in Pedro Ximenez sherry casks – both approaches add sweetness, dried fruit, and other flavors picked up from the lingering port and sherry soaked into the wood. Michigan's New Holland Brewing turned the concept of bourbon-barrel-aged stout beer on its head with the introduction of stout-barrel-finished bourbon!

Prior to bottling, distillers may choose to "proof" the spirit, which involves adding water to decrease the proof from cask strength (125 proof) to as low as 80 proof. Again, alcohol content affects the bourbon's taste, so decreasing the alcohol content also cuts some heat and spice.

Whatever distillers decide, what they can't do is add flavor or color to their whiskey if they want to call it bourbon.


  1. Awesome article. Most detailed explanation of the source of the flavors I've seen. Thanks for taking the time to put it together. Now go upgrade your stairs.


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