Whiskey 101: A Simple Guide to Aging Bourbon

What Makes Bourbon… Bourbon? It may appear as if there are many seemingly arbitrary rules that define what can or can't be classified as bourbon. However, to create the distinct flavor of bourbon that you know and love, there are a few essential steps that distillers must take. 

  • A bourbon mash bill (the mixture of grains from which the spirit is distilled) must contain at least 51 percent corn.
  • Bourbon must be made in the United States. Contrary to popular belief, it doesn't have to be produced in Kentucky, although 95 percent of the supply is.
  • The whiskey cannot enter the barrel at higher than 125 proof. It cannot enter the bottle at a proof less than 80. Sometimes distillers add water to lessen the ABv when necessary. 
  • And finally, aging must take place in a new, charred oak barrel. Afterward, the bourbon barrels are shipped to other countries to serve as "used" barrels for Scotch, aged rum, and even tequila.

Bourbon aging is the focus of this latest iteration of our long-running whiskey series.

Why Does Bourbon Need To Be Aged?

Aging is an essential part of the process of making whiskey. This is what transforms the clear, distilled spirit into a much loved amber elixir, loaded with flavor and intense complexity.

After the distillation process, the whiskey distillate has a very high alcohol content and very little in the way of flavor.

The aging process helps to mellow out the harshness of the alcohol (such as sulfur) while also imbuing the bourbon with its distinctive flavor. The flavor notes are drawn from the wood itself; the longer the bourbon sits in the barrel, the more rich and complex its flavor profile will be. 

But there's a limit.

Unlike wine, which usually gets better the longer it ages, bourbon has an aging lifespan. If bourbon is over 15 years, it tends to lose some of its flavor complexity and take on a bitter taste.

How Long Should Bourbon Be Aged?

For most bourbon, there's no stipulation on how long it should be aged, with two exceptions:

  • When bourbon is labeled "straight bourbon," it has been aged for at least two years. Straight bourbon aged between two to four years must display an age statement on the bottle. Straight bourbon aged over four years requires no age statement.
  • When bourbon is labeled "bottled-in-bond," that's a guarantee it has been aged at least four years and at 100 proof (or 50% ABV).

If your favorite bottle of bourbon doesn't have an age statement, there's no reason to worry. The majority of the bourbons sold today are blends of different ages and barrels, which allows producers to maintain a consistent flavor profile. These typically don't have any age stated on the label.

Critical Factors To Bourbon Aging

The Wood

Aging is really about exposure to the wood — this is what creates the look, smell, and taste of the bourbon we love. A bourbon barrel must be a new, charred oak cask. The most commonly used wood for bourbon barrels is usually American white oak. Also, while the size of the barrel isn't dictated, traditional bourbon barrels are 53-gallons (200L).

To char a barrel, flames are shot through it for a certain amount of time. The majority of distillers use a process known as No. 4 char—the barrel is toasted for about 55 seconds at high heat levels (284°F and above). 

Generally, most barrels aren't charred for more than 1 minute. A No.1 Char is 15 seconds, No.2 is 30, No.3 is 35 seconds, and the No.4 Char is 55 seconds. The No.4 Char is also known as the "alligator char," since the interior of the oak wood staves is left with a rough, shiny texture similar to that of an alligator skin.

The purpose of charring is to change the oak's nature and yield the best possible reaction between wood and whiskey. As the flames lick the sides of the barrel, they cause the sugars in the woods to caramelize. 

As the bourbon is aged, these caramelized sugars will impart their distinct flavor profiles to the distilled alcohol. The typical flavor notes are vanilla, caramel, baking spices, and coconut. 


The external environment where the whiskey barrel is stored significantly impacts how rapidly it ages, how much interaction there's with the wood, and how much evaporation takes place.

The seasonal temperature fluctuation in Kentucky causes the bourbon barrels to impart more color and flavor to the whiskey. During the warmer seasons, the heat expands the spirit causing it to seep into the wood's pore.

When the temperature levels drop, the wood contracts, forcing the liquid out. This cycle is repeated multiple times during the aging period.

The bourbon barrels also "breathe." The unique physiological makeup of oak allows oxygen to pass directly through the wood while remaining liquid-tight. Oxygenating the spirit is the integral step of the whiskey maturation process.

This releases volatiles and further integrates the wood's flavor and aroma compounds into the whiskey.

The level the barrel is stored in the warehouse also affects how quickly the bourbon ages. Typically, there is no temperature regulation in a whiskey aging warehouse, popularly known as a rickhouse.

The higher floors are typically warmer and less humid which leads to more water evaporation. This results in a higher concentration of flavor and aroma in the whiskey. In the humid and cooler bottom floors, it’s the alcohol that evaporates faster, giving rise to the Angel's Share phenomenon.

Either way, except in the case of cask strength spirits, the bourbon distiller will usually add water to the spirit before bottling to ensure the proof is within the recommended level.

Read more: 10 Whiskey Terms That Everyone Should Understand.

Does Bourbon Age In the Bottle?

Once bourbon is bottled, the aging process stops. Unopened, a bottle of bourbon won't go bad. You can store it for decades with no worries. Just make sure to keep the bottle in the dark, dry place where the temperature won't fluctuate too much.

Have you left a glass of bourbon out overnight and tried to drink it in the morning? How did it taste compared to the night before?

Once a bottle of bourbon is opened, you need to be careful about oxidation. If your bottle cap isn't sealed tight, you might experience some evaporation, and the whisky's aroma will gradually change (for the worse). That's why it's important to make sure the lid is always screwed on tight.

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