An Insider’s Look at the Whiskey Aging Process

From malting, the distillation process, aging, and finally, bottling, a lot goes into making your favorite whiskey. But a big part of the distinct aromas and flavors can be attributed to the aging process.

What is Whiskey Aging?

Aging is the process of storing distilled spirits in barrels for a specific period of time. The purpose of doing this is to remove harsh flavors from the raw alcohol while adding distinct flavor characteristics found in the barrel's wood.

Whiskey aging has not always been there. People used to drink whiskey young or even right out of the still. But that all changed in the middle of the 19th century when a great wine blight (wide-spread crop disease) struck Europe, destroying vines across the continent.

As Cognac, brandy, and wine supplies plummeted the aristocracy and the higher classes had to find an alternative. That alternative was whiskey. But, in order to get the whiskey to the market, distillers had to put it in barrels. The longer the journey took, the better the whiskey tasted. Over time aging whiskey in barrels became the norm.

Types of barrels used 

The practice of making casks is a highly skilled and traditional art called coopering. The barrels or casks are often made of oak. This is because oak adds to the taste and aroma while removing undesirable elements like sulfur compounds from newly distilled whiskey.

The varieties of oak used are:

American white oak-this is the most commonly used wood in barrel production. It often imparts mellow, caramel, vanilla, and soft flavors on the whiskey. This oak is grown in the Eastern United States and parts of Canada. It’s a fast-growing tree making it less expensive than the European oak. In fact, all Bourbon whiskey has to be aged in charred American oak barrels.

European oak- tends to add bitter, spicy, and vanilla flavors to whiskey during aging. It grows all over Europe as well as Turkey and Russia.

Mizunara/Japanese oak- provide the vanilla, coconut, spicy rye characteristics, oriental incense, and sandalwood notes. Japanese oak is rare and takes 200 up to 500 years to grow. This makes it very expensive and, as a result, is hardly used anymore in whiskey aging.

Other than oak, other woods used to make whiskey casks include hickory and maple wood, French oak, redwood, chestnut.

whiskey barrel wood

The toasting and charring of whiskey casks

Charring is more aggressive and breaks the surface of the wood. The wood is burned to a crisp, resulting in a rough surface known as “alligator skin.” The interior of charred barrels are blackened and have much more ash residue, resulting in a much darker colored whiskey.

As far as flavor goes, charred wood imparts sweeter flavors like caramel and honey. This is because the wood sugars are caramelized when heavily burned and leach into the whiskey during the aging process. The carbon in the ash also acts as a filter for the harsher elements of the liquor resulting in a smoother mellower flavor.

Toasting heats the wood gently and only results in a brown layer on the surface of the barrel.  Toasted barrels add spicy accents to the whiskey. Because the wood hasn’t been burned to a crisp, the sugars haven’t had time to caramelize, making the whiskey a bit sharper on the tongue.

To char or to toast?

It really comes down to a matter of preference. If you like a smooth, sweet-tasting whiskey, then seek out whiskies aged in charred barrels. If you enjoy a strong, spicy drink, then a toasted barrel whiskey would be more to your liking.

Main Factors that Affect How Whiskey Ages in a Barrel.


The external climate where whiskey barrels are warehoused greatly impacts how rapidly the whiskey ages. As it gets hotter, the whiskey expands, seeping into the pores of the barrels and reacting with the oak to extract color, flavors, and aroma.

When the whiskey cools in the winter, it shrinks and flows back out of the wood along with all those extracted compounds from the barrel. This swing from cold to hot is called a “heat cycle”. Without it, whiskey takes longer to develop deeper, more decadent flavors.

Warmer climates accelerate aging: a whiskey aged a year in Kentucky shares the same chemical compounds as a whiskey aged two or three years in Scotland.

Size of barrel

There is no limit when it comes to barrel sizes. Some are small, while others are double their sizes or even more. Large casks include the Madeira Drum(172 gallons) and  Butt(132 gallons). The American Standard Bourbon(53 gallons) and Cognac(79 gallons) are medium-sized barrels.

The small casks include the Quarter (33 gallons) and Bloodtub(13 gallons). The size of a barrel determines the level of exposure the whiskey has to the wood. As a result, whiskey in smaller barrels will age faster.

whiskey barrel sizes

New vs. used

As mentioned above, by law, bourbon is aged in new charred American oak barrels. But why "new"? A virgin barrel is one that has never been used to age whiskey. This type of cask imparts stronger flavors than a barrel, which is being reused and has already soaked up whiskey for years.

Scotch distillers primarily use ex-bourbon barrels, as well as ex-sherry barrels, some of which were specifically "seasoned" with sherry for the sole purpose of eventually aging whiskey.

First-fill barrels, which have only been previously used once, second-fill barrels, previously used twice, and those repeatedly used beyond that, offer lower levels of their original flavors.

Final Thoughts

A lot of factors determine the final taste of whiskey other than the time spent in the barrel. From choosing the perfect wood used to make the barrel to the climate, aging whiskey is no easy feat.

As you take a sip of your favorite whiskey or enjoy a whiskey cocktail, you can’t help but appreciate the craftsmanship that goes into bringing out those intricate aromas and flavors that make whiskey so popular.


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