American Oak and its Contribution to Irish Whiskey

November 1, 2021

by Aaron Kendeall

The chances are high that the Irish whiskey you last enjoyed spent a large portion of its maturation period (3 years +) sleeping soundly in a wooden barrel made of American oak that had previously been used to age bourbon – Irish whiskey’s American cousin.    

The American white oak tree – Latin name, Quercus Alba – is plentiful in the United States with forests in the Ozark Mountains of Missouri providing much of the lumber coopers use to produce oak staves. It has many properties that have led to it long being a go-to for barrel making.

Compared to other types of oak trees, the American white oak grows tall and straight, with a stout trunk and linear branches. This makes for excellent lumber, free of knots and makes the cooper’s job much easier when building the barrel. Although oak is strong, it can also be formed and bent through the application of steam and heat.

White oak trees also naturally produce tyloses – an important globular substance that gives the cells of the wood a closed structure making it water-tight and resistant to rot, insects and mold.

History of the Oak Barrel in America

The qualities of the American white oak led to its early adoption in the bourbon making process though early on, its use was not mandated. In fact, other woods like chestnut were initially used to create barrels used for whiskey maturation.

Barrels were used not only for storing, shipping and maturing whiskey, but historically on the American frontier it was the storage container of choice for all manner of goods due to the abundance of oak. Barrels were also easily transported – despite weighing up to 500 pounds, a 53-gallon bourbon barrel could be rolled and stacked by one or two people using ramps.

The practice of charring (applying a flame to the inside of the barrel) was also a product of happenstance. Merchants reused barrels for as long as they could and would alternate contents in the same way shipping containers are used to transport various products today. To prevent flavors or chemicals affecting the flavor of the spirit inside, whiskey makers would often expose the inside of the barrel to flame to burn off any residuals from the previous inhabitant. It was only after contents were compared at the beginning and end of the journeys to market did distillers realize the positive flavor and color impacts charred barrel maturation had on the finished product.

While the use of charred American white oak barrels happened organically, the practice was codified into law after the repeal of prohibition in 1933. Lawmakers made the use of new charred oak barrels mandatory in order to gain support for the new legislation to make alcohol production legal among the powerful logging industry and coopers’ unions.

Making a Bourbon Barrel

When making a barrel, the wood must first be seasoned. This removes the moisture naturally found in the plant’s cells and also plays in important role in helping to break down tannins within the wood. These tannins help impart important flavor and mouthfeel qualities to the finished spirit but can leave the bitter qualities if the wood is not first properly seasoned.

The wood used to make the barrels used to mature wines and other types of beverages are often air seasoned or toasted. Bourbon barrels are set apart by the requirement they are seasoned through charring – a process in which they are exposed to open flame for a duration. Coopers use a scale of 1 through 5 to describe the length of time the inside of the barrel has been exposed to flame. A level 1 ‘flash char’ is exposed for a matter of seconds, while a level 5 ‘alligator char’ can be exposed to flame for over a minute. The amount of time the inside of the barrel is exposed to flame determines the thickness of the char layer inside the barrel.  

Inside the Barrel

While the char layer greatly affects the bourbon inside the barrel, by the time Irish whiskey is matured in the ex-bourbon barrel much of the effects have been neutralized.

As the whiskey matures inside the barrel, a number of chemical reactions take place including oxidation, evaporation and flavor extraction. Oxidation and evaporation are possible due to the semi-porous nature of the wood cells, which retain liquid but allow gasses to pass through. In this way, the oak staves act as a semi-permeable membrane, allowing oxygen to interact with the liquid inside, allowing aromatic compounds to infuse the spirit.

At the same time, evaporation takes place, allowing some of the high-volatile ‘heads’ spirit that have made it through the distillation process to burn off, improving the overall quality. The climate of the surrounding atmosphere – especially temperature and humidity – affect the evaporation process. In humid climates, a greater proportion of the evaporate will be alcohol – leading to a lower-proof spirit — while in lower-humidity climates a greater proportion will be water – leading to a higher-proof spirit. And higher overall temperatures lead to a greater loss to the angel’s share – what distillers call the percentage of liquid in the barrel lost over the years to evaporation.

Ireland’s temperate and even-keel climate allow for a steady, slow maturation process. Ireland’s climate is much less extreme in high and low temperature swings than many parts of the United States – notably Kentucky and Tennessee. The use of traditional dunnage warehousing — single-storied stone-lined warehousing in Irish whiskey allow the spirits to slowly mature when compared to multi-stories American rickhouses that have huge temperature swings between the top and bottom floors. While some distilleries in both Ireland and the United States have turned to climate-controlled warehousing to enable maturation in a finely-tuned climate, the majority of warehouses are non-climate controlled.

The flavor extraction process that takes place during the whiskey maturation process has perhaps the greatest influence on the flavor of the final product. Inside the barrel, chemical processes take place between the liquid spirit and the wood’s char and red layers that impact color and flavor. American white oak boasts a high vanillin content, which imparts the familiar vanilla flavors and aromas whiskey for which whiskey is known. Other compounds inherent in American oak are lactone, which is associated with coconut flavor, and wood sugars, which add sweetness and a rich character to the finished whiskey. 


Bourbon barrels’ usage in Ireland

Irish whiskey can be matured in any size barrel up to 700 liters, or about 185 gallons. The standard barrel size in the United States is the 53-gallon barrel – which makes it the de facto industry standard in the whiskey industry across the globe. But it’s important to note that the 53-gallon barrel is not mandated by law in the U.S. or Ireland, and Irish whiskey makers utilize many barrel sizes to mature their spirits.

Bourbon barrels began to be imported into Ireland in the 1960s due to their widespread availability initially and subsequently when it was realized just how compatible these barrels were with the styles of whiskey produced in Ireland. Over the ensuing decades with the consolidation of distilling on the Ireland in Old Bushmills Distillery and Midleton Distillery, these ex-bourbon American oak barrels became the most used barrel type used to mature Irish whiskey. In 2020 more than a quarter of a million ex-bourbon barrels were shipped to Ireland from cooperages in the United States to continue their lives in Irish weather.

Ireland permits the use of wood “such as oak” in the maturation of Irish whiskey, an interesting definition that leaves the door wide open to other woods which we’re increasingly seeing the usage of – chestnut, mulberry, acacia to name but a few.

While these wood types contribute interesting flavor compounds to our Irish whiskey, they are unlikely to replace American oak as the favored material for long-term, cost-effective maturation of our favorite spirit any time soon.