Whisky is famous for its maturation process— barrel ageing. Large wooden barrels are filled with fresh distillate and left to mature over many years, allowing the flavour to develop.
Some estimates say up to 70% of a whiskies flavour develops while it is being barrel-aged. So the selection of wood, barrel treatment, and length of maturation are all important factors that impact flavour. Whisky Barrels are serious business for distilleries and their coopers.
Below we explore how different types of barrels impact flavour in whisky, and how distillers make decisions on how to treat the barrel.
Type of Wood
Oak is the ideal choice of wood for ageing spirits. It is durable enough to last for long periods of time without leaking, yet porous enough to allow liquid to soak into its structure and impart flavour.
There are several types of Oak used around the world, each with its own unique impact on flavour.
American Oak – Sweeter vanilla, caramel, and brown sugar flavours.
French Oak – Subtle pepper spiciness and notes of vanilla.
European Oak – Spicier with more earthy wood input.
Japanese Oak – Complex notes of sandalwood, coconut, spice and incense.
Size of the Barrel
Coopers produce barrels in many sizes, largely based on tradition (with old-world names to boot).
It’s not critical that you remember each different barrel size, as it won’t help you determine the flavour of a whisky. However, it can be interesting to remember this rule of thumb; maturation takes place faster in a smaller cask— as more of the whisky has direct contact with the wood than in a larger cask.
The most popular barrel sizes for whisky are:
- Large – Butt 500L
- Medium – Bourbon 200L, Hogshead 250L
- Small – Quarter cask 125L
Before any liquid is put inside a barrel, the wood goes through two heat-treatment processes Toasting & Charring. These treatments help activate the flavour compounds in the wood and prepare the Oak to receive spirit for ageing.
Charring adds a thin layer of charcoal to the inside surface of the barrel, opening up the wood and exposing more surface area for the whisky to contact. The charcoal also helps to filter any impurities out of the whisky, and mellow any sharp flavours from the distillation.
Barrel life and fillings
The flavour profile of a wooden barrel develops and matures along with its contents. Understanding a barrel’s origin, and its impact on flavour is an indicator of what you can expect from a bottle.
New Oak barrels have a strong impact on whisky, with many of the intense vanilla & spice notes transferring to the whisky in the first few years— perfect for younger whiskies such as Bourbon.
First-fill barrels, refer to the first contents of a given barrel— often Bourbon, Rye, Red Wine or Sherry. Barrels retain residue from their first-fill, which adds its character and flavour to the second-fill whisky.
Second-fill barrels, suit whiskies that are going to age for a longer time (10+ years), as the impact of the wood is less intense and will be added gradually over time. Refill barrels also take on the characteristics of the first-fill liquid— ex-Bourbon barrels add vanilla and a light sweetness, while sherry casks add richer sweetness with dried fruit notes.
Distillers can also choose to age whisky in barrels that have already been used multiple times, older barrels impart a subtle flavour to the whisky. This allows distillers to “Finish” the ageing process in a more interesting barrel, such as Pedro Ximénez, Apera, Muscat, Port, or Tawny casks.
Storing the cask
Whisky is stored in large warehouses at the distillery. As the temperature fluctuates with the seasons, the wood expands and contracts, drawing whisky in and out of the wood. This effect is more pronounced in warmer climates that see bigger seasonal changes in temperature.