What is the purpose of charring the inside of a whisky cask?

The majority of a whisky's flavour comes from the wooden barrel it is aged in. There are a few factors at play, that determine the nature and degree of flavour the cask will add, including:

• How long the whisky is aged in the whisky cask
• The size of the cask
• What was in the cask before the whisky was added
• And, the species of wood used to make the whisky barrel

There's a fifth factor that's also very relevant: charring. In this article, we're going to look at charring whisky casks in further detail.

First up, what is charring?

Charring refers to the process of burning the inside of a wooden barrel for a short period of time. This is done before the casks are filled with new spirit for maturation. The inside of the barrel is exposed to flames for a specified time, causing the wood to char or partially burn. The depth of this char varies, leading to different "levels" of charring. In turn, the level of charr will then influence the whisky's flavour to a greater or lesser degree. 

Why char whisky casks?

There are a few key reasons why distilleries will char their whisky casks, all of which relate to the end influence on the whisky itself.

1. Caramelisation: As the wood chars, it starts to caramelise. This process breaks down the cellulose in the wood, releasing sugars. These caramelised sugars contribute to the whisky's sweetness, adding flavours like vanilla, toffee, and caramel.
2. Activated carbon: Charring opens up the oak's pores and creates a layer of activated carbon. This acts as a natural filter, absorbing some of the less desirable elements from the spirit, such as sulfur compounds.
3. Releasing tannins: Charring releases tannins from the oak. These compounds contribute to the whisky's structure and complexity, adding notes of spice, dark chocolate, and dried fruits.
4. Increased surface area: Particularly with higher levels of char, the surface of the wood will start to develop ridges and grooves. This increases the surface area of the cask. And the greater the surface area, the greater the level of interaction between the cask and whisky - and the more flavour that is added.

Different levels of char

Different distilleries use different levels of char, from a light toast to a deep char. Each level brings different influences to the end whisky.

• Light toast: A lightly toasted barrel provides subtle, delicate flavours. Whisky matured in these barrels often exhibits hints of fresh fruits, floral notes, and a lighter colour.
• Medium char: A medium char strikes a balance. It releases a mix of caramelised sugars and tannins, resulting in a whisky with a balance of sweet and spicy notes.
• Heavy char: Also known as "alligator char" due to its rough, cracked appearance, a heavy char adds bolder flavours. It often brings rich notes of dark chocolate, deep caramel, and smokiness.
• Re-char: One way to breathe new life into a cask that has previously been used is to re-char it. As the name may suggest, this is effectively re-charring the cask to introduce a new layer of charred wood within the cask and boost the flavours it will help create.

Spiritfilled's New Era of Cask process

We are the first and only scotch whisky bottler to use a process called New Era of Cask, or NEOC for short.

The NEOC process takes place in Cognac, managed by specialist Cooper ASC. It’s rather like re-char de-char. But, NEOC is even better at producing superb results.

Expert coopers break down the casks into individual staves. The barrel staves are then carefully shaved to expose the deeply ingrained seasoning that earlier fillings couldn’t reach.

The cask is then reassembled and gently ‘toasted’ by fire, for 45 minutes, which is a longer toasting time than the traditional re-char process. The charring process creates an ‘active’ layer of toasted oak with new, rich flavour compounds.

We then decant our whisky from their first fill casks into these special NEOC casks. The refilled casks are left for 6 to 12 months during which the whisky is enriched and takes on new flavour profiles.

Read more and find our NEOC whiskies here.

Finding your ideal cask

If you'd like to learn more about whisky casks, we'd be pleased to help. As well as bottling our own independent whiskies using our NEOC process, we also help private clients from around the world to buy their own whisky casks.

You may find our free Whisky Cask Investment Guide interesting, and you can speak with our experts by contacting us here. We'd be glad to help.