How Charring Affects Ageing Spirits

Published 25 May 2023

In previous articles we have considered the impact of seasoning and toasting on the chemical properties of wood casks and how this influences the flavours of the spirit ageing inside. In this article we will consider the effects of charring.

When a whisky cask is made at a cooperage, heat is needed to shape the staves, so the barrel is shaped over fire. The fire not only makes the oak more malleable, but toasts and chars the wood on the inside of the cask. Charring the inside of a cask has a significant impact on the chemistry of the whisky aging inside, affecting its colour, flavour, and aroma profile. The exact characteristics of the resulting whisky depend on factors such as the type of oak used, the intensity of the charring, and the duration of the ageing process. Here are some of the key ways that charring affects the chemistry of whisky:

  1. Lignin breakdown: The heat from charring breaks down lignin, a structural component of the oak, into smaller compounds such as vanillin and guaiacol. These compounds contribute to the flavour and aroma of whisky and are responsible for the vanilla, smoky, and spicy notes often associated with oak-aged spirits.

  2. Cellulose breakdown: Charring releases oak lactones, which are chemical compounds that contribute to the woody and coconut flavours in oak-aged whiskies. These compounds are formed as a result of the interaction between the heat and the cellulose in the oak.

  3. Creation of a "red layer": During charring, the wood's sugars are caramelized, forming a layer known as the "red layer" or "caramelized wood layer." This layer is rich in compounds that contribute to the whisky's colour, flavour, and aroma. The depth of the red layer depends on the intensity of the charring, and it acts as a source of compounds like furfural, which adds sweetness and almond-like flavours, and 5-hydroxymethylfurfural, which imparts a toffee-like note.

  4. Charcoal filtration: During the charring process, some of the oak is converted into charcoal, which can filter out impurities in the whisky. This can help to smooth out the flavour and reduce harshness or bitterness.

  5. Oxygenation: Charring creates small fissures in the oak, which allows the whisky to interact with the air as it ages. This exposure to oxygen can cause further chemical changes in the whisky, such as the oxidation of alcohols into aldehydes and ketones, which can contribute to the flavour and aroma profile of the finished product.

  6. Tannins: Oak also contains tannins, which are astringent compounds that can contribute to the bitterness of the whisky. Charring can reduce the amount of tannins in the oak by breaking down some of these compounds, which can help to balance out the flavour of the whisky.

  7. Increased surface area: Charring the inside of a cask increases the surface area of the wood in contact with the whisky. This increases the rate of extraction of wood compounds and accelerates the maturation process.

  8. pH and mineral content: The charring process can also affect the pH and mineral content of the oak, which can in turn influence the chemistry of the whisky as it ages. For example, charring can increase the pH of the oak, which can reduce the acidity of the whisky and make it taste smoother.

The level of char on the barrel can have a significant impact on the flavour and colour of the whisky. Here are the different levels of char that are commonly used: 

No. 1 Char: This is the lightest level of char and is also known as "toast." The inside of the barrel is lightly charred to give the whisky a subtle wood flavour. 
No. 2 Char: This is the most common level of char used in whisky barrels. The inside of the barrel is charred to a moderate level, creating a layer of charcoal that helps to filter and flavour the whisky. 
No. 3 Char: This level of char creates a heavier layer of charcoal on the inside of the barrel, which can impart a smoky or burnt flavour to the whisky.
Alligator Char: This is the heaviest level of char and is sometimes referred to as "Level 4 Char." The inside of the barrel is heavily charred, creating a deep layer of charcoal that can impart a strong smoky flavour and dark colour to the whisky. 

Overall, the charring of oak barrels is a complex process that can have a significant impact on the flavour, aroma, and chemical composition of whisky as it ages. The degree of charring, the type of oak used, and other factors can all influence the final product, making each cask of whisky unique.

Author: Liam Hirt