The 'Gazette'

The charring stage of a barrel

Charring is one of the last steps in barrel manufacturing. It impacts the interactions between whisky and the wood in several ways. By heating at a high temperature, large cracks are created on the surface of the staves. Therefore, charring allows whisky to deeply penetrate the oak, extracting more of certain molecules and facilitating the whisky’s development during maturation.

What is the difference between toasting and charring?

When comparing toasting and charring, the main difference is the intensity of the heat applied to the barrel.

The temperature range for toasting varies between 110 and 230°C depending on the desired organoleptic profile, the heat of the flame is maintained during the process without burning the wood.

During charring however, the temperature is generally kept above 280°C. The barrel ignites, forming a layer of charcoal on the staves through the wood’s pyrolysis giving it a black and ashy appearance.

Charring step

Evolution of the wood’s surface structure (left to right) depending on the charring level

Three beneficial effects of the charring stage

Charring a barrel has a triple benefit for whisky professionals, improving the organoleptic quality of the final product. Let’s look at these three points in more detail.

– Filtration: when the distillate exits the still and is ready to be barreled, it generally has some less desirable aromatic characteristics, signs of its immaturity. The charcoal layer created through charring acts as a filtration system, adsorbing the compounds responsible for the sulphur or fusel oil character.

– Extraction: the changes in the whisky’s sensory characteristics linked to the charring stage inevitably derive from its impact on the wood’s chemical profile. With the increase in temperature, there is a progressive formation of different by-products resulting from the breakdown of lignin.

– Alcohol penetration: charring helps to further break up the oak’s structure allowing the alcohol to easily penetrate the wood and encourages reactions with the deeper uncharred layers. The higher the alcohol content of the whisky, the more interactions there will be between the wood and the spirit, which in turn impacts the extraction of the wood’s components.

Steps of charring

The three barrel charring effects on whisky

The oxidation phenomenon

Oxidation is another phenomenon that must be considered to explain the profile (and the colour development) of whiskies aged in charred barrels. During its maturation, the whisky is in contact with oxygen in the air because wood is porous. The lignin’s ethanolysis reaction facilited by charring leads to the formation of alcohols that are successively oxidised to finally form vanillin and syringaldehyde.

By creating an active layer of charcoal, charring increases the whisky’s quality by absorbing congeners with unpleasant aromas. For economic reasons, barrels are often recycled and the inside reworked, by scraping and re-charring, to refresh the previously charred wood. But how often can barrels really be reused?

Would you like to find out more about the charring?

Clyne, J., Conner, J. M.; Patterson, A. & Piggott J. R. (1993). The effect of cask charring on Scotch whisky maturation. International journal of food science &technology, 28(1), 69-81
Sarni, F., Moutounet, M., Puech, J. L., &Rabier, P. (1990). Effect of heat treatment of oak wood extractable compounds. Holzforschung-International Journal of the Biology, Chemistry, Physics and Technology of Wood, 44(6), 461-466
Puech, J. L. (1981). Extraction and evolution of lignin products in Armagnac matured in oak. American Journal of Enology and Viticulture, 32(2), 111-114

Maturation | January 2025 | Dr. Magali Picard

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