T – like Toasting and Charring

What does toasting or charring mean?

These terms refer to the treatment of the inner surface of wooden barrels with indirect or direct heat. After the cask lids have been sealed and checked for leaks, the casks prepared in this way are then filled with spirit so that it can mature into whisky.

Which casks are involved?

As a rule, these are casks made of oak, which are used for maturing whisky. In Scotland, for example, only oak casks must be used. In the EU, and therefore also in Germany, other types of wood are also permitted. However, the vast majority of wooden casks used worldwide for maturing wine (red wine, sherry, etc.) or spirits (whisky, rum, brandy, etc.) are made of oak. The most common casks are made from American white oak and European oak. The European species mainly include French oak, but Spanish, Hungarian and Palatinate oak are also popular.

How does toasting work?

During the so-called toasting process, the inside of the cask is treated with indirect heat. This is usually the radiant heat of a flame. The temperature is so low that the wood does not ignite. The intensity of toasting is generally controlled by the time, resulting in different degrees of toasting. After 5 to 10 minutes of exposure to heat, the degree of toasting is said to be light, after 10 to 15 minutes it is said to be medium and after 15 to 20 minutes it is said to be heavy. This thermal stress on the wood surface usually leads to a 2-3 millimetre thick toast layer in the cask staves. The temperatures and times for toasting are not standardised, so there can be considerable differences between individual cooperages.

What happens to the wood visually during toasting?

If you look at the staves of a cask after toasting, you will only notice a darkening of the brownish colour of the wood surface. However, no black discolouration due to charring caused by heat or any other physical change to the wood can be seen with the naked eye.

How does charring work?

In contrast to toasting, the inside of the wooden cask is directly ignited with a flame during charring. The wood is left to burn for a certain amount of time before being quenched with water. Depending on the burning time of the wood surface, a distinction is generally made between four types of charring levels, also known as char levels. About 15-20 seconds of flame time are required for char level 1 and about 25-30 seconds for char level 2. For char level 3, the wood may burn for about 35-40 seconds. If the wooden surface inside the cask is ignited for up to one minute, visible charcoal plates will already form on the surface of the staves at this char level 4. As this resembles the skin of a crocodile or alligator, char level 4 is also known colloquially as an “alligator char”. As with toasting, there are no standard times for charring, as each cooperage uses its own heat regime. The direct flame creates an additional layer of toasted oak under the black charred layer due to the heat. Depending on the length of the charring, this toasting layer can be up to 12 millimetres deep.

Which casks are toasted or charred?

Casks for storing or maturing wine, sherry and port, but also for brandy and cognac, are usually toasted. Fresh American white oak barrels intended for maturing bourbon, on the other hand, must be charred on the inside.

What happens to the wood from a chemical point of view during heat treatment?

Toasting or charring the casks has an enormous influence on the ingredients in the oak, as the heat triggers chemical reactions in the wood. Like all plants on earth, oak wood is made up of three main components: cellulose, hemicellulose and lignin. The heat treatment chemically breaks down the three polymer components of the wood in particular, forming colouring and aromatic substances that are released into the distillate during the maturation process. Charring also has the effect of physically breaking up the structure of the oak, allowing the maturing spirit to penetrate more easily and more deeply into the staves and release flavours.

Which flavouring substances are formed in the wood through thermal treatment?

When exposed to heat, the wood component lignin transforms into a wide range of chemical compounds with aromas of spices (cinnamon, cloves), chocolate, smoke and fruit. In addition, lignin is thermally degraded to vanillin, the main flavouring agent in vanilla. The vanilla flavour is very characteristic of American bourbon whiskey. Hemicellulose is made up of various sugars that caramelise during the toasting/charring process and form chemical compounds that impart aromas of nuts (walnut, almond, hazelnut), caramel, liquorice and buttery notes. Their quantity increases as the temperature rises. Cellulose, on the other hand, consists exclusively of one type of molecule, namely glucose. This forms long chains that develop into a three-dimensional network. Cellulose is more resistant to heat, but at high temperatures – similar to hemicellulose – it produces flavourful products that give off notes of dried fruit and roasted almonds. In addition, the sugar molecules in the staves released by the heat can enter into a chemical reaction with the naturally occurring amino acids (components of proteins or proteins) in the oak wood, which is called the “Maillard reaction” after its discoverer. This also produces an abundance of colourful and aroma-intensive products in the cask wood, which are released into the maturing distillate.

Does the carbon layer that remains after the charring also have a function?

Of course! The black layer of burnt wood, which is clearly visible on the surface of the wood and protrudes a few millimetres into the staves, is activated charcoal. This layer of activated charcoal contributes little or nothing to the flavour and colour of the maturing spirit. Nevertheless, it plays an important role in removing unwanted flavours from the spirit, as this activated carbon layer acts like a filter. Through a combination of adsorption and oxidation, it can reduce the amount of unripe aromatic substances in the distillate – such as the odour-intensive sulphur compounds or unpleasant fusel oils – and thus contribute to improving the quality of the matured whisky.

Can the toasting and charring processes also be combined?

Yes. Some cooperages in the USA first toast their fresh white oak barrels and then carry out a charring process. As a result, more aroma and colouring substances are formed in the oak wood. This is intended to give the maturing bourbon a more intense flavour and a deeper colour.

Why are casks for maturing red wine not charred?

The reason why red wine casks are only toasted but not charred is, on the one hand, the formation of the additional layer of activated charcoal during the charring process. This is because this activated charcoal can adsorb the colouring substances in red wine, making the colour of the wine paler. Depending on the duration of storage or maturation in the cask, the wine could even be discoloured. In addition, the activated charcoal layer is also able to remove a certain amount of the aromas and flavours desired in red wine, which winemakers naturally want to avoid at all costs. On the other hand, the charring leads to the increased formation of vanillin in the wood, which is released into the wine and – due to its low odour perception threshold – can be perceived as very intense. Winemakers, on the other hand, only want a very small amount of vanilla in their wine, if any at all.