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. The barrels pre-treated in this way are then filled with spirit after the barrel lid has been rimmed and the tightness checked so that it can mature into whiskey.

What kind of barrels are we talking about?

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

How does toasting work?

Toasting involves treating the inside of the barrel with indirect heat. This usually involves the radiant heat of a flame. The temperature is so low that the wood does not ignite. The intensity of the toasting is generally controlled by time, resulting in varying degrees of toasting. After 5 to 10 minutes of heat exposure, one speaks of a light degree of toasting, after 10 to 15 minutes of a medium degree of toasting, and after 15 to 20 min of a heavy degree of toasting. This thermal stress on the wood surface usually results in a 2-3 millimeter thick toast layer in the barrel staves. Toasting temperatures and times are not standardized, so there can be considerable differences between individual cooperages.

What happens visually to the wood during toasting?

If you look at the staves of a barrel after toasting, you will only see a darkening of the brownish hue of the wood surface. However, no black discoloration due to charring from heat or any other physical change in the wood is visible to the naked eye.

How does the charring work?

Unlike toasting, charring involves directly igniting the inside of the wooden barrel with a flame. The wood is left to burn for a certain time before it is extinguished again with water. Depending on the burning time of the wood surface, a distinction is generally made between four types of char levels, which are also called char levels. For Char-Level 1, about 15-20 seconds of flame time is required and for Char-Level 2, about 25-30 seconds. For char level 3, the wood may burn for about 35-40 seconds. If the wood surface inside the barrel is ignited for up to one minute, visible charcoal plates will already form on the surface of the staves at this Char-Level 4. Since this resembles the skin of a crocodile or alligator, char level 4 is also colloquially referred to as "alligator char." As with toasting, no standard times exist for charring, as each cooperage uses its own heat regime. The direct flame creates an additional layer of toasted oak under the black char layer as a result of the heat. Depending on the length of the charring, this toasted layer can have a depth of up to 12 millimeters.

Which barrels are toasted or decarburized?

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

From a chemical point of view, what happens to the wood during heat treatment?

The toasting or charring of the barrels has an enormous influence on the ingredients in the oak wood, because the effect of the heat triggers chemical reactions in the wood. Like all plants on earth, oak wood is composed of three main components: cellulose, hemicellulose and lignin. Heat treatment chemically breaks down the three aforementioned polymeric components of the wood in particular, forming color and aroma substances that are released into the distillate during barrel aging. The charring also causes the structure of the oak wood to be physically broken down and the maturing spirit to penetrate more easily and more deeply into the staves and release aroma substances.

Which aroma substances are formed in the wood by the thermal treatment?

The wood component lignin, when exposed to heat, transforms into a wide range of chemical compounds that exhibit aromas of spice (cinnamon, cloves), chocolate, smoke and fruit. In addition, lignin is thermally degraded to vanillin, the main flavoring agent in vanilla. The vanilla aroma is very characteristic of American bourbon whiskey. Hemicellulose is composed of various sugars that caramelize during toasting or charring and form chemical compounds that impart aromas of nuts (walnut, almond, hazelnut), caramel, licorice and buttery notes. At the same time, 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 form a three-dimensional network. Cellulose is more stable to heat, but at high temperatures - like hemicellulose - it yields aroma-rich products that give off notes of dried fruit and roasted almonds. In addition, the sugar molecules in the staves released by heat can undergo a chemical reaction with amino acids (components of proteins or albumen) naturally occurring in oak wood, called the "Maillard reaction" after its discoverer. This also produces an abundance of colored and aroma-intensive products in the barrel wood, which are released into the maturing distillate.

Does the carbon layer obtained after charring also have a function?

But yes! The black layer of burnt wood that is clearly visible on the surface of the wood and protrudes a few millimeters into the staves is activated charcoal. This layer of activated charcoal contributes little to nothing to the flavor and color of the maturing spirit. Nevertheless, it plays an important role in removing unwanted flavors in the spirit, as this activated charcoal layer acts like a filter. Through a combination of adsorption and oxidation, it can reduce the amount of immature aroma compounds in the distillate - such as the odor-intensive sulfur compounds or unpleasant fusel oils - and thus help to improve the quality of the matured whiskey.

Can the Toasting and Charring processes also be combined?

Yes. Some cooperages in the U.S. first toast their fresh white oak barrels and then perform charring. This creates more aroma and color compounds in the oak. This is said to give the maturing bourbon a more intense flavor and a deeper color.

Why are barrels for aging red wine not charred?

The reason why red wine barrels are only toasted but not charred is, on the one hand, the formation of the additional activated charcoal layer during charring. This is because this activated charcoal can adsorb the color-giving substances in the red wine, which makes the color of the wine paler. Depending on the duration of storage or aging in the barrel, it could even be discolored. In addition, the activated charring layer is also capable of removing the aroma and flavor substances desired in red wine to a certain extent, which winemakers naturally want to avoid at all costs. On the other hand, charring leads to the increased formation of vanillin in the wood, which is released into the wine and - due to its low odor perception threshold - can be perceived as very intense. Winemakers, on the other hand, would like to have only a very small amount of vanillin in their wine, if any.