Explainer: Toasting
While toasting might conjure up images of raising a glass in celebration, in the Scotch whisky world, it refers to a specific technique applied to the casks used for maturing the spirit. Toasting the cask plays a significant role in developing the rich, complex flavours that define a fine Scotch whisky.
🔥 What is Toasting?
Toasting involves gently heating the interior of oak barrels, causing the wood's natural sugars to caramelise. (This process is distinct from charring, which burns the wood surface more intensely.) Toasting is a more delicate approach, aimed at bringing out nuanced flavours and aromas rather than creating a charred layer. With toasting, the inside of the cask is heated for a specified amount of time, typically about 15 minutes at 350°C.
Vic Cameron, one of our teachers, says: "Toasting is one of the heat treatments that we can do to the inside of a cask. It's when we heat the wood and we do some break down of the lignin in there to release compounds like sugars and aldehydes that can then go into the spirit."
🧑🔬 The Science Behind Toasting
When the inside of an oak barrel is toasted, the heat causes chemical changes in the wood. Lignin, cellulose, and hemicellulose—the primary components of wood—break down and transform:
Lignin: Toasting breaks down lignin into compounds like vanillin, contributing to vanilla and spice notes.
Cellulose and Hemicellulose: These components caramelise, adding sweet, caramel, and toasty flavours to the whisky.
Burning the surface of the oak in this way browns the wood and allows heat to penetrate in, creating what is known as the red layer. This red layer (pictured) is where lignin degradation has occurred and is about three millimetres thick, between the char and the untouched wood.
Additionally, toasting helps open up the wood's pores, allowing the whisky to interact more deeply with the barrel. This interaction is crucial for the maturation process, as it allows the whisky to absorb flavours from the wood while also imparting its own character.
🥃 The Impact on Flavour
The toasting level can significantly influence the final flavour profile of the whisky. Distillers can choose between a light or heavy toast which is achieved by altering the length of toasting time:
Light Toast: Imparts subtle vanilla, floral and fruity notes.
Medium Toast: Adds richer flavours like caramel, toffee and spice.
Heavy Toast: Enhances deep, robust flavours such as chocolate, coffee and dark fruits.
Toasting will generally result in more colour and extractives in the spirit than with charring, although this is not an exact science and is dependent upon the wood, levels of heat treatment and the spirit being matured.
🎬 Want to learn more about Scotch whisky and wood?
Watch this video with one of our teachers Dr Gordon Steele:
