Whisky words: toasting
Toasting is a heat treatment whereby the inside of the cask is heated for a specified amount of time, typically about 15 minutes at 350°C.
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.
Toasting essentially splits the cellulose into wood sugar which caramelises and a portion of the lignin is converted to a number of compounds, including vanillin.
Toasting is generally classed as light, medium or heavy. Distillers can choose between a light or heavy toast which is achieved by altering the length of toasting time.
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.