Let's talk about... wood

One of our whisky lecturers, Dr Gordon Steele, discusses the different wood treatments – charring and toasting – in bourbon and sherry casks and considers how these methods alter the flavour profile of Scotch whisky during maturation.
By law, Scotch whisky must be matured for at least three years in oak casks. It is only through maturation that the pungent, clear, new make spirit becomes the Scotch whisky – with that lovely golden-brown colour – we're all familiar with.

You can often read that the wood contributes 50, 60, 70 percent to the flavour of the Scotch whisky. I find that deeply unhelpful. How can you quantify a flavour in such a way? It makes no sense. I think it’s more like an orchestra; the wood provides the violins and you can't imagine that orchestra without the violins.

Almost no oak is used in Scotch whisky that is untreated in some way (if you think of its first use as being ‘a treatment’). And, really, we're talking of only two types of cask; ex-bourbon and ex-sherry. Both types undergo a heat treatment before they're used for their primary purpose. That heat treatment is absolutely essential to get the flavour congeners out of the wood.

Bourbon casks undergo two types of heat treatment; charring and toasting. What do we mean by those terms? On this stave section, as an example, the blackened areas on the surface is charring. Looking at the edge, you will see the toasting layer which is indicated by the line (the spirit ingress into the wood) where there is a difference in colour. That line is a partial thermal breakdown of the wood that has happened underneath the char layer.

Charring is done over a gas flame and is very spectacular with flames licking up towards the roof. And you can specify how much char you get. You can then put it over a toaster – literally an electric toaster element – and that determines the depth of the toast layer that goes in.

The toast layer is the partial breakdown of the wood congeners, particularly of the cellulose, hemicellulose and lignin. Lignin in particular creates a lot of flavour congeners and a lot of colour too. While toasting increases those congeners, charring destroys them so getting the balance right is important when it comes to the style of whisky being made.

The char removes compounds from the maturing spirit, particularly the compounds containing sulphur. In other words, it takes away the pungency and some roughness of the whisky. Some manufacturers like a deep char layer and that allows them to have a new make spirit which is very sulphury (which can be quite unpleasant as a new make spirit).

You can initially lose flavour as maturation begins and compounds are absorbed onto the char layer. This can be quite worrying for the manufacturers who think: "My goodness, we're losing flavour! We're not actually gaining flavours through maturation!" But within a year we begin to see compounds coming out from the toast layer - diffusing out - and those buttery, vanilla notes which are typical of bourbon wood start to grow within the maturation of the whisky.

It's not the char layer that gives colour, it's the toast layer underneath. And you can see it coming out. On this example, the toast layer is lighter because it has lost congeners and colour out into the spirit.

Sherry wood is not charred, only toasted. Usually over the flames of a little oak fire for about an hour. Once again, you can specify how much toast you get within that layer.

Interestingly, it is the same compounds that come out of the bourbon wood that come out of the sherry wood. How come it gives a different flavour in terms of maturation? That is due to the interaction with other congeners within the maturing spirit. It is also due to the different ratios of the compounds that come out. In the sherry wood, you get more of that dried fruit, Christmas pudding and sherry flavour. Whereas the bourbon wood is a much drier wood flavour, you often get a pencil shaving type of note as well as all of the more familiar vanilla and buttery tones.