E-pistle 2007/067 – The Influence of Wood on Whisky

By Paul Dejong, Belgium

The boring part: Chemistry.

Some experts say the influence of wood on, and the interaction of wood with whisky makes up for more than 70% of the aroma’s in the final product! So, if wood is so important, the question arises: What is wood made of?

The building stones of wood are the following:

* The main ingredient and building stome of wood is: CELLULOSE (C6H10O5). (compare this to the most important supplier of bio-energy: glucose C5H10O5)  Cellulose is good for about  40 to 50% of the total mass of the wood. (incidentally, cellulose is the main part of the total bio-mass on this planet!) Unfortunately, under normal circumstances  most multicellular living organisms cannot digest Cellulose. Only some bacteria can break down Cellulose!

* A second, important constituent of wood is HEMI-CELLULOSE: Hemicellulose is a “conglomerate” of eight sugars (all with a similar chemical formula, C5H10O5 –  C6H12O6) which are essential for human nutrition, being Xylose, galactose, glucose, mannose, N -acetylglucosamine, N-acetylgalactosamine, fucose, and sialic acid. While cellulose is crystalline, strong, and resistant to hydrolysis, hemicellulose has a random, amorphous structure with little strength. It is easily hydrolyzed by dilute acid or base as well as myriad hemicellulase enzymes.

* The third ingridient of wood is LIGNIN. Lignin is a very complicated molecule (polymer) and is -together with pectin- the fysical bond between Cellulose and Hemi-cellulose. It is the substance that makes wood strong and hard. When burned it forms what is chemically known as Methoxyphenols. The main methoxyphenols are guaiacol and syringol…and they are very wel known to the average whisky-affectionado as: phenols!

How does wood get its colour?

Why ask this?
Simply because the main ingredients of wood are clear or colourless (cellulose and hemicellulose)… and whisky is said to inherit it’s colour from the wood. Several molecules and substances give colour to the wood;

* the first is LIGNIN: Lignin becomes so strong and hard because of a chemical concept called Resonance. Resonance is also what  makes lignin look brownish…

* the second group of molecules are the TANNINS or POLYPHENOLES: different acids that bond with sugars such as glucose to become tannins. The best known are Gallic Acid + glucose = Gallotannin and Ellagic Acid + Glucose = Ellagotannin. These tannins have a dark reddish-brown colour

* also a part of the colour of wood are some acids (most noticably acetic acid and Formic acid) and Whisky-lactones (wonder how they got their name?)

As we all know, most Single Malt whisky is maturing in “second hand” casks that previously held Sherry (or other wines) or Bourbon.  As we all know, bourbon, like Scotch is a cristal clear liquid, and imparts no colour into the wood. Sherry or Wine however, usually have a colour of their own (from the start, or thru oxidation) and could impart colour into the wood, or leave chemical residu behind that has an influence on colour. An important chemical substance in wine is Tartaric Acid (E334), which, when bonded with Potassium, forms Potassium Bitartrate or wine -diamonds… these cristals, once formed , are UNDISSOLVABLE , and form a layer on the inside of a winecask (winecasks used to be the most important industrial source of these cristals) , thus possibly forming a barrier between wood and wine (or whisky)

Other well known, and almost equally boring trivia….

Casks used for the maturation of Scotch Whisky are usually made out of European red oak (Quercus robur and quercus sessilis) or out of American white oak (usually Quercus Alba). Could it be that these names were chosen for a reason? Could it be that European oak is actually red, and American oak is actually White? Bingo! Yes it could! European oak is actually darker in colour than it’s American counterpart, because it contains a lot more lignin, and a lot more tannins! Hence “red” Oak. European Red Oak has a “looser” structure than American oak. European oak staves are usually split instead of sawed, making them a lot thikker and more difficult to work with. American oak has a denser structur, that allows it to be sawn in thinner, easier to modulate staves. Hence, an American casks holds less wood, and is easier and cheaper to make.

We also know that Scotch whisky matures either on Bourbon-casks out of American White Oak,  or on ex-wine-casks, usually out of European Red Oak. Extraction of Tannins, Lignin, Vannillin, Sugars happens better with a higher alcohollevel (up to appr. 66% this is true,above that level the extraction becomes less effective).

– New Make has an average ABV of 63,5%
– Wine (Sherry) has an average ABV of 15%

What does the Cooper do?

Oak is used to make casks, because it has a unique trait: It bends well when heated (an oak stave can be bend almost 90°).  The wooden staves are bend using steam, and once the cask is put together,  it is toasted or charred. For this, the cask is put over a heatsource with the ‘lid’ on. It heats up inside but the heat consumes the oxigen… then, when the lid is taken of, and the oxigen comes in, the wood ignites… when left to burn for appr. 5 to 15 seconds it is called toasted, when it is longer (appr. 20 tot 40 seconds) it is called charred. This toasting or charring has two purposes, it effectively sterilises the cask, and -more importantly- the heat pyrolises the lignin (giving phenols) and caramellises the hemicellulose-sugars (forming vanillin, amongst others)

– Most American Oak is Charred,
– Most European Oak is Toasted

So, how to distill this into a tasty, nicely coloured whisky?
After visiting some bodegas in Xerez, and after noticing that almost all the samples of Oloroso cherry we saw were, at best, as dark as an average whisky blend of today, we started wondering where the colour of a so-called dark oloroso matured whisky came from! None of the oloroso’s we saw were even half as dark as that. Only pedro-Ximinez came close. But Bodega’s rarely sell Ximinez casks, so dark oloroso actually being Ximinez was not really an option. So, How do these extra dark (black Bowmore, aso) whisky’s erupt out of a relatively lighter (in colour) oloroso cask? Surely the whisky is further diluting the colour of the Sherry…

A couple of assumptions and conclusions:

– Wood colour is determined by the amount of lignin, Tannins, acids and lactones in the wood.
European red oak contains more of these colouring agents than American white oak

– Wine casks have matured a low alcohol drink, the wine extracts only very little colour from the wood.
Bourbon casks are maturing a high alcohol spirit at 63,5%, the optimum for extracting colour and taste from the wood (which is why most single barrel bourbons are so dark in colour)

-Wine casks were lightly toasted, whereas Bourbon casks are heavily charred.
This charring releases more caramel and lignin/vanillin and allows deeper penetration of new make into the wood. Colour in bourbon comes from the charring, taste in bourbon comes from the charring (vanilla-aroma’s, maple syrup from the caramel,  relatively low in tannins)

– European oak is more porous than American oak, allowing for greater penetration (and sometimes leakage) of the newmake in the wood, allowing it the extract more colour and tannins from the wood. This is why Oloroso sherry casks are so dark, the colour and taste come from the wood (less vanilin and caramel due to less toasting, more tannins, hence some bitterness and more colour)

– American Oak and bourbon are maturing in a relatively warm and sometimes even hot conditions, which gives more expansion and contraction of the wood, speeding up the maturation process and often allowing the alcohol-level to go up, instead of down.

Due to the shortage of European oak (it’s protected) , and the benefits of American white oak (cheaper, easier to handle, aso) more and more wine casks nowadays are made out of American white oak. Lightly toasted American oak that held wine is going to impart a completely different taste in whisky…. could this be the change?

So I guess you could say I’m with Dave in the ongoing e-mail discussion within the maniacs.
Although Michel’s paxarette question is indeed food for thought!