What is the difference between American and European oak?

American oak in cooperage is almost always Quercus alba, the white oak, and European oak is Quercus robur, the pedunculate oak, or Quercus petraea, the sessile oak.

The three species grow differently and therefore yield wood with different properties. American white oak grows in North America and supplies the wood for bourbon casks. European oak comes from European forests and is used for, among other things, sherry and wine casks.

Casks that previously held bourbon are therefore, as a rule, American oak. Casks that previously held sherry do not have to be.

American white oak has tyloses that seal the wood vessels, so that sawn staves stay leak-free, whereas European oak has to be split for that.

Chatonnet and Dubourdieu described that difference in 1998. In the European species, the tyloses do not form an adequate barrier against seepage, so the staves must always be split along the grain.

Splitting takes more wood than sawing. The difference in processing is therefore also reflected in the amount of tree needed per cask.

American oak contains more beta-methyl-gamma-octalactone than European oak, the substance that is called whisky lactone in the specialist literature.

Chatonnet and Dubourdieu write that the American species have a greater aromatic potential than European oak, pointing to the high content of methyl-octalactone and to two isomers of 3-oxo-retro-alpha-ionol.

What a drinker tastes of it depends on the length of maturation, the charring of the cask and the whisky itself.

European pedunculate oak releases large amounts of ellagitannins, far more than sessile oak, and sessile oak in turn more than American white oak.

That order is set out in the 1998 research by Chatonnet and Dubourdieu. Ellagitannins are polyphenols from the wood; they help determine how the drink feels on the tongue and how quickly it changes color.

For that reason the researchers call pedunculate oak the most suitable for maturing spirits, and sessile oak and white oak for maturing fine wine.

A sherry cask refers to the previous contents of the cask, not to the tree species it is made from.

The Scotch Whisky Association explains that Scotch may mature in new oak casks and in oak casks that previously held wine, beer or spirits, with exceptions for, among others, stone fruit. The rules set requirements on the previous contents and on the result, not on the oak species.

Anyone who wants to know which wood was used must therefore ask the bottler or the distillery.

The properties of oak vary by tree, by growing site and by batch, both in fresh wood and in previously used casks.

Mosedale set out that variation in 1995 in Forestry, a journal on forestry research. Chemical and anatomical properties of the wood differ, and on top of that comes the history of the cask: what it held before, how often it has been used and how heavily it has been charred.

Two casks of the same type and from the same supplier can therefore give a noticeably different result.