Changes in wood extractives from oak cask staves through maturation of scotch malt whisky

Abstract Cask staves, prepared from American white oak, Quercus alba, were obtained from unused, new charred Bourbon, used Bourbon, first‐fill Scotch and exhausted Scotch casks. The aim of the experiment was to determine the effect of repeated maturations of whisky on concentration and location of phenolic extractives in the cask wood. Contents of vanillic and syringic acids and the respective aldehydes, arising from hydrolysis of guaiacyl‐syringyl lignin, and whisky lactones (cis‐ and trans‐β‐methyl‐γ‐octalactone), arising from charring, were determined. Staves were sampled at 5 mm intervals from the inner char surface to 25 mm depth and wood extracted with chloroform, heptane and aqueous ethanol. Contents of material absorbing at 520 nm, total phenols and vanillic and syringic acids, vanillin and syringaldehyde, were determined by high‐performance liquid chromatography; both isomers of whisky lactone were quantified by high‐resolution gas chromatography. New cask wood had a maximum for coloured material in the char layer, but maxima for other compounds at 5 mm below the char. As casks were reused in maturations of whiskies, the contents of aromatic aldehydes and acids were reduced in the first 20 mm of the wood and maxima for both acids and aldehydes shifted towards the centre of the staves. This suggested that repeated exposure to aqueous ethanol resulted in compositional changes in the lignin in the wood. The newly charred wood had a maximum for cis‐β‐methyl‐γ‐octalactone at 5 mm below the char and the trans isomer at 15 mm. In the first Bourbon maturation both isomers were largely depleted and with successive extractions the maxima moved into the cask until in the exhausted cask wood no lactone was detected.