A connoisseur's palate is no longer the only accurate judge of the maturity of fine Scottish whisky. Chemists at the University of Texas at Austin have created a dye complex that reveals the age of scotch at a glance and their work has been published in Journal of the American
Chemical Society. Sheryl Wiskur and Eric Anslyn have developed a complexing agent that reacts with compounds that accumulate in whisky as it matures in oak casks. This reaction triggers a colour change, the intensity of which denotes the whisky's stage of maturity. The method was tested on several single-malt whiskies, aged between 5 and 16 years, from the distillery on the Scottish island of Islay. The colour change correlated well with the known ages.
Old whiskies can command high prices. Dealers often
validate distillers' claims of a whisky's age by measuring how much gallic acid
a whisky contains. Water reacts with tannin compounds in the wood, producing
gallic acid, which leaches into the whisky. The longer the spirit has been in
the cask, the more gallic acid it contains. However, the level of gallic acid is
not a very accurate age test. It varies with the type of wood, with how many
times the cask has been used and with what the cask has previously stored. Oak casks release several tannin-derived organic acids
into the whisky. No single one provides a reliable measure of age. The new
method evaluates the concentrations of several tannin acids at once. The
researchers add a dye complex to the whisky. Key chemical structure features of
the complexing agent are a guanidinium embedded in an aminoimidazoline group and
two boronic acid groups. The dye is pyrocatechol violet. Organic acids in the
whisky replace the dye in the complex triggering a colour change from maroon to
yellow. The intensity of the change depends on how much dye is released, which
in turn depends on tannin acid concentration in the whisky. The same method might work for oak-aged wines, which
contain similar tannin-derived organic acids. However, a complexing agent more
specific to tannin-derived acids may be needed.
