Now, two researchers at Linnaeus University in Sweden using computer simulations of ethanol, water and an guaiacol to understand how diluting whisky with water affects its taste. Apart from water and alcohols, whiskies contain different organic compounds that contribute to their taste. Apparently, scientists have studied this extensively. Many whiskies, especially those that are made on the Scottish island of Isley, have a smoky taste that develops when malted barley is smoked on peat fire.
The scientists tell us that chemically that distinctive scotch flavor is attributed phenols, and in particular one called guaiacol, which their research found is much more common in Scottish whiskies than other whiskies. The concentration of guaiacol was found to be 3.7–4.1 parts per million in two unnamed Scottish whiskies that were previously studied. Such a small concentration of guaiacol can be smelled and tasted by us.
Guaiacol is a small and mostly hydrophobic molecule that is able to interact with water by hydrogen-bonding and polar-aromatic interactions. Scotch whisky is distilled to around 70% alcohol by volume then diluted to about 40 % when bottled. The scientists carried out molecular simulations of guaiacol-water-alcohol solutions at various ethanol concentrations to understand why the taste of whisky changes upon the addition of water. They simulations revealed that ethanol and water mix non-ideally generating clusters of ethanol molecules.
The scientists found that water and ethanol do not mix completely. They observed a microscopic phase separation occurs in the mixture, and at lower ethanol concentrations they found that guaiacol was more likely to be present in the liquid air interface. In a glass of Scotch whisky, which typically exhibits alcohol concentrations of 45%- 27% alcohol, guaiacol will be found near the liquid surface, where it greatly contributes to both smell and taste of the Scotch. At cask-strength concentrations of 59% or higher, ethanol interacts more strongly with the guaiacol and guaiacol is therefore driven into the solution.
Thus, the taste of guaiacol and similar phenol compounds will be more pronounced when whisky is diluted in the glass from the 40% alcohol content of the bottle. This taste-enhancement is counteracted by the dilution of guaiacol’s concentration. Overall, there is a fine balance between diluting the whisky to enhance the taste and diluting the Scotch whisky too much. This balance will depend on the concentration and types of taste compounds that are characteristic for each whisky; though the scientists found that 27% ethanol by volume was the limit of dilution before the guaiacol was too dilute to be appreciated.
Karlsson, Bjorn C. G., Friedman, Ran, 2017/08/17; Dilution of whisky - the molecular perspective, Scientific Reports, 6489.