Molecular dynamics simulations of flavour molecules in Scotch whisky

Abstract

Molecular dynamics simulations are used to investigate the interfacial properties of molecules related to flavours of Scotch whisky, in ethanol/water solutions with different values of alcohol-by-volume (ABV). The propensity of flavour molecules to accumulate at the liquid-vapour interface is linked to the evaporation of those molecules into the head space, and consequently their perception when nosing or tasting. The simulation approach is first validated for ethanol/water solutions, without flavour molecules, by comparing simulated and experimental values of the ethanol surface excess and the interfacial tension. The chosen values of ABV are 0% (pure water), 20% (typical dilution for sensory evaluation), 30% (diluted spirit), 40% (typical bottle strength), 50%, 65% and 73% (representing a range of cask strengths), and 100% (pure ethanol). Then, flavour molecules are considered, ranging from hydrophobic to hydrophilic: octane (alkane); octan-1-ol (alcohol); octanal (aldehyde); octanoic acid (carboxylic acid); and ethyl hexanoate (ester). The primary focus is whether there is a positive excess of such molecules at the interface, or whether the molecules remain fully solvated by the liquid layer. The dependence of this excess on ethanol content can be correlated with how flavour molecules are released into the head space on dilution with water, and hence the tasting experience. Additional molecular-level details are presented, such as how organic molecules are oriented with respect to the interface. This illustrates how molecular simulations could be employed to improve our understanding of the links between composition and flavour perception, or to aid the development of low-alcohol spirits with similar sensory characteristics to full strength equivalents.