Release of distillate flavour compounds in Scotch malt whisky

Equilibration of a flavour volatile between a distilled spirit and the head space is a two-stage process. The first equilibrium occurs between the bulk solution and the headspace spirit interface, and the second between the su rface layer and the headspace. The first stage is represented by the activi ty coefficient of the flavour volatile, which, for hydrophobic compounds, i s greatly reduced by the aggregation of ethanol molecules in aqueous soluti on. The second equilibrium is governed by the vapour pressure of the solute and the ambient temperature and pressure. In mixed saturated solutions the composition of the surface layer and consequently the headspace is determi ned by the concentration and activity coefficients of the mixture component s. Components of wood extract were found to act principally on the first eq uilibrium. Ethanol lignin acted in the same manner as high molecular weight esters and alcohols of the distillate, displacing volatile components from the surface layer. The suppressant effect of ethanol lignin was lost at 37 degrees C and consequently would only be important in nosing of spirit sam ples. Wood extract was found to decrease the critical point for the aggrega tion of ethanol, resulting in reduced activity coefficients for ethyl decan oate from 5 to 30% ethanol at both 25 and 37 degrees C. This effect would r educe the spirit-mouthspace partition coefficient with the resulting decrea sed release of flavour volatile in the mouth. This mechanism would explain the decreased impact of undesirable, immature aromas when wood matured spir its are consumed. (C) 1999 Society of Chemical Industry.