Vapor-liquid equilibria of aroma compounds highly diluted in hydroalcoholic solutions: measurement of new isobaric data for a better understanding of the distillation of spirits

International audienceThe knowledge of vapor-liquid equilibria (VLE) of aroma compounds in hydroalcoholic mixtures is essential to better elucidate and simulate their behavior during the distillation of spirits and thus control the quality of the distillates. A distillation process is driven by the distribution of the different components of the mixture which, in turn, depends on physical properties of the system and on molecular interactions between the different components. From a thermodynamic perspective, wine and alcoholic spirits are non-ideal mixtures of ethanol and water containing hundreds of highly diluted aroma compounds, which, despite their low concentration, will dictate the aromatic quality of distilled spirits. Amongst aroma compounds, terpenes are a class of key compounds for the aroma profiles of hydroalcoholic beverages that are present in wine in their free or glycosylated form. One of such terpenes, α-terpineol, is deemed to play a major role in the aroma of gin[1] and tequila[2] and has been detected in cognac, calvados, armagnac[3] and cachaça[4]. During wine distillation, α-terpineol is formed via acid hydrolysis of its glycosylated counterpart and is further concentrated in the distillate along with ethanol[5].Given the scarcity of reliable and accurate equilibrium data in distillation conditions in the literature and the limitations of predictive models for some compounds, especially in aqueous media, experimental measurements remain a reliable alternative to determine VLE data for infinitely dilute aroma compounds. This work focuses on the experimental acquisition of new VLE data for α-terpineol highly diluted in ethanol-water mixtures in a Gillespie-type recirculating still operating at adiabatic and isobaric conditions. The compositions at equilibrium are determined for a temperature interval from 353 to 370 K at 101.3 kPa using a densimeter and a gas chromatograph with a flame ionization detector (GC-FID). The distribution of aroma compounds between the vapor and liquid phases is then defined by their partition coefficients, or absolute volatilities, as depicted in Figure 1, and their volatilities in relation to ethanol/waterratios. Low-pressure activity coefficient model NRTL and predictive model UNIFAC are then compared to describe VLE in the hydroalcoholic mixtures and estimate binary interaction parameters. These parameters allow to employ simulation tools that help to understand and predict the influence of distillation parameters on the final composition of the distillates