Effect of colloidal properties of oil-in-water emulsions on ethanol liquid-vapour partition

Ethanol vapour-liquid partition was studied in oil-in-water emulsified model systems prepared with two different emulsifiers (beta-lactoglobulin (beta-Lg) and Tween 20) and 2% w/w of ethanol at varying oil quantity (10-75% w/w). The role of some structural and physical properties of the emulsified system on the thermodynamic equilibrium of ethanol in the vapour phase was studied by dispersion degree, interfacial area and viscosity evaluations. Results of the ethanol vapour pressure (P(e)) of emulsified systems are discussed on the basis of those obtained in hydro-alcoholic models having composition similar to that of the aqueous phase. Composition and structural characteristics of emulsified model systems resulted to affect liquid-vapour partition of ethanol. In particular, oil content affected P(e) as a consequence of the variation of both the oil/water weight ratio and the physical properties of the dispersed state. The lower P(e) and partition coefficient (K(e)) values of emulsified systems as compared with those of aqueous models at similar ethanol concentration could be attributed mainly to ethanol adsorption/retention phenomena taking place during emulsification process at interfacial level. This result is the consequence of the surface free energy of ethanol which leads it to easily to adsorb at interfaces of phases characterised by different polarity (water-air, oil-water). The presence of Tween 20 and beta-Lg in aqueous systems, and in particular the former one, significantly increased the volatility of ethanol. On the contrary, their use in the preparation of the emulsified systems resulted to influence the ethanol liquid-vapour partition only at oil concentration above 40% w/w, even if they affected their structural properties and, in turn, their viscosity. Taking into account the marked effects of the presence of ethanol itself on the structural and rheological properties of the emulsified system, results of the present study could be of interest in the development and optimisation of new formulated emulsified foods and beverages containing this alcohol as well as their stability and sensorial acceptability.