Preparation of Monodisperse Food-Grade Oleuropein-Loaded W/O/W Emulsions Using Microchannel Emulsification and Evaluation of Their Storage Stability

W/O/W emulsion is an emerging system in developing new functional and low-calorie food products. The aim of this study is to produce food-grade monodisperse water-in-oil-in-water (W/O/W) emulsions loaded with a hydrophilic bioactive oleuropein. W/O/W emulsions were prepared via high-pressure homogenization and subsequent microchannel (MC) emulsification. The internal aqueous phase was a 5-mM sodium phosphate buffer containing d(+)-glucose (5 wt.%) and oleuropein (0.1-0.7 wt.%). The oil phase consisted of soybean oil and tetraglycerin monolaurate condensed ricinoleic acid esters (TGCR; 3-8 wt.%). The external aqueous phase was a 5-mM sodium phosphate buffer containing d(+)-glucose (5 wt.%) and decaglycerol monolaurate (1 wt.%). Oleuropein-loaded submicron W/O emulsions with average droplet diameters as small as 0.15 mu m and monomodal droplet size distributions were prepared by high-pressure homogenization when applying high TGCR concentrations of 5-8 wt.% and low oleuropein concentrations of 0.1-0.3 wt.%. Monodisperse oleuropein-loaded W/O/W emulsions with average W/O droplet diameters of around 27 mu m and coefficients of variation of below 5 % were successfully prepared when using a silicon MC array plate with wide channels of 5-mu m depth and 18-mu m width. The monodisperse W/O/W emulsions prepared at high TGCR concentrations and low oleuropein concentrations were the most stable during 40 days of storage. The adsorption behavior of oleuropein at the internal aqueous-oil interface was relevant to W/O/W emulsions microstructure and stability. The results are believed to provide useful information for successfully preparing stable monodisperse W/O/W emulsions loaded with hydrophilic functional compounds. The surface activity of the loaded material seems to be a key parameter in optimizing the formulation of W/O/W food emulsion.