Batch and continuous ultrasonic emulsification processes on both lab and pilot scales were investigated using Tween 80 or milk protein isolate (MPI) as emulsifiers. The process parameters of processing volume, residence time and ultrasonic amplitude, as well as emulsion formulations, emulsifier type and concentration, were studied for the effect on emulsion droplet size. Emulsions prepared with ultrasound yielded submicron droplets, similar to 200 nm, with Tween 80 and MPI, utilising all processing methodologies. Inverse power laws were obtained correlating emulsion droplet size with respect to energy density, highlighting the efficiency of the continuous over batch processing. This efficiency is ascribed to the smaller processing volumes, associated with continuous ultrasonic emulsification. Longer processing times were required for MPI to achieve submicron droplets (<200 nm) in comparison to Tween 80 as greater times are necessary for interfacial adsorption and surface stabilisation, shown by interfacial tension measurements. (C) 2015 The Authors. Published by Elsevier Ltd.