Comparative assessment of the effect of ultrasound treatment on protein functionality pre- and post-emulsification

The effect of ultrasound treatment (∼34 W cm−2 for 2 min) on unadsorbed and adsorbed milk protein isolate (MPI) and pea protein isolate (PPI) was investigated in terms of the ability to form and stabilise emulsion droplets. Submicron emulsions were prepared using a microfluidiser with a single pass at 100 MPa with untreated proteins (control), proteins ultrasound treated prior to pre-emulsification (unadsorbed) or proteins sonicated post pre-emulsification (adsorbed). Emulsions were also prepared with a low molecular weight surfactant, Tween 80, for comparative purposes.

Ultrasound treatment reduced the size of both MPI and PPI to the nanoscale (∼150 nm) from micron sized aggregates (∼20 μm). Emulsions prepared with ultrasound treated post pre-emulsification MPI yielded significantly smaller emulsion droplet sizes than those prepared with untreated or unadsorbed MPI. This behaviour is ascribed to rearrangement of interfacial protein allowing for the formation of smaller emulsion droplets. In contrast, emulsions produced with PPI yielded no significant differences, regardless of treatment, in emulsion droplet size, this was attributed to the more highly structured nature of PPI in contrast to MPI. Nevertheless, emulsions prepared with ultrasound treated PPI, both unadsorbed and adsorbed, yielded significantly more stable emulsion droplets than untreated PPI. This behaviour is associated with an enhanced interfacial layer and greater electrostatic repulsive forces as observed by an increased zeta-potential.