The effect of ultrasound treatment on the structural, physical and emulsifying properties of dairy proteins

The effect of ultrasound treatment on the structural, physical and emulsifying properties of three dairy proteins: sodium caseinate (NaCas), whey protein isolate (WPI) and milk protein isolate (MPI) was investigated. The pH of untreated NaCas, WPI and MPI solutions was 7.1, 6.8 and 6.7, respectively. Protein solutions at different concentrations (0.1 – 5 wt. %) were treated by ultrasound radiation for 2 min at a frequency of 20 kHz and with a power intensity of ∼34W.cm-2. The structural and physical properties of the untreated and ultrasound treated proteins were studied in terms of changes in protein size, molecular structure and hydrodynamic radius using dynamic light scattering (DLS), SDS-PAGE and intrinsic viscosity, respectively. The emulsifying properties of the ultrasound treated proteins were compared to the untreated proteins and to a low molecular weight surfactant, Tween 80. Ultrasound treatment reduced the micelle size and hydrodynamic volume of the proteins as measured by DLS and intrinsic viscosity, while SDS-PAGE showed that there was no measurable reduction in molecular weight. 10% Rapeseed oil-in-water emulsions prepared with untreated NaCas and WPI had submicron sized droplets (∼120 nm) at all concentrations, while the emulsions produced with untreated MPI and Tween 80 had micron sized droplets (> 1 μm) at the lower concentrations studied. Unexpectedly, the emulsions produced with ultrasound treated NaCas and WPI had the same submicron droplet sizes as the untreated proteins at all concentrations, despite the observed reduction in micelle size and reduction of intrinsic viscosity (i.e. increase in hydrophobicity) of the sonicated proteins. These results suggest that ultrasound treatment did not affect the rate at which the sonicated proteins were adsorbed at the oil-water interface, since no significant changes in interfacial tension were measured between the untreated and sonicated NaCas and WPI. Emulsions prepared with sonicated MPI at concentrations ≤ 1 wt. % had smaller droplet sizes than the emulsions produced with untreated MPI at the same concentrations. This effect was consistent with the observed decrease in interfacial tension for ultrasound treated MPI, which will facilitate droplet break-up during emulsification.