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

Abstract

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 were sonicated for 2 min with a power intensity of ∼34 Wcm-2. The structural and physical properties of the dairy proteins were studied in terms of changes in protein size, molecular structure and hydrodynamic radius using DLS, SDS-PAGE and intrinsic viscosity, respectively. The emulsifying properties of the sonicated proteins were compared to the untreated proteins and Tween 80. Ultrasound treatment reduced the size and hydrodynamic volume of the proteins, while there was no measurable reduction in molecular weight. Emulsions prepared with untreated NaCas and WPI had submicron sized droplets (∼120 nm), while emulsions produced with untreated MPI and Tween 80 had micron sized droplets (> 1 µm) at lower concentrations. Unexpectedly, emulsions produced with ultrasound treated NaCas and WPI had the same droplet sizes as untreated proteins at all concentrations, despite the reduction in protein size of the sonicated proteins. Emulsions prepared with sonicated MPI at concentrations ≤ 1 wt. % had smaller droplet sizes than the emulsions produced with untreated MPI. This effect was consistent with the observed decrease in interfacial tension for ultrasound treated MPI, which will facilitate droplet break-up during emulsification.