The effect of ultrasound upon the physicochemical and emulsifying properties of wheat and soy protein isolates

The effect of ultrasound upon the physicochemical and emulsifying performance of wheat protein isolate (WhPI) and soy protein isolate (SPI) was investigated. Protein solutions (0.1–3 wt. %) were sonicated with an acoustic intensity of ∼34 W cm−2 for 2 min. The physicochemical properties were assessed in terms of changes in protein aggregate size, hydrodynamic volume and molecular structure. The emulsifying performance of ultrasound treated WhPI and SPI was compared to their untreated counterparts, and a low molecular weight surfactant, Tween 80, for comparative purposes. Ultrasonic processing significantly reduced the aggregate size of both proteins, whilst no reduction in the primary structure molecular weight profile was observed in both instances, ascribed to insufficient energy to hydrolyse the peptide bond. Emulsions prepared with both untreated proteins yielded submicron emulsion droplets (∼150 nm) at concentrations ≥ 0.75 wt. %. Emulsions fabricated with both sonicated proteins at concentrations <0.75 wt. % demonstrated significantly (P < 0.05) smaller emulsion droplets and long term emulsion stability in comparison to their untreated counterparts. This effect is consistent with the observed reduction in the equilibrium value of interfacial tension between untreated and ultrasound treated proteins.