Effect of low frequency ultrasound on the functional characteristics of isolated lupin protein

Abstract

The impacts of sonication on functional properties of lupin protein isolates derived from lupin flour was investigated. The lupin protein isolates of pH 5 and 9 were then treated with low-frequency ultrasound throughout an energy density range of 457–2746 J/mL. The physicochemical and functional properties including particle size, solubility, zeta potential, rheological, thermal properties and molecular weight of the samples were measured. Ultrasound treatment was effective in reducing the particle size of the lupin protein isolates for the pH 5 sample to less than 10 μm, and less than 1 μm for the pH 9 suspension. Ultrasound also increased the solubility of lupin protein across the pH profile. Zeta potential wasn't affected to any significant extent, as it remained unchanged at −5 for the pH 5 protein isolate, and −27 mV for the pH 9 protein sample after ultrasound treatment. Following sonication, the viscosity of the pH 5 lupin protein increased, where the highest viscosity measurement of 1000 mPa s being achieved for the sample sonicated at an energy density of 1830 J/mL; in contrast, viscosity decreased with higher energy densities for the pH 9 sample. The storage modulus behaviour demonstrated the efficacy of ultrasound for improving the gelling properties of the lupin protein samples at pH 5, with 4000Pa being measured upon sonication at an energy density of 2746 J/mL; on contrast, the pH 9 sample possessed poor gelling properties despite ultrasound treatment. Finally, the DSC curves indicated an inverse correlation between the energy density of ultrasound treatment and enthalpy.