Modification of pea protein isolate for ultrasonic encapsulation of functional liquids

Abstract

This study reports on the ultrasonic processing of pea protein isolate (PPI) in phosphate-buffered saline (PBS, pH 7.4) and Tris/HCl (pH 8) buffer systems in order to modify its properties for use in the encapsulation of functional liquids. Tetradecane-filled microspheres were synthesized using ultrasonically-modified PPI as a shell material under high intensity 20 kHz ultrasound irradiation. Tetradecane was used as a model liquid, which could in principle be replaced by functional liquids such as vitamins, fish oil, etc. The solubility of water-insoluble globulin present in PPI was significantly improved in the first sonication step, which was confirmed by solubility measurements and sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis. The hydrodynamic diameter measurements indicated that the dissolved pea proteins formed soluble aggregates. The size, size distribution, shell thickness, mechanical strength and yield of PPI microspheres were controlled by the variation of ultrasonic parameters in the first step. In terms of stability, the microspheres maintained a core–shell structure and their size remained unchanged after one-month storage at 4 °C. Most of the microspheres had a spherical shape with a smooth surface morphology. The shell thickness varied with the surface activity and solubility of PPI, which in turn were affected by sonication time. Average stiffness ranging from 9.5 to 22 mN m⁻¹ and average Young's modulus from 0.58 to 2.35 MPa were obtained by using atomic force microscopy (AFM). Disulphide crosslinking and noncovalent interactions played a role in the shell formation, also facilitating the storage stability of PPI microspheres.