pH and ionic strength responsive core-shell protein microgels fabricated via simple coacervation of soy globulins

The influence of the salt concentration, pH, protein concentration and temperature was investigated on spontaneous reversible coacervation in soy protein isolate (SPI) solutions. Coacervation was characterized by formation of protein enriched micro-domains and was most significant at 0.1 M NaCl. It was predominant below 40 degrees C and favored by lowering the temperature. Decreasing the pH caused the spherical coacervates to form irregular flocs. Increasing the protein concentration led to the formation of larger coacervates, but lowered the rates of coacervation. Irreversibly crosslinked microgels were fabricated by heating suspensions of coacervates at 80 degrees C. The effects of the pH and the salt concentration on the morphology and the protein composition of the microgels were analyzed. The microgels swelled and displayed a core-shell structure when the electrostatic repulsion between proteins was increased by removing salt or adjusting the pH away from the isoelectric point. Reducing the electrostatic repulsion induced de-swelling of the microgels and disappearance of the core-shell structure. By tuning the electrostatic interaction, the SPI microgels could bind or release positively charged lysozyme. These results show that SPI microgels are promising bioactive compound carriers.