Characterization, release profile, and antibacterial properties of oregano essential oil nanoemulsions stabilized by soy protein isolate/tea saponin nanoparticles

This study successfully synthesized soy protein isolate (SPI)/tea saponin (TS)-based nanoparticles (STNPs) using the pH-driven method. The characteristics of the STNPs were investigated as a function of the SPI-to-TS mass ratio (1:0, 4:1, 2:1, 1:1, 1:2, 1:4, and 0:1). The findings demonstrated that the smallest particle size and highest surface charge appeared when the SPI-to-TS mass ratio was 2:1. Additionally, hydrogen bonding, hydrophobic interactions, and electrostatic interactions were found to contribute to the formation of STNPs. Subsequently, the synthesized STNPs were used to create oregano essential oil (OEO) nanoemulsions for further investigation of the effects of SPI-to-TS ratios on the characteristics and functional properties of nanoemulsion systems. The results revealed improved adsorption of STNPs at the oil-water interface with the increase in the proportion of TS, and, consequently, the droplet size and zeta potential of the nanoemulsion reduced significantly. Moreover, the nanoemulsions exhibited excellent rheological behaviors, high physical stability, sustained OEO release, and strong antibacterial activity when the SPI-to-TS ratio was increased from 1:0 to 2:1. However, when the SPI-to-TS mass ratio was less than 1:1, the nanoemulsion became unstable due to the formation of TS micelles in the aqueous phase, exhibiting nonuniform droplets, low stability, and rapid release of OEO. Furthermore, antibacterial analysis results indicated that the primary mechanism of antibacterial activity for nanoemulsions containing OEO involved disruption of the bacterial cytoderm, leading to leakage of intracellular contents.