The impact of soy protein isolate-dextran conjugation on capsicum oleoresin (Capsicum annuum L.) nanoemulsions

The purpose of this study was to conduct an in-depth investigation of the effects of soy protein isolate (SPI)-dextran (molecular weights 10, 40, 70, and 150 kDa) bonding on physical properties and environmental stability of oil-in-water emulsions containing capsaicin. Results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Fourier transform-infrared (FT-IR) spectroscopy confirmed that SPI covalently links to dextran in dry-heated Maillard reactions (60 degrees C and 79% relative humidity). Dextran and SPI have the maximum conjugation yield at a molecular weight of 40 kDa. The capsaicin nanoemulsions were prepared using a high-pressure homogenization method with native SPI, the SPI-Dex mixture, and conjugates in an aqueous phase, and oleoresin capsicum in an oil phase. The mean particle diameter (Dz) of the capsaicin nanoemulsions stabilized by the SPI-Dex conjugates was significantly lower than that stabilized by SPI and the SPI-Dex mixture, particularly at 40 kDa. The pH, thermal stability, and storage stability of the capsaicin nanoemulsions stabilized by the SPI-Dex (40 kDa) conjugate were markedly improved and no appreciable creaming or flocculation was observed at 25 degrees C or 50 degrees C for 30 days. We concluded that the SPI-Dex conjugates formed by covalent bonding improved the physical stability of the nanoemulsion, which depended on the molecular weight of dextran. These results provide new insight into preparing highly stable nanoemulsions with good encapsulation efficiency.