A novel method to prepare protein-polysaccharide conjugates with high grafting and low browning: Application in encapsulating curcumin

One significant challenge during the conjugation between protein and polysaccharide via Maillard reaction is to achieve high grafting degree and low browning degree. We developed a novel method called cyclic continuous reaction to resolve this challenge. We prepared soy protein isolate (SPI)-dextran conjugate and used the conjugate to encapsulate curcumin to form nanoparticles to improve curcumin's solubility for enhanced antioxidant capacity. The obtained SPI-dextran conjugates showed a higher grafting degree (42%) and a lower level of browning (0.053 cm−1). Chemical reaction kinetics modeling suggested that Maillard reaction during SPI-dextran conjugation is a pseudo zero order chemical reaction. The secondary structure analysis of conjugates suggested that the content of random coil structure increased, indicating that conjugation with dextran increased protein's structural disorder. The tertiary structure analysis indicated that the surface of conjugate molecules showed enhanced hydrophilicity. The curcumin was indeed encapsulated by SPI-dextran conjugate and the formed nanoparticles were stable and possessed a uniform particle size (polydispersity index 0.148). The antioxidant capacity of conjugate-curcumin nanoparticles was more than double of curcumin alone. This novel method opens up tremendous opportunities for the broad protein-polysaccharide synthesis community.