Abstract:

Electrostatic interactions between proteins and polysaccharides are a fundamental physico-chemical phenomenon in vivo. These interactions are believed to be the main driving force for the self-assembly. In addition, a proper understanding and control of this interaction should enable to design desired structures. Herein, the influences of mass ratios of chitosan(CS) to soybean protein isolates(SPI), pH, ion strength and temperature on the interactions between SPI and CS were investigated. The strength of electrostatic interactions between SPI and CS were mainly influenced by the pH values of solutions. The aggregation of SPI and CS occurred at pH varying from 5.5 to 6.6. At the low ion strength, a compact CS/SPI aggregates was formed, whereas, CS/SPI aggregates are dissociated at higher ion strength. As the heating induced denaturation of SPI, hydrophobic amino acid residues on the peptide chains are exposed to the solution, which results in an increase in hydrophobic interactions between CS and SPI. Furthermore, CS/SPI particles and bilayer film were prepared by adjusting the factors above. DLS reveals that nanoparticles are formed by the self-assembly between CS and SPI, and the hydrodynamic diameter is about 200 nm with uniform distribution. After the denaturation of SPI induced by heating, the hydrodynamic diameter is increased. Cross-linking by glutaraldehyde inducs a little decrease of hydrodynamic diameter. SEM results show that there are some cracks in the surface of chitosan monolayer film with thickness of ca 300 nm, and the cracks disappear after the formation of the bilayer film(CS/SPI) with thickness of ca 500 nm.

Key words: soybean protein isolates, chitosan, electrostatic interactions, nanoparticles, bilayer film