The Grafting Modification Study Of Soy Protein Isolate

In this paper, the modification and application of soy protein isolate (SPI) were mainly studied. The graft copolymerization of SPI with two water-soluble monomers Methacryloyloxyethyl Trimethyl Ammonium Chloride (DMC) or 2-Acrylanmido-2-Methyl propane Sulfonic Acid (AMPS) was studied, and the graft modification mechanism is also studied. The solution behavior of SPI and the grafted copolymers (SPI-g-NH-PAMPS) were as studied in detail such as viscosity, surface charge, solubility, surface hydrophobicity, aggregate forms, etc.. Further, the emulsifying properties study on SPI and the grafted SPI were carried out as a preliminary exploration.The radical graft copolymerization of SPI with DMC were carried out in the 8mol/L urea cushioning solution and under the protection of nitrogen (N₂), using ammonium cerous and nitrate-potassium persulfate as an mixture initiator, andβ-mercaptoethanol as protein unfolding agents. The FTIR spectra of SPI and SPI-g-PDMC indicated that the DMC had been grafted onto the SPI. The effects of Ce⁴⁺/S₂O₈^2-, DMC content, reaction time on the graft copolymerization were studied by determining the grafting parameters such as grafting percentage (GP%) and grafting efficiency (GE%). The optimum reaction conditions were obtained as following: [Ce⁴⁺/S₂O₈^2-] =5: 11, [DMC] = 16mmol, reaction time is 6h. The results of Zeta potential study were: within the entire studied pH value range (2 to 12), the surface of SPI-g-PDMC is filled with positive charges.The graft copolymers (SPI-g-NH-PAMPS) were prepared by a graft reaction between the free carboxylic acid groups of soy protein isolate (SPI) and the amino groups of poly(2-Acrylanmido-2-methyl propane sulfonic acid) (H₂N-PAMPS) using 1-(3-(dimethylamino)propyl)-3-ethyl-carbodiimide hydrochloride (EDC·HCl)/ N-Hydroxysuccinimide (NHS) as the condensing system in a buffered solution. The structures of H2N-PAMPS and SPI-g-NH-PAMPS were fully characterized by GPC, FTIR, 1H-NMR and 13C-NMR spectra. The results proved that H2N-PAMPS had been successfully grafted onto the SPI. The point and mechanism of the gaft reaction catalysed by EDC/NHS were explored and researched.The aqueous solution properties of SPI and SPI-g-NH-PAMPS were studied in detail. Compared with the SPI, the surface charges of SPI-g-NH-PAMPS changed to negative charge, the viscosity was increased, the surface hydrophobicity reduced. And SPI-g-NH-PAMPS have better solubility in the wide pH range (2 to 12), especially increasing about 40 percent in isoelectric point. SPI-g-NH-PAMPS aggregates displayed the core-shell structure. The average size of SPI-g-NH-PAMPS aggregates was also bigger than that of SPI, and increased from 180 nm to 220 nm.The emulsifying properties of SPI and SPI-g-NH-PAMPS were preliminarily studied by optical microscopes and rulers. The result was: both SPI and SPI-g-NH-PAMPS were able to form oil in water emulsions. In the studied concentration range (11～1 mg / mL), higher the concentrations of protein solution, better the emulsion performance. In acidic pH range, both the emulsifying activity and stability of SPI-g-NH-PAMPS were better than those of SPI.