Studies On Graft Modification Of Soybean Protein Isolate And Their Solution Behaviors

In this paper, the graft copolymerization of Soybean protein isolate (SPI) withmethacrylic acid (MAA) was studied, and the solution behavior of SPI and theirgrafted copolymers (SPI-g-PMAA) were also studied.In this graft copolymerization of SPI with MAA, the ammoniumpersulphate (APS) was used as an initiator, and the 8mol/L urea andβ-mercaptoethanol (ME) were used as protein unfolding agents. The FTIR spectraand ¹³C-NMR spectra of SPI and their grafted copolymers indicated that the MAAhad been grafted on the SPI. The effects of APS, MAA, ME, reaction temperatureand reaction time on graft copolymerization were studied by determining the graftingparameters such as grafting percentage (GP %) and grafting efficiency (GE %). Theoptimum reaction conditions were obtained as following: [MAA] = 2.4mol/L,[APS] =16mmol/L, [ME] = 0.05mol/L, reaction temperature is 80℃, and reactiontime is 4h. And then the reasonable mechanism of graft copolymerization was putforward by means of the theory deduction.On the basis of the success of graft copolymerization, the solution behavior ofSPI and SPI-g-PMAA were investigated. Firstly, the rheology behaviors of thesolutions of SPI and SPI-g-PMAA were studied by the rheometer. The experimentresults indicated that the urea solutions of SPI were Newtonian fluids, but theSPI-g-PMAA solutions were non-Newtonian fluids and their non-Newtonianproperties increased with the increase of solution concentration. After adding a littlesalt (NaCl), the viscosity of grafted copolymer solution decreased and thenon-Newtonian properties were weakened, but the viscosity would increase withincreasing of salt concentration. The viscosity of SPI and grafted copolymer solutionincreased when the temperature decreased or the graft percentage (GP %) increased.The viscosity of SPI and grafted copolymer solution had lowest value aroundisoelectric point (pH≈5);the viscosity of them increased with increasing of pH valuewhen pH﹤8, and then the viscosity of SPI solution varied slightly and that of graftcopolymer solution increased with increasing of pH value when pHï¹¥10.And then, Zeta potential of SPI and the SPI-g-PMAA solutions were alsostudied by Zeta potentiometer. The static charges on the surface of SPI moleculeswere increased clearly after graft modification. And the values of the Zeta potential ofSPI and SPI-g-PMAA solutions increased with the decrease of the solutionconcentration, or decreased strongly with the increase of pH values. But the additionof salt influenced slightly on the Zeta potential of the solutions.