Research On Citrus Pectin-Soy Protein Isolate Electrostatic Interaction,Characteristic Of Interfacialadsorption And Emulsifying Stability

The present research aims to study the interaction of SPI and CP in bulk phase and interface, the stability of emulsion produced with these two bio-polymers, thus to provide more theoretical evidence for their application in practice. The present research is divided into three aspects as the following:1. Interaction of SPI and CP in solution at total bio-polymer concentration of 1mg/mL: By means of ζ-potential, dynamic light scattering, turbidity, rheology and phase state of mixed solution at different pH value and SPI/CP mass ratio, the region at which soluble complex is formed has been determined: When pH value was higher or equal to7.0, SPI was co-soluble with CP whatever the ratio is, and the mixed solution is clear,both SPI and CP molecule were negatively charged(ζ<-25mV), there was no complexation occurred; When pH value is lower than 6.0, the ζ- potential decreased, Dh and turbidity started to increase, which meant the formation of complex. At all ratios,ζ-potentials of solution reached highest value at pH 5.5, suggesting the negatively charged protein had combined to CP molecule; Further reducing pH leaded to phase separation,suggesting formation of insoluble complex, and the starting point for phase separation relies on ratio, relatively higher percent of SPI content leaded to easier precipitation near iso-electric point of protein(pI). At the same pH value, system with more SPI was prone to form larger size complex and made higher turbidity, while the value of ζ-potential was lower, suggesting intense interaction between these two bio-polymers, more complex combine together to form inter-molecular complex. As the result of ζ-potential experiment,the formation of soluble complex was due to the presence of non-adsorption CP molecule in the solution, which inhibited the further aggregation of complex by electrostatic repulsion. The addition of SDS and urea to mixture solution had confirmed that the formation of complex was driven by electrostatic interaction predominantly, while there was a little contribution of hydrophobic interaction between bio-polymers to complex formation at pH value higher than pI, but the electrostatic interaction was most important.The result of rheology also suggested that the formation of complex would intenselydecrease viscosity of mixture solution, and raise the fluid index, which was more similar to Newtonian Fluid.2. Research on emulsifying stability of SPI-CP mixture at different condition.Results suggested that mixture under neural pH could not effectively be produced to stable emulsion, due to depletion flocculation, and the emulsion made by insoluble complex was highly inclined to flocculate, only the soluble complex was proper to produce finely stable emulsion. In addition, the emulsion made with soluble complex was more stable than that of SPI alone at same concentration, and the former had lower interfacial protein adsorption content than the latter. Among the parameters of ultrasonic machine in producing emulsion, ultrasonic time had more effect on final particle size.Further study suggested that extremely high content of CP(r=0.25, pH5.5) in complex solution was not beneficial for emulsion stability. Emulsion stability with the addition of200 mmol/L NaCl showed that the emulsion at pH under pI could resist unfavourable solution environment, while the opposite was even more unstable than SPI emulsion at pH 7.0. After heating at 100 °C for 30 min, the emulsion at lower pH value(pH3.5) was more inclined to aggregation, suggesting more hydrophobic group unfolds during intense complexation, which leaded to more flocculation during heating. Volume percent of oil phase had considerable effect on emulsion particle size and stability, extremely lower(1%and 2.5%) or higher(20%) content of MCT was susceptible to unstability due to depletion flocculation and bridging flocculation. At the condition of proper pH, ratio and MCT content, the ultrasonic parameters had less influence on particle size except for initial several minutes of emulsion preparation(0-12min).3. The study on interfacial adsorption of two mixed bio-polymers suggested that SPI had much more tendency to adsorb on oil-water interface compared with CP, and dominate the formation of interfacial film in the presence of SPI and CP, while CP still had different kind of effect on SPI adsorption and film formation. At pH 7.0, where the mixture was co-soluble, both of SPI’s adsorption rate and film elasticity were promoted when CP was present at r=20, which was due to limited thermodynamic incompatibility;While at r=0.5 or 0.25, the film elasticity was much weakened in the presence of CP due to competitive adsorption scheme, although the SPI diffusion rate at the initial stage ofadsorption and the final interfacial pressure was raised. When pH value was lower than pI(r=0.25, pH3; r=0.5, pH3.5), complex was formed, both adsorption rate and equilibrium surface pressure were decreased considerably, but the final interfacial film was much more elastic and the interaction between molecule on interface was greatly improved,which suggested the stability scheme of electrostatic complex was in kinetic stability with strengthening film elasticity, but not in thermodynamic stability with lower interfacial tension. When the ionic strength was raised, all systems got improved interfacial adsorbing ability, but interfacial molecular interaction was greatly limited, which was not in favor of interface stability.