Study On The Formation Mechanism Of Soybean Protein/Gelatin A Complex Coacervation

Heteroprotein complex coacervation is one of the methods to develop new functional biomaterials.An ideal complex coacervate can be formed by adjusting the relevant parameters of heteroprotein complex coacervation.However,in-depth exploration of the formation rules,phase behaviors,impact factors,kinetic and thermodynamic formation mechanisms of heteroprotein complex coacervation are required to investigate.In this study,positively charged soy protein(included soy protein isolate(SPI)and soy ?-conglycinin(SC)),and negatively charged gelatin A(Gel)were materials to systematically study the related mechanisms of the complex coacervation,which were analyzed by the effects of temperature,pH,ionic strength and mixing ratios on the complex coacervation.This study was aimed to provide a theoretical basis for the development of new heteroprotein complex coacervation materials.(1)The effects of pH and mixing ratios(SPI:Gel,w/w)on the phase behaviors,formation rules and thermodynamics of the SPI/ Gel complex at 25°C were studied.The soluble complex was formed when the mixing ratios were 1:1 and 1:14,respectively.The liquid-liquid phase separation occurred when the mixing ratios were 1:2 ? 1:9 to form a complex coacervate at pH6.5.When the pH increased from 6.5 to 7.0,the turbidity,hydrodynamic diameter,and absolute value of ?-potential of the complex decreased.Isothermal titration calorimetry(ITC)showed the complex coacervation of SPI and Gel was a spontaneous(?G < 0)and thermal(?H < 0)reaction,driven by electrostatic interaction(|?H| > |T?S|).The stoichiometric ratios of the complex at pH 6.5,6.75 and 7.0 were 1:9.5,1:10.3,and 1:13.5,respectively.Fourier transform infrared spectroscopy showed both electrostatic interaction and hydrogen bonding were involved in forming complex coacervation.(2)The effects of temperature,pH and mixing ratios(SC:Gel,w/w)on the phase behaviors,morphology,formation rules and thermodynamics of SC/Gel complex were studied.At the condition of 25°C,the SC/Gel mixture showed solid-liquid separation when the mixing ratio was 1:1 at pH 6.0 and 1:3 at pH 6.5,while liquid-liquid separation occurred in other experimental groups.The stoichiometric ratios of the complex at pH 6.0 and 6.5 was 1:6.9 and1:8,respectively.At the condition of 40?,the solid-liquid phase separation occurred in the SC/Gel mixture at pH 6.0 and the mixing ratio was 1:1,liquid-liquid phase separation occurred in other experimental groups.The stoichiometric ratios of the complex at pH 6.0 and 6.5 were1:7.4 and 1:9.2,respectively.The complex coacervation of SC and Gel was a spontaneous and thermal reaction,dominated by electrostatic interaction.When the pH and temperature of the system increased,the turbidity,formation speed,hydrodynamic diameter,and the absolute value of ?-potential decreased,|?H| and |T?S| decreased.(3)The effects of NaCl concentration at 40? on SC/Gel phase behaviors,morphological structure,formation rules and formation thermodynamics were studied.The SC/Gel complexes containing 5 m M NaCl at the mixing ratios of 1:4 and 1:5 both showed liquid-liquid separation,whereas the other complexes showed solid-liquid separation.When the NaCl concentration increased from 5 m M to 20 m M,the turbidity,formation speed,hydrodynamic diameter,and the absolute value of the ?-potential decreased.The stoichiometric ratio changed from 1:6.7 at5 m M to 1:8.9 at 20 m M with the decrease of |?H|,|T?S| and |?G|.