Mutual titration of soy proteins and gum arabic and the complexing behavior studied by isothermal titration calorimetry, turbidity and ternary phase boundaries

Complexing between soy proteins (SP) and gum arabic (GA) was achieved by mutual titration of soy protein and gum arabic and was characterized using isothermal titration calorimetry (ITC), turbidity, sedimentation and ternary phase boundaries. In the first section, SP were titrated into GA (SP-to-GA titration) under salt-free condition (no added NaCl) at pH 3.0 and pH 5.6, respectively. ITC experiments displayed exothermic processes at both pH status, but the enthalpy changes (ΔH) at pH 3.0 was −0.70 ± 0.02 cal/g as compared to −0.10 ± 0.01 cal/g at pH 5.6. For SP-to-GA titration at pH 3.0, a sudden turbidity increase was observed at the critical SP/GA mass ratio (rφ) of 0.42, which was approximately equal to the charge density ratio of GA and SP (0.36), indicating the charge compensation was achieved at phase separation point. In the second part, GA was titrated into SP (GA-to-SP titration) under salt-free condition at pH 3.0. An immediate turbidity increase was observed when GA was added into SP, while the sedimentation ratio measurement showed that the complex was unstable only in the GA/SP mass ratio range of 0.3–0.6. The ITC result showed a much higher ΔH than that for SP-to-GA titration. Effect of NaCl concentration on the complexing behavior by SP-to-GA titration at pH 3.0 was studied in the last. ΔHs and binding isotherms changed monotonically with the increase of salt concentration from 0 to 250 mM. However, turbidity measurement and phase boundaries revealed that the maximum phase separation was obtained at salt concentration of 100 mM.