Charge Compensation, Phase Diagram, and Protein Aggregation in Soy Protein–Gum Arabic Complex Formation

Mixtures of soy protein (SP) and gum arabic (GA) formed an electrostatic complex in a relatively narrow pH range at very low ionic strength. The conditions under which the complexes could be formed were determined using turbidimetric measurements first. In salt-free condition and 1:1 SP/GA mixture, critical pH values with the formation of soluble (pH c = 4.40), insoluble (pH φ1 = 3.55), and maximum (pH opt = 3.15) complexes were observed. As SP/GA ratios increased from 1:4 to 8:1, critical pH values shifted toward higher pH. Charge densities (ZN) for SP and GA were calculated from electrophoretic mobility using soft particle analysis theory. Results showed that a 1:1 charge ratio at pH φ1 was found at any SP/GA ratio, indicating that charge compensation was fulfilled for SP/GA insoluble complex formation. A SP–GA–water ternary phase diagram was built at pH 4.0. The influence of the total biopolymer concentration (0–10% w/w) and SP/GA ratio was represented in the phase diagram. At a total concentration of 0.10%, results were consistent with the turbidity measurement; that is, no phase separation occurred at an SP/GA ratio lower than 1:2 at pH 4.0. Salt effect (NaCl, 0–500 mmol/L) on SP/GA complexes was discussed. Results indicated that SP/GA complexing, which led to the formation of turbidity peaks at pH 3.2, was suppressed when NaCl concentrations were ≥50 mmol/L, whereas the remarkable increase in turbidity around pH 5.0 was caused by the aggregation of soy protein molecules on which gum arabic could be adsorbed.