Formation and characterization of the complex coacervates obtained between lentil protein isolated and xanthan gum

Interactions between proteins and polysaccharides within foods are of the upmost importance to the food industry because of their influence on ingredient functionality (e.g., emulsification, foaming and gelling/ thickening), rheology/texture, structure and processing. The use of proteins of vegetable origin has been increasing due to their technofunctional properties. Therefore, the aim of this research was to study the best conditions to formation of complex coacervates obtained between lentil protein isolated (LPI): xanthan gum (XG). The complexes formation were studied using zeta potential, phase diagram and isothermal titration analysis. The analyzes were used to define the pH, proportion and concentration of biopolymers. The zeta potentials of LPI and XG were conducted as a function of pH (2.5–6.0) and the strength of the electrostatic interaction (SEI) was estimated. The highest SEI was 1146.07 mV² at pH 3.5, indicating that this pH is ideal for complex formation. To construct the phase diagram, five concentrations (0.1%; 0.25%; 0.5%; 0.75%; 1.0%) and five proportions of biopolymers (1:1; 2:1; 4:1; 8:1; 10:1, LPI:XG) were studied. The analysis was done visually, observing the formation and precipitation of complexes. It was observed that in solutions with a higher concentration of biopolymers there was greater precipitation, however, in the 1:1 ratio, the complexes remained soluble. The only ratio in which there was precipitation at all concentrations was 8:1 (LPI:XG). ITC was applied to further understand interactions of LPI–XG, including binding model and thermodynamics of complex coacervation process. The titrating was realized at pH 3.5 and 25 °C. The following values were obtained: 3.77, 1.80 x 10-8 M, -191.2 kcal/mol, -605.9 cal/ mol K, -10.55 kcal/mol, for the reaction stoichiometry (N), binding constant (K), enthalpy (ΔH), entropy change (ΔS), and Gibbs free energy change (ΔG), respectively. ΔG was calculated from the equation ΔG = ΔH – TΔS, where T is the temperature in degrees Kelvin (298.15). The thermogram of heat rate versus time presented titration profile was exothermic, and the exothermic peaks decreased regularly to a steady state after 11th injection of LPI with molar ratio of around 5.0, indicating that the binding between the the biopolymers was saturated. The regular decrease of peaks was associated to a reduction in free XG remaining in the reaction cell after successive injections of LPI. Both ΔH and TΔS values were negative which serves as the evidence for the enthalpically favorable and entropically unfavorable of LPI–XG complexation process. A negative ΔG indicates that the reaction was spontaneous. Based on the results, it was observed that the best conditions for the formation of the coacervate complex were pH 3.5, ratio of 8:1 (LPI:XG) and concentration of 1.0%.