Evaluation of pH, NaCl and CaCl2 salts on Solubility, Zeta Potential and air - water interfacial properties of the protein isolate from lupin seeds

Lupin is a legume seed rich in proteins and presents high nutritional and health benefits but is little used in the human diet. In this work, lupin proteins were extracted using an alkaline-saline solution followed by dialysis. Then, globulins were separated by isoelectric precipitation to give lupin globulin isolate (LGI). The solubility, zeta potential and interfacial properties of LGI were evaluated at pH of 3.4 and 6.8 in the presence of NaCl or CaCl₂. The solubility of LGI was dependent on the pH and type of salt, with lower solubility in the isoelectric region (pH ∼ 5), with lower solubility in the presence of NaCl (42.81 ± 1.23 %). The lowest value of interfacial tension was at pH 3.4 and absence of salts (41.79 ± 0.63 mN m⁻¹). Indicating the type of salt changes the electrostatic shielding of the LGI electrical double layer, which leads to a decrease in electrostatic potential at the surface of LGI. The rate of diffusion is highest at pH 6.8 under conditions without salts or in the presence of NaCl (0.57 ± 0.01 and 0.55 ± 0.05 mN m⁻¹ s^−0.5 respectively). When the salt was changed to CaCl₂ diffusion was decreased, probably due to protein-protein interactions. On the other hand, the protein adsorption rate is lower at pH 6.8 under all conditions studied. Apparently, it is influenced by the type of surface electrical charge of the proteins and the rate of rearrangement is greater than the rate of adsorption. The results obtained demonstrate that the lupin protein isolate can be used in multiphase food systems, such as foams, due to its interfacial properties.