Effect of CaCl2 and pH value on demulsification of emulsion from enzyme-assisted aqueous extraction processing of soybean oil

Abstract: To explore the demulsification mechanism of emulsion recovered from crude enzyme-assisted aqueous extraction processing of extruded full fat soybean flour, the effect of salt and pH value on the emulsion stability was investigated respectively by demulsification rate, Zeta potential, apparent viscosity, particle size distribution and mean oil particle size. For evaluating the efficiency of salt demulsifying the emulsion, CaSO4, CaCl2, MgCl2, NaCl were separately added into the emulsion at concentration 0.06 mol/L, reaction time 10 min, and temperature 80 ℃. The experiment results showed 4 salts significantly destabilized the emulsion, but the highest free oil yield was recoveried from the emulsion added with CaCl2, followed by CaSO4, MgCl2, NaCl. Although the emulsion added with CaCl2 was completely broken (100% free oil recovery) at 60, 70, 80 ℃ respectively, demulsification effect was impacted by CaCl2 concentration, reaction time, reaction temperature. Demulsification rate progressively rose with CaCl2 concentrations increasing from 0.02 to 0.08 mol/L, reaction times from 0 to 90 min, reaction temperatures from 60 to 80 ℃. However, demulsification rate decreased with the increase of reaction time after the emulsion was completely broken. As Ca2+ neutralised the negative charge of the protein during heating, apparent viscosity and the absolute value of Zeta potential of the emulsion added with CaCl2 declined significantly compared with the control (without CaCl2 addition), which induced oil droplets aggregate and mean oil droplet size increase so that the stability of the emulsion decreased. Due to the greater screening effect of the higher Ca2+ concentration and reaction temperature on the negatively charged groups of proteins, the increase of CaCl2 concentrations ranging from 0.02 to 0.08 mol/L and reaction temperatures from 60 to 80 ℃ further reduced the absolute value of Zeta potential and apparent viscosity of the emulsion, and promoted the oil droplets aggregation. This resulted the higher free oil recovery. With pH value decreasing from 9 to 3 at 50 ℃, the absolute value of Zeta potential and apparent viscosity of the emulsion declined significantly, the average oil particle size increased so that the stability of the emulsion reduced, and the free oil recovery increased. Specially, the stability of the emulsion was the lowest and free oil recovery achieved the highest (100% free oil recovery) at pH value 3-4 because the absolute value of Zeta potential reached the lowest level (closed to 0), which meant the electrostatic repulsion forces between protein molecules of the emulsion nearly disappeared. When pH value dropped below 3, however, the absolute value of Zeta potential and apparent viscosity of the emulsion increased again so that the mean oil droplet size and demulsification rate decreased. Light micrographs showed that there were significant differences in the microstructure and oil recovery of the emulsion before and after demulsification. Oil drop diameter of the emulsion apparently enlarged, and more free oil was released after demulsification and centrifugation. This research can provide the theory foundation of destabilization strategies for the emulsion formed during enzyme-assisted aqueous extraction processing of soy oil.