Performance of Quillaja bark saponin and β-lactoglobulin mixtures on emulsion formation and stability

The emulsifying properties of a mixture of Quillaja bark saponin (QBS) and β-lactoglobulin (β-lg) were evaluated under different pH conditions (7–9) and NaCl concentrations (0–200 mmol·L−1) and compared to the individual components. The formation and stabilization of oil-in-water emulsions were evaluated through visual analysis, droplet size distribution, droplet surface electrical charge (ζ-potential), and emulsion rheology. Both pH and NaCl concentration affected the properties of these emulsions in different ways, depending upon the QBS:β-lg ratio. QBS and/or β-lg emulsions had a relatively high negative droplet charge at pH ranging from 7 to 9 (−76.7 to −17.8 mV), which decreased in magnitude with decreasing pH or with increasing NaCl concentration. All emulsions were polydisperse and presented relatively small average droplet diameters (236–491 nm). Steady-state flow measurements revealed the non-Newtonian, shear-thinning behavior of all emulsions, which was properly described by the Herschel-Bulkley model with a small yield stress (12.9–214.8 mPa) and low apparent viscosity at 100 s−1 (1.81–2.97 mPa s). The emulsions were characterized as weak gels by dynamic oscillatory measurements. Most of the emulsions comprising QBS, β-lg, or a mixture of both were stable against droplet coalescence over a pH and NaCl concentration range. Regardless of the tested emulsifier, phase separation did not take place, although droplet creaming was observed. Emulsions comprising QBS and β-lg, both independently and mixed, showed similar emulsifying properties. However, the mixture of these appeared to provide emulsions with improved stability when compared to QBS and β-lg emulsions. The good stability of these emulsions can be attributed to the increased electrostatic repulsion and steric stabilization conferred by the two emulsifiers concurrently.