Biosurfactant-Protein Mixtures: Quillaja Bark Saponin at Water/Air and Water/Oil Interfaces in Presence of β-Lactoglobulin

The adsorption kinetics of mixtures of a biosurfactant Quillaja Bark Saponin (QBS) with a globular protein, beta-lactoglobulin (beta-LG) at the water/air and water/tetradecane interfaces was investigated by measuring dynamic interfacial tension with axisymmetric drop shape analysis (ADSA) and maximum bubble pressure (MBP) techniques. With bulk concentration of beta-LG fixed at 10(-7) M, the most pronounced synergistic effects in the rate of the QBS adsorption at both interfaces were observed at low biosurfactant concentrations (S X 10(-7)-1 X 10(-5) M). The synergistic effect due to a protein-biosurfactant complex formation is dearly noticeable, yet less pronounced than, e.g., previously studied QBS/lysozyme mixtures. The surface pressures attained at water/oil interface are higher than in the water/air system, although, at high biosurfactant/protein ratios, the presence of beta-LG decelerates adsorption of the QBS/beta-LG complex onto the water/tetradecane interface. In analogy to mixtures of synthetic surfactants with proteins, the adsorbed layer gets dominated by QBS at higher biosurfactant concentrations, although the presence of beta-LG affects the surface pressures attained even at QBS/beta-LG ratios as high as 10(4). The synergistic effects are much less noticeable in foamability and emulsion formation/stability, as probed by the modified Bikerman's and dynamic light scattering (DLS) techniques, respectively.