Particle size and morphological features of lupin protein-pectin complexes affect the oil-water interfacial properties and emulsion stabilization

Lupin proteins are novel plant-based stabilizers, but their solubility and functionality are dramatically reduced at acidic pH, which can be improved by forming protein-polysaccharide complexes. The physicochemical properties of protein-polysaccharide complexes can affect the properties of oil-water interfaces stabilized by them. Here, we prepared lupin protein-pectin complexes at ratios 1:1, 0.5:1, and 0.25:1, with different particle sizes and morphological features, to elucidate the role of these properties in the oil-water interface and emulsion stabilization, and compare the behavior of the complexes to that at air-water interfaces. The molecular properties of complexes were analyzed with DLS and surface hydrophobicity measurement. Morphological features of the complexes were imaged by AFM. Mechanical properties of the oil-water interfaces were investigated using large amplitude oscillatory shear (LAOS) and dilatation (LAOD). The LAOD data were analyzed by general stress decomposition (GSD). The emulsifying properties of the complexes were studied by measuring droplet size and testing emulsion stability under flow. Complexes with a 0.25:1 ratio had the largest size (∼580.4 nm) with highly cross-linked structures and formed the stiffest oil-water interface (E_d’ = 67.5 mN/m) and the most stable emulsion against flow-induced coalescence. Complexes with a 1:1 ratio had the smallest size (∼275.6 nm) with a less cross-linked structure, formed the weakest oil-water interface (E_d’ = 35.0 mN/m), and had lower stability in flow. This study reveals the role of particle size and morphology of complexes in fluid-fluid multiphase systems and shows that the behavior of complexes is remarkably different at oil-water interfaces compared to air-water interfaces.