Interfacial behavior of lupin protein and its complexes with polysaccharides at different oil-water interfaces analyzed by general stress decomposition

In this study, we studied the interfacial behavior of lupin proteins (LPI) and lupin protein-polysaccharide complexes (LPI-PS) (with sodium alginate, pectin, and κ-carrageenan), at different oil-water interfaces using interfacial dilatational rheology. Interfacial mechanical properties were investigated using large amplitude oscillatory dilatation (LAOD) and analyzed with the general stress decomposition (GSD) method. LPI and LPI-PS complexes adsorbed faster at apolar oil-water interfaces than at more polar oil-water interfaces. A significant change in the GSD parameters, E_τ1L and E_τ4, was observed across different hydrophobic subphases (i.e., more polar oil, apolar oil, and air). At more polar oil-water interfaces, the E_τ4 moduli were highly positive (1.6–5.2 mN/m), and E_τ1L was very low (11.4–17.9 mN/m). At more apolar oil-water interfaces, the E_τ4 moduli became slightly negative (between −2.7 and  −3.7 mN/m), and E_τ1L was considerably increased (37.8–51.4 mN/m). At air-water interfaces, the E_τ4 moduli were most negative (between −11.9 mN/m and −13.1 mN/m), and E_τ1L was highest (77.8–150.4 mN/m). These results suggested that the LPI-PS complexes may behave more similar to particles and form soft glass-like structures at polar oil-water interfaces, and more gel-like networks may form at apolar oil- and air-water interfaces. At the air-water interface such networks have previously been observed using atomic force microscopy. LPI showed a more substantial increase in E_d’ with reduced oil polarity than LPI-PS with lower structural flexibility. Emulsions prepared with more polar oils also showed worse emulsion flow stability than the others, due to the lower stiffness of their oil-water interfaces.