Air-water interfacial and foaming properties of lupin protein-polysaccharide soluble complexes: Role of physicochemical properties, morphological characteristics, and flexibility

Lupin protein isolate (LPI) has poor foaming properties in acidic conditions. The addition of polysaccharides to form electrostatic complexes with LPI at acidic pH was used to improve the foaming properties of LPI. This study mainly investigated the role of morphological properties and flexibility of LPI-polysaccharide complexes in stabilizing air-water interfaces and foams. Three polysaccharides were chosen with different chain flexibility, namely κ-carrageenan (KC), pectin (PC), and sodium alginate (SA), to make electrostatic complexes with LPI at a 1:1 ratio and pH 4.0. Dynamic light scattering (DLS) and atomic force microscopy were used to study particle size and morphology of the complexes. LPI-KC formed a large complex (∼488.7 nm), consisting of several κ-carrageenan chains and large globular protein clusters, and formed a highly cross-linked structure, most likely linked by protein molecules and small protein clusters. LPI-PC formed a “core-shell-like” complex (∼267.2 nm), where the complexes appear to have a dense core with pectin chains protruding from that core. LPI-SA formed a smaller more linear complex (∼197.6 nm) that most likely consisted of bundles of polysaccharide chains held together by several protein molecules through attractive electrostatic interactions. Automatic droplet tensiometer (ADT) and AFM coupled with Langmuir-Blodgett deposition were used to study the interfacial properties of the complexes. LPI-PC and LPI-SA adsorbed faster to the air-water interface but formed interfaces with lower stiffness in the early adsorption phase than LPI-KC. After 3 h adsorption, LPI-KC formed a strong 2d gel-like air-water interface with the highest interfacial stiffness, while LPI-SA formed a soft glassy-like interface with a weaker interfacial stiffness than LPI-KC and LPI-PC. As a result, the LPI-KC stabilized foams showed the highest stability, followed by the LPI-PC stabilized foams, while the LPI-SA stabilized foams showed the lowest stability. Findings from this study revealed the relationship between the conformation of complexes and the air-water interfacial and foaming properties, which could be used to tailor the molecular properties of protein-polysaccharide complexes to achieve their optimal functionality in aerated food products.