Interfacial properties and functionality of lupin protein-pectin complexes at the air-water interface

Lupin protein is a potential novel plant-based food stabilizer. It has low solubility at acidic pH, which can be enhanced by forming electrostatic complexes with pectin, and this may increase its application potential in food systems, such as in foams. Here, we investigated the air-water interfacial properties (including adsorption behavior, and interfacial shear and dilatational rheology), interfacial structure and foaming properties of lupin protein isolate (LPI) and its electrostatic complexes with low methoxyl pectin at pH 4.0, and at an LPI-to-pectin ratio (w/w) of 2:1, 1:1, 0.5:1 and 0.25:1. The soluble fraction of LPI adsorbed faster to the air-water interface than complexes, and displayed a higher interfacial shear modulus than the complexes, while the complexes (except the 2:1 ratio) showed a higher interfacial dilatational modulus than LPI. Based on AFM images, LPI formed a denser interface than all complexes at a low surface pressure of 10 mN/m, while all samples formed comparably dense interfaces at a surface pressure of 20 mN/m. Our results suggest that LPI may form a more glass-like interface that requires a higher shear stress to disrupt the microstructure, while complexes appear to form a thicker polymeric interface that shows a higher resistance to compression or expansion during dilatational deformation. A 0.1% solution of complexes showed better foam stability than 0.1% of the full fraction of LPI (including soluble and insoluble particles). However, 0.1% of only the soluble LPI fraction exhibited higher foam stability than the complexes, probably caused by the higher resistance of the LPI-stabilized interface to shear deformation (important during drainage), and due to a smaller air bubble size. This shows the excellent functionality of lupin with respect to foam stabilization, which is severely impacted by its low solubility at acidic pH. These findings also show that the complexation of LPI and pectin is an efficient way to improve interfacial and foaming properties of LPI, which could promote their applications as plant-based and clean label additives in the production of aerated food products at acid conditions.