Nano and micro lupin protein-grape seed extract conjugates stabilizing oil-in-water emulsions

Conjugation of plant proteins with phenolic compounds has been an efficient strategy to improve the limited capacity of this kind of protein to stabilize emulsions. In this sense, the stabilizing capacity of lupin protein-grape seed extract conjugates in the form of nano (NC) or micro (MC) particles was evaluated in oil-in-water emulsions. Covalent binding between lupin protein isolate (LPI) and grape seed extract (GSE) was confirmed by quantification of total free sulfhydryl groups and fluorescence assays in both NC and MC for all GSE concentrations tested (0.04–0.12 wt%). The main differences between MC and NC were the larger size and surface hydrophobicity of the former, reflecting different stabilizing capacities. The droplet size and microstructure of all emulsions after 28 days of storage suggest that NC- and MC-stabilized emulsions were highly stable to coalescence. However, NC produced emulsions with a higher tendency to flocculation. Increasing the GSE concentration in the conjugates attenuated the droplet flocculation, resulting in emulsions with improved kinetic stability. Results indicate that the dominant stabilizing mechanism of the conjugates is Pickering-type, although others may occur simultaneously. Indeed, MC produced emulsions with better stability with no visible creaming and more complex rheological properties. A gel-like behavior (G’>G’’) was observed from oscillatory rheology, which was considered an additional stabilizing mechanism. In contrast, NC produced emulsions with predominantly liquid-like behavior (G’’>G’). Shear resistance was evaluated after homogenization of the emulsions, showing that the 3D network formed in the emulsions was rapidly recovered, including the highly structured MC-stabilized emulsions. Different properties of emulsions stabilized by NC or MC indicate potential varied applications in the food industry. The conjugation of GSE with LPI showed improvements in the overall stability of the emulsions, demonstrating the great potential for usage of these conjugates in the formulation of highly stable emulsion-based systems, even using a protein of limited stabilizing capacity.