Biopolymer-induced depletion attraction in pulse protein-stabilized viscous oil-in-water emulsions

Depletion interactions in colloids are less extensively studied despite their influence on emulsion stability and functionality. Depletion interaction arises due to excess non-adsorbing species in an emulsion's aqueous phase, which creates localized concentration gradients between oil droplets that force them together, resulting in flocculation. The present research aims to investigate the effect of excess non-adsorbing biopolymers on the depletion interaction-induced aggregation, stability and viscosity of faba bean protein-stabilized canola oil-in-water (O/W) emulsions. A stock 30 wt% O/W emulsion was prepared with an optimum concentration of faba bean protein that would leave no excess protein. This emulsion was diluted with varying ratios of sodium caseinate (SC) solution (5 wt%), mixing gently to not displace the faba protein from the interfaces. The resulting 20 wt% O/W emulsions containing 0 to 2 wt.% SC was characterized by aggregate size, visual observation, viscosity and creaming stability under accelerated gravitation. SC had a significant impact on creaming behaviour; the height of the cream layer increased with an increase in unadsorbed SC concentration in the emulsion. The droplets of all emulsions were aggregated. However, stronger aggregation, which could not be broken down by adding an anionic small molecule emulsifier, was observed at >1wt% SC, indicating a critical effect of excess protein concentration. Cream layer separation under accelerated gravitation increased with the addition of SC, reaching a maximum separation at ≥1 wt% SC. In conclusion, the behaviour of concentrated O/W emulsion can be manipulated by adding excess nonadsorbing biopolymers. In the future, the properties of the non-adsorbing species will be varied such that depletion interaction and, hence, the structure and functionality of the emulsion system can be precisely controlled. While depletion interaction is well-studied in non-food grade or simulated systems, this research serves as a step toward a better understanding of it in viscous food emulsions.