Facile and Robust Route for Preparing Pickering High Internal Phase Emulsions Stabilized by Bare Zein Particles

Abstract

Pickering high internal phase emulsions (HIPEs) stabilized by biological particles have attracted growing interest. However, developing Pickering HIPEs with bare zein particles (ZPs) is still a challenge. Here, we demonstrated a facile method for the development of Pickering HIPEs stabilized solely by bare ZPs. In brief, the pH value was adjusted during the emulsification; thus, the aggregation of ZPs transferred from the continuous phase of HIPEs to the oil–water interface. In detail, the strong hydrophobicity of ZPs was changed into intermediate wettability after changing pH, as indicated by the value of the three-phase contact angle (θ) decreasing from 108.4° to around 90°. Through adjusting the assembly behaviors of ZPs, the interfacial and continuous microstructures in the emulsification system were defined accordingly, therefore forming stable Pickering HIPEs. Generally, with the final NaOH concentration ranging from 0.6 to 2.0 mM in HIPEs (pH 6.6–8.9), HIPEs with an internal phase volume fraction of 80% were prepared successfully using pristine ZPs as sole eco-friendly Pickering stabilizers. The microstructure of HIPEs featured that ZPs deposited in situ at the oil–water interface, forming an effective physical barrier, while the rest of ZPs that remained in water phase functioned as the structural element and stacked into a 3D network architecture, constituting another strong barrier and facilitating the immobilization of oil droplets. This microstructure contributed to satisfactory stability, good viscoelasticity behavior, and ideal thixotropic recovery of HIPEs. Moreover, the HIPEs obtained here served as the encapsulation system for probiotics, and they maintained the viability of Lactobacillus plantarum at 4 °C. The facile method developed here paves the way for preparing Pickering HIPEs stabilized by protein-based particles.