Oil-in-water Pickering emulsions using a protein nano-ring as high-grade emulsifiers

[Display omitted] •A protein nano-ring (SR′) is developed into a biocompatible Pickering emulsifier.•SR′ can adsorb at the oil/water interface and form stable oil-in-water emulsions.•Dispersed droplet size is dependent on emulsion formulation and external factors.•SR′-stabilized emulsions with high oil/water ratios show high gelation propensity.•The SR′ emulsified system is able to protect bioactive but labile compounds. Pickering emulsion-based delivery of liposoluble bioactive ingredients employing protein nanoparticles as biocompatible emulsifiers is a promising choice for food, cosmetic, and medical industries. This paper reports a novel design of a protein nano-ring (termed SR′) derived from chaperonin GroEL as an emulsifying agent, which has a naturally evolved hydrophobic binding rim in addition to its well-defined shape. It is shown that SR′ adsorbed at rosemary oil/water interface and formed stable oil-in-water Pickering emulsions, with dispersed droplet size being dependent on the SR′ concentration and oil/water ratio as well. The optimal formulation yielding stable nano-emulsions was determined to be at a SR′ concentration between 0.30 wt.% and 0.45 wt.%, and an oil/water ratio of 0.05-0.20 (v/v). Meanwhile, we demonstrate that nano-sized Pickering droplets could be easily prepared irrespective of the examined external factors including pH, temperature and ionic strength, with the lowest droplet sizes being produced at pH = 7.0, temperature ≤ 40 °C, and ionic strength (NaCl concentration) ≤ 50 mM. Besides, rheological analysis revealed the gelation propensity of SR′-stabilized emulsions with high oil/water ratios, an advantageous property that would further enhance the emulsion stability. Finally, it is shown that the SR′ emulsified system is able to protect β-carotene, which was used as a model of bioactive but labile compound. This work, in the context of the current drive for biocompatibility and sustainability, is believed to provide opportunities for emulsion-based applications to switch towards greener solutions.