Thermoresponsive structured emulsions based on the fibrillar self-assembly of natural saponin glycyrrhizic acid

We report the novel use of the naturally occurring saponin, glycyrrhizic acid (GA) as a structuring material to transform liquid oil into a soft-solid structured emulsion system. The GA nanofibrils from the anisotropic self-assembly of GA molecules were first used as stabilizers to fabricate olive oil-in-water emulsions using a facile one-step emulsification at high temperature. Then, the obtained emulsions were further self-organized into the emulsion gel by applying a subsequent cooling to trigger the gel network formation, which is mostly due to the enhanced noncovalent interactions among GA fibrils in the continuous phase as well as at the droplet surface. The GA fibrils could adsorb at the interface in a multilayer form, leading to the formation of unique fibril shells with high electrostatic repulsive force, which could provide superior stability for the GA fibril-stabilized oil droplets and thus the whole emulsion gel during storage and heating. The thermoreversible gel-sol transitions of a self-assembled GA fibrillar network in the continuous phase endow the stable emulsion gels with a temperature-responsive switchable behavior. Moreover, the GA fibril-coated oil droplets embedded in the network were found to be closely packed together and connected with the gel matrix. As a consequence, the emulsion gels exhibited many interesting rheological behaviors, including a high gel strength, shear sensitivity, and good thixotropic recovery. These simple and inexpensive smart responsive oil structuring materials based on natural saponins could find novel applications in the fields of food, pharmaceuticals, or cosmetics.