Continuous Production of Z-isomer-rich Astaxanthin Nanodispersions with Stabilized Z-structures Using Selected Ester-Based Nonionic Emulsifiers

The multiple beneficial biological activities of astaxanthin show promise for the application of ​nutritional supplements and cosmetics, but the industrial use of this carotenoid is hindered by its high crystallinity and poor solubility, which result in low oral bioavailability and percutaneous absorbability. These drawbacks are inherent to many carotenoids and are typically addressed through their formulation as aqueous dispersions. Given that carotenoid Z-isomers have greater bioavailability than the corresponding all-E-isomers, we herein realized the continuous production of Z-isomer-rich astaxanthin nanodispersions and determined the optimal conditions for Z-isomer stabilization. The thermal isomerization of (all-E)-astaxanthin was performed in a flow reactor using subcritical ethyl acetate as a solvent, and a total Z-isomer ratio of >60% was achieved in only 15 s. The mixture of isomers was dispersed in an aqueous solution of an ester-based nonionic emulsifier using an in-line swirl mixer, and the properties of these dispersions were investigated as functions of emulsifier type. The best performance was observed for sucrose fatty acid ester, which promoted the formation of fine dispersions (average particle size of <10 nm) and stabilized astaxanthin Z-isomers. Thus, the Z-isomer-rich fine dispersions are expected to increase the bioavailability of astaxanthin and hold great promise for its industrial applications.