Unraveling the isomerization and extraction characteristics of ester-form astaxanthin originated from Haematococcus lacustris in supercritical CO2 for practical applications of Z-isomer-rich astaxanthin production

In recent years, the superior bioavailability and biological activity of astaxanthin Z-isomers have become evident, with their application being highly anticipated. Currently, thermal treatment remains the most commonly used method for inducing Z-isomerization in astaxanthin. However, this approach often requires the use of toxic solvents and catalysts to achieve efficient thermal isomerization. Here, we developed environmentally friendly processes for Z-isomerization of ester-form astaxanthin originated from Haematococcus lacustris utilizing the supercritical CO₂ (SC-CO₂) extraction process. Astaxanthin esters with a high ratio of Z-isomers (∼60 %) were successfully prepared without using toxic chemicals through three strategies: Z-isomerization via (i) high-temperature extraction, (ii) heating the extracted liquid, and (iii) heating the raw material before extraction. Furthermore, this study revealed that high-temperature treatment (>140 °C) of astaxanthin esters in SC-CO₂ enhances the production of the valuable 9Z-isomer while also demonstrating that the solubility of geometrical isomers of astaxanthin esters in SC-CO₂ is almost identical.