Rapid and Continuous Astaxanthin Isomerization in Subcritical Ethanol

Recent studies have shown that Z-isomers of astaxanthin have greater bioavailability and biological activity than the all-E-isomer. Hence, it is important to develop efficient and practical Z-isomerization methods. This study attempted to establish a continuous-flow isomerization system for (all-E)-astaxanthin in subcritical ethanol. Specifically, the effects of temperature, pressure, time, and addition of antioxidants and a natural catalyst, allyl isothiocyanate, on the Z-isomerization and degradation were investigated. Continuous high-temperature processing (>140 °C) efficiently promoted astaxanthin Z-isomerization within a few minutes, but simultaneously accelerated its degradation. We found that the thermal degradation could be inhibited by adding specific antioxidants, especially ascorbic acid. Finally, using the continuous-flow system with ascorbic acid, 60.3% of total Z-isomer ratio of astaxanthin was achieved while inhibiting the degradation at 200 °C and 10 MPa in just 30 s. This efficient and environmentally friendly isomerization technology is an effective means for the production of Z-isomer-rich astaxanthin materials.