Enhanced Solubility and Reduced Crystallinity of Carotenoids, beta-Carotene and Astaxanthin, by Z-Isomerization

The impact of the Z-isomer content of carotenoids, beta-carotene and astaxanthin, on their solubility in organic solvents and crystallinity is investigated. Carotenoid samples containing different Z-isomer contents are prepared from the high-purity all-E-isomers by the thermal isomerization and filtering technique: beta-carotene, 27.4% and 89.8% Z-isomer content; astaxanthin, 34.5% and 63.2% Z-isomer content. As the Z-isomer content of the carotenoids increases, their relative solubility markedly improves, e.g., the solubility in ethanol of beta-carotene containing 89.8% Z-isomer is approximate to 250 times higher than that of the all-E-isomer. Moreover, differential scanning calorimetry (DSC), powder X-ray diffraction (XRD), and scanning electron microscopy (SEM) analyses clearly indicate that the (all-E)-carotenoids are present in the crystal state, while the Z-isomers are present in an amorphous state. A number of studies have reported that the Z-isomerization of carotenoids induced changes in bioavailability and antioxidant activity. This may be due to the change in physicochemical properties such as solubility and crystallinity by Z-isomerization. In addition, these changes are important information for improving carotenoid processing. Practical Applications: Carotenoids in plants occur predominantly in the all-E-configuration, whereas the Z-isomers are present in the human body in considerable quantity. Since (all-E)-carotenoids have low solubility in solvents and high crystallinity, efficiencies of those processing such as solvent extraction, micronization, and emulsification are low. However, this study experimentally reveals that solubilities of carotenoids, beta-carotene and astaxanthin, are significantly improved and crystallinities of them are reduced by Z-isomerization. These fundamental data should contribute to improvement of carotenoids processing efficiency.