Microwave-AssistedNatural Catalyst-Promoted Astaxanthin <i>Z</i> -Isomerizationin Deep Eutectic Solvent Systems

Abstract To improve the solubility and bioavailability of astaxanthin while enhancing its processing properties, this study established an eco-friendly, highly efficient microwave-assisted Z-isomerization system. We first conducted a systematic screening of various menthol-based deep eutectic solvents (DESs) to identify the most effective medium for this conversion. Following the selection of the optimal DES, we investigated the catalytic effect of three representative natural sulfur-containing compounds─methyl isothiocyanate (MITC), hexyl isothiocyanate, and dimethyltrisulfide─under microwave irradiation. Furthermore, we systematically evaluated the influence of other key process parameters, including heating method, catalyst dosage, astaxanthin mass concentration, microwave power, temperature, and duration, through single-factor experiments. The results demonstrated that the menthol-acetic acid-based DES provided the most effective reaction environment, and MITC exhibited the highest catalytic efficacy, achieving a peak total Z-isomer proportion of 36.2% at 60 mg/mL, compared to a baseline of 6.82% in the catalyst-free control group. Mechanistic analysis suggests that the electrophilicity of isothiocyanates and the steric effects of their side chains play critical roles in the isomerization process. These findings demonstrate that this green processing system can serve as an efficient and sustainable approach for the production of Z-isomer-enriched astaxanthin.