Beyond Thermal Effect: Offline Microwave Activation of Deep Eutectic Solvents to Enhance Astaxanthin Extraction

This study developed a highly efficient and green technique for astaxanthin extraction by employing an offline microwave activation process for a choline chloride-lactic acid deep eutectic solvent (DES), leveraging the non-thermal effect of microwave. The impact of different microwave treatments on DES viscosity and subsequent astaxanthin yield was systematically investigated. Fourier transform infrared spectroscopy (FTIR) was employed to analyze the changes in the hydrogen bond network within the DES, elucidating the mechanism of microwave activation of DES. Ultrasonic assistance and repeated vacuum drying and grinding were employed to enhance the extraction efficiency of astaxanthin. The results showed that all microwave treatments significantly reduced the viscosity of the DES, which failed to return to the initial level upon cooling to ambient temperature. A high-power 915 MHz microwave system was markedly more effective in viscosity reduction compared with a conventional 2450 MHz microwave oven, because the 915 MHz microwave, with higher power and greater penetration depth, exerted a stronger electric field effect inside the DES. The highest yield of 23.28 μg/g was obtained by extraction in a water bath at 40 ℃ using DES activated by 915 MHz microwave heating to 70 ℃ and then cooled down, which was higher than that obtained using DES with 10% water content with the same treatment. The combined use of ultrasound treatment and vacuum drying resulted in an astaxanthin yield of 33.65 μg/g, representing over 97% of that obtained from freeze-dried samples (34.47 μg/g). Meanwhile, it reduced the drying time by approximately 66% and significantly lowered energy consumption. FTIR analysis indicated that both microwave treatment and water addition narrowed the O–H stretching peak and caused its blue shift, and simultaneously weakened and blue-shifted the C=O peak, collectively signifying a reduction in the strength of hydrogen bonds. Hence, it was inferred that the alternating electric field of microwave disrupts weak hydrogen bonds within the DES's three-dimensional network. This “fragmenting” it into smaller, highly mobile clusters that retain excellent solvation capacity, thereby enhancing astaxanthin extraction. In conclusion, efficient and green extraction of astaxanthin can be achieved through offline microwave activation of DES combined with ultrasound assistance and vacuum drying. This strategy provides a novel technological pathway for the industrial application of DES in astaxanthin extraction from shrimp shell waste.