Optimisation and modelling of ultrasonic-assisted extraction of canthaxanthin from Chromochloris zofingiensis using eutectic solvents

Carotenoids are well-known bioactive compounds and high-value nutritional molecules. Chromochloris zofingiensis, a unicellular green alga, is a promising source of commercial carotenoids owing to its robust growth, easy cultivation, and facile scale-up. However, this microalga has a very rigid three-layered cell wall that hinders extraction. Meanwhile, traditional extraction methods are complicated and cause serious pollution. To address these issues, a eutectic solvent (ES) was identified from 15 candidates for the ultrasonic-assisted extraction of canthaxanthin from C. zofingiensis for the first time. Remarkably, this hydrophobic ES comprising octanoic and decanoic acid displayed novel dual functions of breaking the cell wall and carotenoid extraction. Under the optimised extraction conditions obtained using a response surface methodology (ultrasound at 50 ℃ for 49 min, octanoic acid-to-decanoic acid molar ratio of 2.3:1, and solid-to-solvent ratio of 66.2 mg/mL), the maximum yield of canthaxanthin (60.5 μg/mL) was equal or superior to that using traditional organic extractant after grinding to break the cell wall. Furthermore, only the ES was employed as the extraction solvent, and the canthaxanthin content obtained by pre-grinding under the same condition (70.4 μg/mL) was much higher than that of grinding and extraction with ethanol (62.2 μg/mL). Possible mechanisms of cell wall breakage and component extraction were investigated based on extraction kinetic analysis, molecular dynamics simulation, and scanning electron microscopy observation. This ES is non-volatile and environmentally friendly, making it a simple, sustainable, and effective alternative extractant for carotenoids in microalga. There is considerable potential for its industrial-scale development and applications in various fields such as food, medicine, and cosmetics.