Optimization of astaxanthin extraction from red (Gracilaria corticata) and brown (Sargassum polycystum) macroalgae through ultrasonication and microwave processing

This study aimed at optimizing the extraction of astaxanthin (ASX) by microwave- and ultrasound-assisted methods through an organic solvent (a mixture of ethanol and ethyl acetate), a green solvent (a microemulsion (MEL) of tributyloctylphosphonium bromide ionic liquid in water), and a vegetable oil (sunflower oil) from red (Gracilaria corticate/GC) and brown (Sargassum polycystum/SP) macroalgae. First, MELs were evaluated in terms of density, particle size and conductivity. Then, total carotenoids, ASX content, ASX extraction efficiency and DPPH scavenging activity were measured to evaluate the performance of different extraction methods. According to the results, the density of MEL was 0.97 g/cm³, its diameter was 15.80 nm and the conductivity was 312 µS/cm at 27.1 °C. Based on our findings, the optimal conditions for the extraction of ASX from SP and GC by sonication were the solvent to sample ratio of 20:1, power of 200 W and time of 30 min; for microwave processing, the solvent to sample ratio of 20:1, power of 100 W and time of 5 s. The results showed that in both methods, the ionic MEL solvent showed better performance in terms of ASX extraction, and SP resulted in more total carotenoids compared to GC. Microwave treatment was more effective for extracting ASX from brown algae due to its ability to rapidly disrupt cells from the inside, improve solute–solvent interactions, and overcome structural resistance of algal tissues more efficiently than sonication. Based on the results, red (GC) and brown (SP) macroalgae can be suggested as accessible and inexpensive sources for extracting valuable compounds such as bioactives and the natural pigment astaxanthin.