Sequential recovery of alginate from fucoidan extraction by-products of Nizamuddinia zanardinii seaweed using green extraction methods

The effects of green technologies-ultrasound, microwave and their combined application-on alginate extraction from fucoidan production by-products of brown seaweed (Nizamuddinia zanardinii) were compared with the conventional alkaline method. The impact of the extraction methods on the Fourier-transform infrared spectroscopy (FT-IR) spectra, molecular weight, antioxidant, rheological, emulsifying and foaming properties of the recovered alginates was also evaluated. The highest (15.36 % w/w) and lowest (11.88 % w/w) alginate yields were obtained using the microwave and conventional methods, respectively. Using ultrasound (2362 kDa) and ultrasound-microwave (2608 kDa) led to a significant reduction (p < 0.05) in the average molecular weight of alginate in comparison to the microwave (3015 kDa) and alkaline methods (3021 kDa). The microwave-extracted alginate showed the highest DPPH (2,2-diphenyl-1-picrylhydrazyl) radical scavenging activity (19.98 %-35.60 %) and ferric reducing antioxidant power (FRAP) (0.138-177 abs) of the extracted alginates. The rheological properties of the alginates were affected by the extraction method, resulting in the highest viscosity in the microwave- and conventionally-extracted alginate. Also, all the extracted alginates showed moderate emulsifying and foaming properties. Overall, the findings highlight the great potential of green technologies to enhance the recovery and functionality of alginate from fucoidan extraction by-products, advancing the efficient biorefining of brown seaweed.