Optimization of sequential ultrasound-microwave assisted extraction of polysaccharide from red seaweed (Kappaphycus alvarezii)

This study deals with the optimization of sequential ultrasound-microwave extraction of carrageenan from Kappaphycus alvarezii and its comparison with aqueous and alkali extraction. Optimization of extraction parameters was carried out using Response Surface Methodology. For the evaluation of the parameters Central Composite Rotatable Design with three independent parameters (temperature, extraction time and liquid/solid ratio) at five different levels was studied with yield as a response. ANOVA was performed to find the significance of the model. The highest extraction yield of 61.25 ± 0.70% with 10 min of ultrasound pre-treatment at 400 W followed by 15 min of microwave extraction at 93 °C was obtained using sequential UMAE. The structural characterization of extracted polysaccharide confirmed the presence of kappa-carrageenan. The physicochemical properties such as 3,6-anhydrogalactose, sulfate, protein content, rheology, and gel strength of carrageenan were measured. The carrageenan extracted using MAE and sequential UMAE showed high 3,6-anhydrogalactose content (41.12 ± 0.91%) and low sulfate content compared to aqueous extracted carrageenan. The rheological curves showed shear-thinning behaviour and higher apparent viscosity values for UMAE extracted carrageenan. In addition, sequential UMAE showed the highest values for gel strength (588.03 ± 4.81 g) and antioxidant activity (DPPH method: 28.80 ± 0.43; ABTS method: 32.35 ± 0.35) due to high 3,6-anhydrogalactose content, which was formed on removal of the sulfate group from precursor carrageenan.