Physicochemical Characterization and Antioxidant Activity of Semi-Refined Carrageenan Extracted from Kappaphycus alvarezii

Abstract

Semi-refined carrageenan (SRC) is a lower-processing alternative to refined carrageenan that retains part of the native seaweed matrix. This study characterized SRC extracted from Kappaphycus alvarezii using alkaline pretreatment followed by ultrasound-assisted extraction (UAE). Extraction temperature was varied from 30 to 60 °C for 1 h at 80 kHz and approximately 79% amplitude. The highest yield was 30% at 60 °C. The resulting SRC had a viscosity of 677.80 ± 10.18 cP at 35 °C, sulfate content of 21.73 ± 0.79%, ash content of 17.94 ± 0.86%, acid-insoluble matter of 10.56 ± 0.42%, and pH of 8.75 ± 0.05. Its gel strength was 14.64 ± 0.98 lb ft~ 2 , compared with 59.10 ± 1.15 lb für 2 for refined carrageenan (RC), while viscosity did not differ significantly between SRC and RC under the reported test conditions. SRC showed concentration-dependent DPPH inhibition, reaching 0.864 ± 0.0005% at 5 mg mL _1 , although its activity remained below that of ascorbic acid. FTIR bands were consistent with a sulfated galactan containing hydroxyl, sulfate ester, and 3,6-anhydrogalactose-associated groups. GC-MS detected heneicosane and tricosane among the identified compounds. X-ray diffraction and scanning electron microscopy indicated a less ordered, granular and fibrous material than RC. The findings identify 60 °C as the best temperature among those tested for this extraction procedure and support SRC as a candidate film-forming material for edible coatings, while its lower gel strength and residual insoluble fraction should be considered during formulation.