Structural properties and antioxidant activity of polysaccharides extracted from Laminaria japonica using various methods

Polysaccharides in brown algae, including the kelp Laminaria japonica, are economically important to both food and pharmaceutical industries. The method used to extract polysaccharides significantly influences their characteristics and functions. In this study, L. japonica polysaccharides (LJP) were extracted using four techniques: hot-water extraction (HWE), ultrasound-assisted extraction (UAE), enzyme-assisted extraction (EAE), and acid-assisted extraction (AAE). The highest yield (9.73 ± 0.29 %) and sulfate content (12.47 ± 0.79 %) were obtained via UAE, whereas EAE yielded the highest total carbohydrate content (53.50 ± 1.38 %), with an extraction yield of 9.12 ± 0.13 %. The minimum molecular weight (Mw) was found after AAE, whereas a higher Mw was obtained after EAE and HWE, and a homogeneous Mw was obtained after UAE. All four LJP extracts contained a helix-coil conformation based on Congo red staining, with five monosaccharides, fucose, galactose, mannose, xylose, and glucuronic acid, in varying proportions. Polysaccharides extracted via EAE and UAE exhibited higher antioxidant activity with morphological and conformation changes. A significant correlation was noted between sulfate content and antioxidant activity, and antioxidant capacity was positively correlated to mannose content rather than uronic acid. The results indicate UAE to be the best of the four extraction methods for polysaccharides with homogeneous Mw and high antioxidant activity. This study sheds light on the effects of extraction processes on the physiochemical characteristics of polysaccharides and the relationship between chemical composition and antioxidant activities.