Influence of Extraction Methods on the Chemical Composition and Antioxidant Activity of Polysaccharide Extracts from Discarded Sea Grape (Caulerpa lentillifera)

Abstract

Sea grape (Caulerpa lentillifera) is a green marine macroalga which contains polysaccharides as major components of the cell wall. After harvesting and grading for sale, approximately 60-70 % of sea grapes are considered to have defects and are unsaleable. However, their nutritional values are not affected and thus they should be utilized for other purposes such as a source of natural products. The aims of this study were to investigate the impacts of pH of the extraction solvents and extraction time on the polysaccharide yield and chemical composition of C. lentillifera extracts, and to investigate their antioxidant potentials. Hot water extraction (pH 6) at 90 °C for 20 min was practical and the most cost-effective method compared to acidic or alkaline conditions and compared to longer extraction times. The polysaccharide extracts contained 47 % total carbohydrate, 31 % minerals, and 19 % sulfate by dry weight (dw). The major monosaccharides (% mol) were galactose (37 %), mannose (32 %), xylose (19 %) and glucose (11 %). The high-molecular weight (Mw) polysaccharide extract was determined to be approximately 3,830 kDa relative to dextran standard. Acid hydrolysis with 1 M hydrochloric acid (HCl) at 70 °C for 120 min was performed on the extract to obtain the low-Mw polysaccharide extract (~21 kDa). The antioxidant activities of the low-Mw polysaccharide extract were ten-fold higher than the high-Mw polysaccharide extract. Low-Mw polysaccharide extract had the half-maximal inhibition concentration (IC₅₀) values of 0.08 mg∙mL^-1 for DPPH and 1.07 mg∙mL^-1 for ABTS, which was lower than the high-Mw polysaccharide extract (0.9 and 10.8 mg∙mL^-1). Therefore, the extracted polysaccharide with low-Mw has high potential for use as an antioxidant ingredient in the food industry.