Optimized microwave extraction, characterization and antioxidant capacity of biological polysaccharides from Eucommia ulmoides Oliver leaf

Microwave-induced technique was combined with response surface methodology for optimizing the isolation of polysaccharides from Eucommia ulmoides Oliver leaf. The maximum polysaccharides yield of 12.31% was achieved by microwave extraction at 74 °C for 15 min with a solid to liquid ratio of 1:29 g/mL, which agreed with the predicted value and was 2.9-fold higher than that of the conventional heat-reflux extraction method. The dominant bioactive constituent in extracts was chlorogenic acid (1.3–1.9%), followed by geniposidic acid (1.0–1.7%). The polysaccharides from the optimized extraction had a high molecular weight and polydispersity (M(w) 38,830 g/mol, M(w)/M(n) 2.19), as compared to the fraction prepared in the absence of microwave (M(w) 12,055 g/mol, M(w)/M(n) 1.26). Glucose was the dominant sugar component (38.2–39.1%) of heterogeneous polysaccharides which belonged to a structure of β-type acidic heteropolysaccharides with a glucan group and highly branched degree. The polysaccharides showed a higher DPPH radical scavenging index (0.87–1.22) than BHT (0.41) but lower than BHA (3.56), which can act as a favorable antioxidant in functional food.