Extraction, molecular weight distribution, and antioxidant activity of oligosaccharides from longan (Dimocarpus Longan Lour.) pulp.

Ultrasonic-microwave synergistic extraction (UMSE) was optimized for the extraction of oligosaccharides from longan pulp (OLP). Box-Behnken design was used to evaluate the effects of temperature (35-55°C), ultrasonic time (5-25 min), and water to material ratio (10-30 mL/g) on the extraction efficiency of crude OLP. A regression model was developed and its validity was statistically demonstrated. Significant interaction between temperature and water to material ratio was observed. The following optimal conditions for the extraction yield of crude OLP were determined: extraction temperature 55°C, ultrasonic time 18.52 min, and water to material ratio 10 mL/g. The extracted OLP were purified for the determination of molecular weight distribution and antioxidant activity. Results of matrix-assisted laser desorption ionization time-of-flight mass spectrometry revealed that the molecular weight distribution of the purified OLP ranged from m/z 495.138 to 795.511. The purified OLP exhibited a dose-dependent behavior in 1,1-diphenyl-2-picrylhydrazyl radical scavenging activity.