Résumé
Polysaccharides were extracted from a wild species of Ornithogalum by using three methods: maceration, ultrasound-assisted extraction, and combination of maceration and ultrasound. Extraction conditions were optimized by using response surface method (RSM) with a central composite design (CCD). The optimal extraction yield was 81.7%, 82.5% and 85.7%, and the optimal polysaccharides yield was 74.7%, 75.7%, and 82.8% under the optimum conditions of maceration, ultrasound-assisted extraction and combined extraction, respectively. These results indicate that the combination method significantly improves the extraction and polysaccharides yields compared to traditional extraction methods. The combination method also allows reducing the time of ultrasound treatment and thus its adverse effects on polysaccharides. In addition, these results well corroborate with the theoretically predicted values. The NMR (1H,13C, HSQC, HMBC, and COSY) analysis shows that the extract is composed of fructo-polysaccharides with a backbone of (2 → 6)-linked β-d-fructofuranosyl (Fruf) and (2 → 1)-linked β-d-Fruf branched chains, and terminated with glucose and fructose residues. The antioxidant activities of the extract were evaluated from ABTS radical scavenging activity, total antioxidant capacity, metal-chelating power and β-carotene bleaching test. Data show that the extract presents outstanding antioxidant activities.