Extraction and Separation of Oligosaccharides from Purple Onion

In this paper,the onion oligosaccharides were extracted from fresh purple onion by hot water extraction method and the extracts were isolated and investigated. The extraction process of carbohydrates in the purple onion was optimized by single factor test and orthogonal test. The thin-layer chromatography(TLC)and MS were used to detect the extractive and Bio gel P-2 filtration chromatography was used to separate the carbohydrates. The results showed that the best extract conditions of oligosaccharides from the purple onion were liquid ratio 1:5,extract temperature 70 ℃ and extracting time 80 min. Under these conditions,the total carbohydrates content was up to 3.92 mg/g. TLC analysis indicated that the extracts of purple onion contained six oligosaccharides with different polymerization degree. HPLC analysis showed that the purple skin onion oligosaccharides mainly consisted of fructo-oligosaccharide,and β-fructofuranosidase could better hydrolyze them. The results of gel chromatography showed that Bio gel P-2 gel could be used for the initial separation of onion oligosaccharides.