Study On Extraction, Purification And Antioxidative Activity In Vitro Of Polysaccharides From Onion

Onion belongs to Liliaceae Allium plant, which is called the queen of vegetables. Onion production accounts for about 10% of the total vegetables worldwide, second only to tomatoes. They are unique agricultural and condiment resources, and were planted widely in China. Onions are rich in nutrients, including sulfur compounds, polysaccharides, flavonoids, steroidal saponins and prostaglandins. It is getting research hotspot recently for its physiological functions. Onions were used as raw material in this paper, and its extraction, purification, monosaccharide and antioxidant activity were studied. All the results are showed below:1. The optimum conditions of extracting polysaccharides from onion with cellulose was investigated by response surface methodology and identified temperature 53℃, reaction time 100min, pH 5.0, enzyme 0.3%, solid-liquid ratio 1:40(w/v). Under these conditions the polysaccharides extraction rate was 9.39%, 4.06% higher than extracting with water.2. The results showed that macroporous adsorption resin AB-8 was the better resin to decolorization of polyssacharides from onion. The best conditions for decolorization are determined as follows: the amount of resin is 1g/20mL , the pH value is 5.0, the temperature is 40℃, the content of polysaccharides is 2792.5mg/L, the flow rate is 3BV/h. Under the optimal technique, the decolorization ratios,the polysaccharide reserved ratios are 86.71% and 88.92%, respectively. It is evident that AB-8 is suitable for use in decolorization of polyssacharides from onion.3. The polysaccharide was sequentially purified by ion exchange chromatography on DE-52 cellulose column and a refined polysaccharide fraction was obtained. Then the physical and chemical characters, the monosaccharide compositions of onion polysaccharide were studied. The results showed that the amount of total carbohydrates in onion is 85.32 %. UV-spectrophotometer analysis indicated that onion polysaccharide showed a signal of polysaccharide at 195nm and no signals of protein, nucleic acid at 260 and 280 nm. FTIR spectrometry analysis indicated that onion polysaccharide showed typical signals of polysaccharide, including signals at 3422.25 cm-1,2920.43 cm-1,1627.37 cm-1,1415.55 cm-1,1331.67 cm-1,1091.85 cm-1 and its monomer of backbone isα-pyranose. The monosaccharide constituents of onion polysaccharide were rhamnose, glucuronide acid, galacturonic acid, glucose, galactose, xylose and their molar proportions were 0.32:0.14:1.90:0.20:4.97 and 1.00, respectively.4. The onion polysaccharides have strong scavenging effects on 1,1-diphenyl-2-picrylhydrazyl radicals and hydroxyl radicals, the scavenging effect on superoxide radical was not abvious, its scavenging rate was less than 10% in the selected range of polysaccharide concentration and has no significant change in the increase of the concentration. Moreover, onion polysaccharides have certain reductive capability, and its ability increased with the concentration gradually.