The Optimization Of Welan Gum Fermentation And Study On Its Structure And Capability

Welan gum is a kind of soluble exopolysaccharide which produced by Alcaligenes sp.NX-3 in an aerobic submerged fermentation. As reported that welan gum in aqueous media has excellent rheological properties,imparting viscosity to an aqueous medium when dissolved in low concentrations. It has potential to be used as thickening agent, suspending agent, emulsifying agent in agricultural or industrial applications. Till now the industrialization of welan gum has been realized only in Kelco,a company of American. The report about fermentation is rare while the application patents have more than one hundred. The research of this micro-polysaccharide has not been reported in our country. This thesis is about flask-shaker fermentation of welan gun production using Alcaligenes sp.NX-3. Studies on the nutritional and environmental requirements for optimal polysaccharide production were carried out. The structure and characteristics of welan gum produced by fermentation were studied also. The main works and conclusions are as followed: 1. Fermentation condition optimized The carbon source and nitrogen source are important for welan gum ferment. Through single factor method, sucrose and yeast extract are confirmed as carbon source and nitrogen source respectively. Then central complex design method was applied for the optimization of cultural medium. It was showed that the prime fermentation medium contain: sucrose 49g·L^-1,yeast extract 4 g·L^-1, K2HPO4 2 g·L^-1, MgSO₄ 0.4g·L^-1. According to the optimized medium , the yield increased from 10.78 g·L^-1 to 22.85 g·L^-1. Statistic analyze showed that carbon source was the most remarkable factor for the production of welan gum; also the concentration of K₂HPO₄ was important. The optimized environmental condition as followed: rotate speed of the shaker is 200 r/min, medium volume is 100mL/500mL, initial pH is about 7.5, the welan gum synthesis temperature is 30℃. 2. The structure of welan gum The method of welan gum purification has been established. The components of analytic sample were identified by TLC,GC and chemical method after acid hydrolysis as: glucose, mannose, rhamnose and glucuronic acid. The proportion of mannose,glucose and rhamnose is 1:1.35:2.11. The content of saccharide and glucuronic acid is 87.3%,12.44%respectively. By means of IR and periodate oxidation, it showed that the polysaccharide was mainly consisting of (1→3)and (1→4) glucosidic linkages. 3. The characteristics of polysaccharide solution The relation between viscosity and shear rate,concentration,temperature,pH and salts was investigated. The results showed that the solution of welan gum is a typical pseudoplastic liquid, having high viscosity at low concentrations. It has the excellent stability towards pH2.0¹3.0, also has outstanding property of viscosity retention even at temperatures up to 150℃. Some kinds of salts were cooperated with welan gum as a thickening agent.