Study On Welan Fermentation And Its Rheological Properties

Welan gum, a kind of highly vicious extracellular polysaccharide produced by Sphingomonas sp. ZW-3, has found great potential application in oil recovery, concrete construction industry for its unique rheological properties.The proposal of this thesis is to produce welan gum by optimizing various low-cost raw materials, and study the molecular weight characterization with rheological properties. At first, welan batch fermentation under different nutrition conditions with Sphingomonas sp. ZW-3 was explored, and different modes of fed-batch fermentation were tested. Then, the rheological properties of welan solutions were analyzed, and their average molecular weight and molecular weight distribution, pseudoplasticity and viscosity, storage modulus and loss modulus were compared. In addition, preliminary exploration on welan samples'temperature resistance and salt tolerance was carried out. Outlines of this thesis are as follows:At first, a series of low-cost carbon sources such as sugar molasses, glycerol, fructose syrup and cassava starch were used for welan fermentation. The results indicated that the first three carbon sources could not achieve high welan yield, which needs further research. According to the principles of cassava starch hydrolysis with highly thermostableα-amylase and glucoamylase treatment, welan batch fermentation process with cassava starch hydrolysate of different DE value as carbon sources showed that welan yield from batch fermentation with DE=87% cassava starch hydrolysate reached 21.2 g/L, which was a little higher than that of batch fermentation with glucose as carbon sources.Second, influences of different nitrogen sources (soybean flour, dried corn steep liquor powder and combined nitrogen sources (the mixture of NaNO3 and soybean peptone) and different carbon sources (glucose and sucrose) on welan batch fermentation were investigated. The results showed nitrogen sources had greater impact on welan yield in comparison with tested carbon sources. The highest welan yield of 20.0 g/L was obtained with soybean flour as nitrogen sources. The welan yield of 17.0-18.0 g/L was achieved with dried corn steep liquor powder as nitrogen source. The lowest welan yield of 15.0 g/L was from combined nitrogen sources. The results of fed-batch fermentation suggested that the starting glucose concentration 10.0 g/L was more favorable for welan production as compared to the initial 20.0 g/L glucose, and the final welan yield was 24.8 g/L.Then, according to welan batch fermentation under different carbon and nitrogen sources, polymer rheological properties of the produced welan solutions were analyzed by using zero shear viscosity measurement, steady flow measurement and dynamic viscoelasticity measurement. The resultant data was applied to Carreau model, Power Law model and the linear simulation of complex viscosity log (η*) to log (ω). The analysis indicated that welan average molecular weight and molecular weight distribution were greatly affected by nitrogen sources in comparison with tested carbon sources. Welan samples derived from combined nitrogen media exhibited highest average molecular weight, narrowest molecular weight distribution and highest storage modulus. Welan sample from soybean flour nitrogen sources media was modest, while welan sample produced from dried corn steep liquor powder media exhibited lowest average molecular weight, widest molecular weight distribution, lowest storage modulus but highest loss modulus.Finally, influences of NaCl on viscosity of welan standard sea-water solutions was tested, and their high temperature resistance at NaCl 10%(w/w)was also studied. The results suggested that low NaCl concentration was favorable for improving the viscosity of welan standard sea-water solutions, and high NaCl concentration would decrease the viscosity. The viscosity of welan standard sea-water solutions was higher, and stable at 100℃. But when the temperature was increased to 120℃, the viscosity retention ability of commercial welan sea-water solutions was stronger with viscosity also apparently reducing.