Study On The Chemical Composition Of Exopolysaccharide From Lactic Acid Bacteria

Lactic acid bacteria(LAB) is a large group of probiotics in human body, recognized as safe micro-organisms. Streptococcus thermophilus produce Lactic acid, which is only one probiotic Streptococcus in LAB. The exopolysaccharides(EPS) produced by Streptococcus thermophilus is heter-polysacchatides, which has many special physiological functions, such as enhancing immune functions. So it has some potential value. People focus on it because of all these advantages. However, the production is very low, molecular weight and chemical composition of Exopolysaccharide, which is a bottleneck in industrial production, so the application is limited. To solve this problem, many researches are being done. In fermentation industry, there are many methods to increase the products, analyze molecular weight and chemical composition of Exopolysaccharide, including changing genetic properties of trains, changing conditions of fermenting, extracting method, gel filtration and trimethylsilylated methyl glycosides et al.With Streptococcus thermophilus cultured on the MRS-based medium, the optimal fed-batch fermentation conditions and extraction technology of exopolysaccharides of Streptococcus thermophilus was studied, EPS preparation was separated using DEAE-Cellulose ion-exchange chromatography. Exopolysaccharides preparations from Streptococcus thermophilus was characterized using gel filtration and gas chromatography(GC).By enzymatic, TCA, sevag, enzyme+TCA, TCA+sevag, enzyme+TCA +sevag methods were used to remove proteins form EPS preparation. The results show that the enzyme+sevag method to remove proteins as the best method.Single factor experiment was used to choose ethanol concentration, the temperature and pH factors'level, then the orthogonal design revolving regression analysis method was used to analyze the data. The optimal conditions for pure extracting EPS and the regression equation were obtained. The regression coefficient and the result of notable show the regression equation fits the actual process well. The optimal extraction conditions are ethanol concentration is 90%, the temperature is 15℃, and, ethanol is at pH 6, and the yield of EPS reaches the maximal value 2.9005 g/L. The relative error the experimental values and those predicted from the regression is 2.4%, and it suggests the fitness of the equation for the data is better.EPS preparation was characterized using DEAE-Cellulose ion-exchange chromatography and dextran G-100 gel filtration. UV detection method were used to identify the purity of EPS. Crude EPS with distilled water and 0.1mol/LNaCl through the DEAE-Cellulose ion-exchange column could be divided into neutral EPS and acidic EPS. Using dextran G-100 gel filtration on the purity of acidic EPS was identified by a single peak. Acidic EPS was wide wavelength scanned by UV spectrum scanning, acidic EPS did not contain proteins and nucleic acids, it prove acidic EPS is pure EPS. The molecular mass of EPS is 3.2×104Da.EPS preparations from Streptococcus thermophilus was characterized using gas chromatography(GC). The polysaccharide was highly heterogeneous and was mainly composed of glucose, galactose, fructose, rhamnose, mannose and xylose. Besides, GC analysis revealed that the polysaccharide contained two unknown monosaccharide with two small peak area suggesting low content.