Fermentation exopolysaccharides

In the sucdnoglycan fermentation, exopolysaccharide production commences after cessation of growth. During the 80 hour production phase of the fermentation, there is a linear increase in product concentration and a linear decrease in residual glucose. 

From a commercial point of view, xanthan gum is the most important microbial exopolysaccharide currently being manufactured. Therefore, we shall consider the fermentation of this product by Xanthomonas campestris in some detail. 

There are two main methods for the recovery of exopolysaccharides from fermentation liquors ... 

Polar organic solvents readily precipitate exopolysaccharides from solution. The solvents commonly used are acetone, methanol, ethanol and propan-2-ol. Cation concentration of the fermentation liquor influences the amount of solvent required for efficient product recovery. In the case of propan-2-ol, increasing the cation concentration can lead to a four-fold reduction in die volume of solvent required to precipitate xanthan gum. Salts such as calcium nitrate and potassium chloride are added to fermentation broths for this purpose. 

Ultrafiltration has the advantage that there is removal of low molecular weight fermentation products and medium components during concentration of the exopolysaccharide. In addition, biological degradation is minimised because fluid is held only for a short time during the filtration process. Other advantages lie in file fact that there is no requirement for solvent recovery and the process is carried out at ambient (not elevated) temperature. 

Glucose limits sucdnoglycan production (ammonia limits growth). It is necessary to limit sucdnoglycan production to avoid an excessively viscous fermentation liquor which would affect mixing and subsequent downstream processing to recover exopolysaccharide. 

Lactic acid bacteria are used to produce fermented milk products, and the exopolysaccharides produced by the bacteria influence the texture of the resulting products. More importantly, these exopolysaccharides are thought to have several health benefits. There is evidence they lower cholesterol, modulate the immune system, help prevent colon cancer, and fight ulcers [342,343]. There is thus interest in establishing stmcture-function relationships for these stmctures, as well as in metabolic engineering of lactic acid bacteria to produce capsular polysaccharides with the desired properties [342,343]. 

Tieking, M., Korakli, M., Ehrmann, M.A., Ganzle, M.G., and Vogel, R.F. 2003. In situ production of exopolysaccharides during sourdough fermentation by cereal and intestinal isolates of lactic acid bacteria. Appl. Environ. Microbiol. 69, 945-952. 

Most polysaccharides used today are of plant origin. However, also bacteria produce polysaccharides. Especially extracellular polysaccharides (eps s) produced by lactic acid bacteria may find application in foods. Lactic acid bacteria are food-grade organisms and the eps s produced offer a wide variety of structures. The presence of eps is considered to contribute greatly to texture and structure of fermented milk products. An exopolysaccharide produced by Lactococcus lactis ssp. cremoris B40 was chosen as a subject of study. The eps was a gift from the Dutch Institute of Dairy Research (NIZO), Ede, the Netherlands. The eps had no gelling properties, could not be precipitated in plates by ethanol or cetylpyridinium chloride and did not show interaction with Congo red. 

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