Xanthan gum is a polysaccharide with a β-D-glucose backbone like cellulose, but every second glucose unit is attached to a trisaccharide consisting of mannose, glucuronic acid, and mannose. The mannose closest to the backbone has an acetic acid ester on carbon 6, and the mannose at the end of the trisaccharide is linked through carbons 6 and 4 to the second carbon of pyruvic acid (Figure 2.12). The presence of anionic side chains on the xanthan gum molecules enhances hydration and makes xanthan gum soluble in cold water. Xanthan gum is produced by the bacterium Xanthomonas campestris, which is found on cruciferous vegetables such as cabbage and cauliflower. The negatively charged carboxyl groups on the side chains because the molecules to form very viscous fluids when mixed with water. Xanthan gum is used as a thickener for sauces, to prevent ice crystal formation in ice cream, and as a low-calorie substitute for fat. Xanthan gum is frequently mixed with guar gum because the viscosity of the combination is greater than when either one is used alone. The acetylation and pyruvylation levels vary depending on fermentation conditions but typical values. Typically pyruvate residues can be found on 30-40% of the terminal mannose residues whereas 60-70% of the internal mannose residues may contain acetate groups. Recent work has looked at the properties of GM modified strains of xanthan gum that are either deficient in acetate groups, pyruvate groups or both (R.Rosalam and R.England, 2005).
Figure 2.12 The repeating unit of xanthan gum (R.Rosalam and R.England, 2005).
Xanthan gum solutions have ability to form highly viscous solutions at low concentrations and stable viscosity over a wide range of environmental conditions namely ionic strengths, heat and pH as well as enzymes. The effect of salt on viscosity depends on the concentration of the xanthan gum in solution. At low gum concentrations (below 0.3 % w/w), mono-valent salts such as NaCl, it can cause a slight decrease in viscosity. Conversely, NaCl addition at higher gum concentrations increases solution viscosity, the same effects occur with most divalent metals salts. Xanthan has several advantages as a mobility control agent in enhanced oil recovery. It is high pseudoplasticy (shear thinning properties), flocculent, stable to pH and temperature changes and to high salt concentrations, effective lubricant, thermal stability, salt compatibility and allows easy injectability. The special rheological properties of xanthan are technologically suitable for the ‘Enhanced Oil Recovery’ (EOR) applications. At low concentration, the gum forms high viscosity solution that exhibit pseudoplasticity. For the efficient displacement of oil the pumping of xanthan gum solution in the rocks is necessary. As a result oil held in the pores of the sand stone rocks is displaced. Currently, the world wide consumption of xanthan gum is approximately 23 million kg/y, approximately 5 million kg/y are used as a drilling fluid viscosifier in the oil industry. The petrochemical industry uses other plant-derived polysaccharide and synthetic polymers instead of xanthan gum based
on the relative costs of xanthan gum to the other polymers (F.kamal et. al. 2003 and P.adhikary et.al. 2004).
Several researchers have investigated using less expensive carbon sources to produce xanthan gum. S.D. Yoo, et. al. 1999 used waste sugar beet pulp to produce xanthan gum. O.S.Azeez, 2005 demonstrated that cashew tree latex could be used to produce xanthan gum. Lopez and Cormenzana, 1996 investigated the use of olive- mill wastewaters as low cost substrate for xanthan gum production. Variation in the amount of amylose and amylopectin in a starch changes the behaviour of the starch. The amylose component of starch controls the gelling behaviour since gelling is the result of re-association of the linear chain molecules. Amylopectin is usually larger in size. The large size and the branched nature of amylopectin reduce the mobility of the polymer and their orientation in an aqueous environment. Figure 2.13 (a) and (b) shows the structures of the amylose and amylopectin components of a starch molecule. The abundance in hydroxyl groups in the starch molecules impart hydrophilic properties to the polymer and thus its potential to disperse in water. Starch is the second most abundant biomass found in nature, next to cellulose.
Figure 2.13 Building Units of Starch (a) Amylose and (b) Amylopectin
(a)
Xanthan gum is a polysaccharide with a β-D-glucose backbone like cellulose, but every second glucose unit is attached to a trisaccharide consisting of mannose, glucuronic acid, and mannose. The mannose closest to the backbone has an acetic acid ester on carbon 6, and the mannose at the end of the trisaccharide is linked through carbons 6 and 4 to the second carbon of pyruvic acid (Figure 2.14) The presence of anionic side chains on the xanthan gum molecules enhances hydration and makes xanthan gum soluble in cold water.
Xanthan gum is produced by the bacterium Xanthomonas campestris, which is found on cruciferous vegetables such as cabbage and cauliflower. The negatively charged carboxyl groups on the side chains because the molecules to form very viscous fluids when mixed with water. Xanthan gum is used as a thickener for sauces, to prevent ice crystal formation in ice cream, and as a low-calorie substitute for fat. Xanthan gum is frequently mixed with guar gum because the viscosity of the combination is greater than when either one is used alone. The acetylation and pyruvylation levels vary depending on fermentation conditions but typical values. Typically pyruvate residues can be found on 30-40% of the terminal mannose residues whereas 60-70% of the internal mannose residues may contain acetate groups. Recent work has looked at the properties of GM modified strains of xanthan gum that are either deficient in acetate groups, pyruvate groups or both.
Xanthan is produced in its native state as a twin stranded, right handed five fold helix. The stability of the helix is strongly affected by the ionic environment. Upon heating the xanthan helix goes through a transition to a disordered state and upon cooling it reverts to a helical structure. However it is believed that native xanthan exists in a form where chains are paired and once that has been lost and the xanthan molecules allowed to reorder the exact pairing cannot be retained and a partially crosslinked structure is formed as helices twist around various neighbors.
Xanthan gum solutions have ability to form highly viscous solutions at low concentrations and stable viscosity over a wide range of environmental conditions namely ionic strengths, heat and pH as well as enzymes. The effect of salt on viscosity depends on the concentration of the xanthan gum in solution. At low gum concentrations (below 0.3 % w/w), mono-valent salts such as NaCl, it can cause a slight decrease in viscosity. Conversely, NaCl addition at higher gum concentrations increases solution viscosity, the same effects occur with most divalent metals salts.
Xanthan has several advantages as a mobility control agent in enhanced oil recovery. It is high pseudoplasticy (shear thinning properties), flocculent, stable to pH and temperature changes and to high salt concentrations, effective lubricant, thermal stability, salt compatibility and allows easy injectability.
The special rheological properties of xanthan are technologically suitable for the ‘Enhanced Oil Recovery’ (EOR) applications. At low concentration, the gum forms high viscosity solution that exhibit pseudoplasticity. For the efficient displacement of oil the pumping of xanthan gum solution in the rocks is necessary. As a result oil held in the pores of the sand stone rocks is displaced.
The biosynthesis of microbial heteropolysaccharides such as xanthan is a complicated process involving a multi enzyme system. The initial step in the biosynthesis of xanthan is the uptake of carbohydrate, which may occur by active transport or facilitated diffusion. This is followed by phosphorylation of the substrate with a hexokinase enzyme that utilizes adenosine 5’-triphosphate. The biosynthesis
involves conversion of the phosphorylated substrate to the various sugar nucleotides required for assembly of the polysaccharide-repeating unit via enzyme such as UDP- Glc pyrophosphorylase. UDP-glucose, GDP-mannose and UDP-glucuronic acid are necessary for the synthesis of xanthan with the appropriate repeating unit (P.adhikary et.al 2004).
CHAPTER 3