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Polysaccharides – Xanthan Gum
M.Sc. Biotechnology Part II (Sem III)Paper III - Unit III
Mumbai UniversityBy: Mayur D. Chauhan
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Polysaccharides
• They are long chains of polymeric carbohydrates.
• Monosaccharides are linked together by Glycosidic bonds.
• Homogenous Polysaccharide & Heterogenous Polysaccharide.
• Properties depend upon the monosaccharide units.
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Types of Polysaccharides
Storage
Acidic
Structural
Bacterial
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1. Storage Polysaccharides
• Starch: Starch is glucose polymer in plants in which glucopyranose units are bonded by alpha-linkages. It is made up of a mixture of amylose (15–20%) and amylopectin (80–85%)
• Glycogen: Glycogen is the analogue of starch and is sometimes referred to as animal starch, having a similar structure to amylopectin but more extensively branched and compact than starch. Glycogen is a polymer of α(1→4) glycosidic bonds linked, with α(1→6) linked branches
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2. Structural Polysaccharides
• Arabinoxylans: They are found in both the primary and secondary cell walls of plants and are the copolymers of two pentose sugars: arabinose and xylose.
• Cellulose: Cell wall component of plants. Cellulose is a polymer made with repeated glucose units bonded together by beta-linkages.
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• Chitin: Chitin is one of many naturally occurring polymers and it forms a structural component of many animals, such as exoskeletons. It is closely related to cellulose in that it is a long unbranched chain of glucose derivatives.
• Pectins: Pectins are a family of complex polysaccharides that contain 1,4-linked α-D-galactosyluronic acid residues. They are present in most primary cell walls and in the non-woody parts of terrestrial plants.
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3. Acidic Polysaccharides
• Polysaccharides that contain carboxyl groups, phosphate groups or sulfuric ester groups.
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4. Bacterial Capsular Polysaccharides
• Pathogenic bacteria commonly produce a thick, mucous-like, layer of polysaccharide. This "capsule" cloaks antigenic proteins on the bacterial surface that would otherwise provoke an immune response and thereby lead to the destruction of the bacteria.
• Mixtures of capsular polysaccharides, either conjugated or native are used as vaccines.
• Bacteria and many other microbes, including fungi and algae, often secrete polysaccharides to help them adhere to surfaces and to prevent them from drying out.
• Examples: X. campestris, X. phaseoli, X. malvacearum, X. carotae, X. juglandis etc.
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Xanthan Gum
• Polysaccharide secreted by the bacterium Xanthomonas campestris.
• Xanthomonas campestris was originally isolated from the Rutabaga plant.
• It produces viscid and gummy colonies on agar media.
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X. Campestris grown by using carbohydrate as substrate
X. Campestris grown by using Cabbage extracts as substrate.
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• Commercially produced Xanthan Gum was identical to naturally produced Xanthan Gum.
• Essential requirements during the process of Fermentation: 1. Glucose, Sucrose, Starch, Corn sugar, Acid whey (Cheese production) as Carbon Sources.2. Ammonium chloride, Magnesium sulphate and certain trace elements were required for optimum growth.3. pH should be maintained
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• During fermentation, the pH of the medium decreases due to the formation of metabolic acids.
• Xanthan gum also has acidic functions but if the pH reaches a critical point like 5.0, the gum production decreases drastically.
• A nearly neutral pH allows the gum synthesis to continue until all the carbohydrate substrate gets utilized and the fermentation is complete.
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• Optimum temperature should be around 280 C• Also at 1-5% glucose concentration has been
found to provide best xanthan gum yields.• Aeration and Agitation should be proper
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Structure of Xanthan Gum
• It is either a single or multi stranded helix• It is a high molecular weight natural
polysaccharide.• Molecular weight is 2-20 x 106 Daltons• It is a 5 sugar repeating unit – D-glucose, D-
glucuronic acid, D-mannose and 2 types of carboxyl groups acetate and pyruvate.
• It exists as either a rod like shape or a worm like shape but with low flexibility.
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Protocol
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• Polysaccharide are precipitated by using Isopropyl alcohol or acetone.
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Properties of Xanthan Gum
• 1. Rheological Properties: Most important property of Xanthan gum is to control the rheological properties of fluids.
• Xanthan gum gets dissolved in hot or cold water upon stirring to form a high viscosity even at low gum concentrations.
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• At low concentrations of Xanthan Gum (0.1%), addition of NaCl causes a slight reduction in viscosity.
• At concentrations of Xanthan gum of 0.25% or higher, addition of NaCl causes an increase in viscosity.
• So the higher the Xanthan gum concentration, the higher the concentration of salt required to achieve maximum viscosity.
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• The Viscosity of Xanthan gum in aqueous solutions is Pseudoplastic.
• Pseudoplastic means reduction of viscosity when shear forces are applied.
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2. Effect of Heat on properties of Xanthan Gum
• Normal solutions show a decrease in the viscosity when they are heated.
• Xanthan gum solutions show an increase in the viscosity when they are heated, after an initial decrease.
• Reason is transformation of a double helical structure to a random coil which leads to increase in the volume of the molecule.
• Xanthan gum solutions have excellent thermal stability.
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3. Effect of pH
• The viscosity of Xanthan gum solutions in the presence of a low level of salt, 0.1% NaCl, is independent of pH over the pH range 1.5-13
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Compatibility of Xanthan gum with other solutions
• Dissolves directly in many acid solutions like 5% sulfuric acid, 5% Nitric acid, 5 % acetic acid, 5% acetic acid, 10% Hydrochloric acid, 25% phosphoric acid.
• Solutions remains thermally stable at ambient temperature for several months.
• Also compatible with methanol, ethanol, isopropanol and acetone.
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• Enzymes like protease, cellulase, hemicellulase, pectinase and amylase will not degrade xanthan gum in solution.
• It can be degraded by strong oxidizing agents like peroxides, persulfates and hypochlorites.This degradation is elevated at high temperatures.
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Interaction with Galactomannans
• Xanthan gum can also react with Guar Gum and Locust bean gum.
• The viscosity of combination (Xanthan + Guar) is greater than would be expected from the individual viscosities.
• In the case of locust bean gum, the synergistic viscosity increases at low gum concentration but as the concentration of the gum is increased, a heat reversible gel is formed.
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