1

Polysaccharides – Xanthan Gum

M.Sc. Biotechnology Part II (Sem III)Paper III - Unit III

Mumbai UniversityBy: Mayur D. Chauhan

2

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.

3

Types of Polysaccharides

Storage

Acidic

Structural

Bacterial

4

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

5

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.

6

• 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.

7

3. Acidic Polysaccharides

• Polysaccharides that contain carboxyl groups, phosphate groups or sulfuric ester groups.

8

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.

9

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.

10

X. Campestris grown by using carbohydrate as substrate

X. Campestris grown by using Cabbage extracts as substrate.

11

• 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

12

• 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.

13

• 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

14

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.

15

16

Protocol

17

• Polysaccharide are precipitated by using Isopropyl alcohol or acetone.

18

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.

19

20

• 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.

21

22

• The Viscosity of Xanthan gum in aqueous solutions is Pseudoplastic.

• Pseudoplastic means reduction of viscosity when shear forces are applied.

23

24

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.

25

26

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

27

28

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.

29

• 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.

30

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.

31

32