Microbial polysaccharides

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Microbial polysaccharides
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During the Second World War (1940), the
usefulness of microbial polysaccharides was
recognized with the discovery of dextran
polysaccharides, which found application as a
blood plasma extender. It was after two decades
i.e. in the year 1960 that another useful
microbial polysaccharide, xanthan gum, was
developed as one of the most widely used
industrial polysaccharides, by the group of
scientists headed by Miss A. Jeans of Kelco
Company in the USA.
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Xanthan Gum

• A polysaccharide
• Used as food additive rheology modifier.
• Production involves fermentation of glucose or
  sucrose by the bacterium, Xanthomonas campestris

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STRUCTURE

• M.W. varies from 1 million - 50 million
  depending upon preparation
• Backbone
• 2 ß-D-glucose units linked through the 1 and 4
  positions.
• Side chain
• 2 mannose units 1 glucuronic acid unit
• Linked to alternate glucose of the backbone at
  the 3 position.
• In all, thus, the chain has repeating modules of
  five sugar units.

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• 40-50 of terminal mannose units have a pyruvic
  acid group linked as a ketal to its 4 6
  positions.
• The other mannose unit has an acetyl group at the
  6 position.
• 2 of these chains aligns to form a double helix
• A rigid rod configuration
• High efficiency as a viscosifier of water

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Structure of Xanthan
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Preparation

• Inoculating a sterile aqueous solution of
  carbohydrate(s), a N2 source, dipotassium
  monohydrogen phosphate some trace elements
• Medium is well-aerated stirred
• Xanthan is produced extracellularly in the medium
• Final concentration of xanthan 3-5 by weight
• Fermentation for 4 days
• Precipitation of Xanthan from the medium by
  addition of isopropyl alcohol
• Dried and milled to give a powder that is readily
  soluble in water or brine.

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Applications

• Even 1 can greatly increase in the viscosity of
  a liquid
• In foods, used at 0.05 0.5.
• Viscosity decreases with high shear rates
  (mixing, shaking, chewing) Pseudoplasticity
• In salad dressings sauces
• Thicker at rest in the bottle to keep the mixture
  homogeneous
• Shear forces due to shaking thins it, so, easily
  poured
• After exiting the bottle, no shear force. So,
  thickens back clings to salad.
• Stable under a wide range of temperatures and pH

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• Helps to stabilize the colloidal oil and solid
  components against creaming.
• In frozen foods and beverages, creates the
  pleasant texture
• In toothpaste, serves as a binder to keep the
  product uniform
• In gluten-free baking, to give the dough / batter
  a "stickiness"
• Gluten in wheat to be omitted
• For thickening liquids for those with swallowing
  disorders, since it does not change the color or
  flavor of foods or beverages.

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• In the oil industry, to thicken drilling fluids
• Fluids serve to carry the solids cut by
  horizontal drilling bit back to the surface
• Also been added to concrete poured underwater, in
  order to increase its viscosity and prevent
  washout.
• In cosmetics, to prepare water gels usually in
  conjunction with bentonite clays.
• In oil-in-water emulsions to help stabilise the
  oil droplets against coelesence
• Has some skin hydrating properties.