Microbial polysaccharides
Xanthan Gum
• A polysaccharide
• Used as food additive & rheology modifier.
• Production involves fermentation of
glucose or sucrose by the bacterium,
Xanthomonas campestris
STRUCTURE
• M.W. varies from ~ 1 million - 50 million
depending upon preparation
• Backbone:
– 2 β-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.
• 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
Structure of Xanthan
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.
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
• 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.
• 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.