Microbial polysaccharides

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.

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.