Polysaccharides - Xanthan

11 important questions on Polysaccharides - Xanthan

What are 2 properties that all bacterial polysaccharides have?

  • Organised chemical structure due to presence of repeating units
  • Controlled fermentation process yielding rather standardised products

How are bacterial polysaccharides produced?

By fermentation. Examples are: xanthan, gellan and curdlan.

What is the source of xanthan?

The micro-organism: Xanthamonas campestris

What is the structure of xanthan?

  • Beta-1,4 (cellulose) backbone (stiff)
  • Every second glucose has a side chain (charged) of:
    • mannose --> with acetyl group
    • glucuronic acid
    • mannose --> with pyruvate group

What is the conformation of xanthan influenced by?

  • Temperature
  • Ionic strength
  • pH
  • Primary xanthan structure
    • Acetyl content
    • Pyruvate content

What are the unique flow properties of xanthan?

  • At rest: disordered structure, with yield values
  • Low shear: semi-ordered structure, free flowing
  • High shear: ordered structure, flowing

Independent on pH and T

What is the effect of the solvent conditions  (salt) on xanthan conformation?

At higher salt concentration (10 mM NaCl) you need 70 C to have 100% disordering/unfolding of the xanthan. At lower salt concentration (2 mM NaCl) or water you already have some disordering at 20 C, but more disordering when the temperature rises.

Why is xanthan highly valued in food texturising?

  • Quite soluble
  • Highly stable over wide pH and T range
  • Exhibits highly pseudo plastic flow properties
  • Shows moisture control in baked goods
  • Is a viscosifier alone, but shows strong interactions with galactomannans giving transparent, thermo-reversible gels

What is the source of gellan?

The micro-organism Pseudomonas elodea.

What is the structure of gellan?

It is a linear anionic heteropolysaccharide. It consists of rhamnose - glucose (substituted by acetyl and glyceryl) - glucuronic acid - glucose.

Can you explain why xanthan, being a polymer with a cellulose-like backbone, is highly soluble and has good viscosifying and stabilising properties?

Xanthan has despite having a beta-1,4 backbone, also short side chains that enhance solubility.
The negative charges and the 'apolar' acetyl groups influence the molecular interactions, resulting in unique viscosity properties.

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