Physical aspects - Particle and polymer gels

13 important questions on Physical aspects - Particle and polymer gels

What is a gel?

A space-spanning network, enclosing a continuous phase.

What is the symbol for the viscous (liquid) part of a gel?

G", loss modulus.

What are 2 types of gels?

  1. Particles gels
  2. Polymers gels

How are particle gels formed?

By aggregation of particles, often globular proteins.

  • Acid induced (yoghurt)
  • Enzyme induced (cheese)
  • Heat induced (whey, soy, hen egg)
  • Ca2+ induced (cold set gelling of whey protein)

Which types of bonds exist in particle gels?

  • S-S bonds (heat induced aggregation of proteins)
  • Salt bridges (cold set gels)
  • Hydrogen bonds
  • Hydrophobic interactions (globular protein gels)

What is the effect of pH during aggregation on beta-lactoglobulin (from whey protein)?

When the solution is heated, the proteins unfold and will aggregate through attractive hydrophobic interactions. Attractive interactions must be stronger than repulsive interactions (caused by pH).
  • close to pI --> random aggregates (low repulsion)
  • far from pI --> strand-like or even rod-like aggregates (high repulsion)

What is the effect of pH during aggregation of beta-lactoglobulin (from whey protein) on the colour and structure of the gel?

  • Close to pI --> random aggregates --> gels are opaque, more brittle network
  • Far from pI --> fibrillar structures --> gels are more transparent, more elastic network

What are examples of polymer gels in food?

  • Polysaccharides (pectins, alginates, galactomannans, etc.)
  • Gelatin (flexible proteins)

How are hydrogen bonds formed as junction zones in polymer gels?

Between the different groups of gelatin or polysaccharide.

  • Gelatin (triple helix)
  • Xanthan (double helix)
  • Carrageenan

How are salt bridges formed as junction zones in polymer gels?

Salt bridges are divalent ions, and can be present in all types of polysaccharides.
  • Pectins
  • Alginates

How are hydrophobic interactions formed as junction zones in polymer gels?

Most polysaccharides are mainly hydrophilic, so then no hydrophobic interaction. Only for block copolymers in pharma and controlled release applications.

How are covalent bonds formed as junction zones in polymer gels?

Disulphide bridges (S-S). Obtained through a chemical reaction between groups that contain sulphur.

What is the elasticity of a polymer gel dependent on?

The flexibility of the chains.
  • H-bonds --> loose --> high flexibility --> elastic
  • Salt bridges --> many junction zones --> flexibility decreases --> decrease elasticity --> more brittle gel

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