Gel - Gel formation
27 important questions on Gel - Gel formation
Which interactions form junction zones?
- Covalent links
Physical interactions
- Hydrogen bonds --> helix formation (gelatin, carrageenan, xanthan)
- Salt bridges --> addition of calcium salts (pectin, alginate)
- Hydrophobic interactions --> between hydrophobic CH3-groups
- (Attractive) electrostatic interactions --> 1 positive + 1 negative
Which mechanisms to induce gelation are used for polysaccharides? And which not?
- Ionic strength
- pH
Not used
- Temperature
- Enzymes
How is gelation induced by temperature?
2-step process
- Heating --> unfolding or dissociation (due to large energy input) --> exposure of reactive sites (hydrophobic groups)
- Association/aggregation of the unfolded/dissociated molecules to form larger 'aggregates'.
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Which proteins give thermo-reversible gels and which not? And which interactions play a role?
Not thermo-reversible --> globular proteins; hydrophobic interactions, disulphide shuffling
How is gelation induced by ionic strength?
- Monovalent ions --> screening of charges, reduction electrostatic repulsion --> attractive van der Waals or hydrophobic interactions
- Multi-valent ions --> additional salt bridges
- proteins --> cold gelation
- polysaccharides --> hydrophobic interactions between CH3 or salt bridges
How is gelation induced by pH changes?
- Reduction of repulsive forces (by pH) leads to aggregation.
- Attractive forces between the molecules (+ and -) --> reduction of forces (by pH) --> less aggregation and gel formation
How is gelation induced by enzymes?
Enzymes can hydrolyse proteins. Cheese --> chymosin enzyme cleaves of the k-casein from the casein micelles. This removal, removes the stabilizing effect (steric layer), resulting in attractive interactions between casein micelles, aggregation and gelation.
What happens when the concentration of polymers is below the minimum concentration?
Which gel types are there?
- Single systems
- Particle gels
- Polymer gels
- Multi-component systems
- Mixed gels
- Filled gels
How are particle gels made?
Which bonds play a role in particle gels?
- Covalent S-S bonds
- Hydrophobic interaction
- Salt bridging
- H-bonds
How are polymer gels made?
What is the difference between an interaction site and a junction zone?
Junction zone --> part of polymer chain that has the ability to make several bonds with another polymer molecule
Which polysaccharides are able to form helix structures? And by what interactions are these helices stabilised?
Hydrogen bonds
Which protein is able to form helix structures?
What kind of shape do polysaccharides need to have to be able to form junction zones?
What is the difference between the concentration needed for particle gels and polymer gels?
Particle gels --> higher weight fraction needed for gelation
How does the type of polymer/polysaccharide determine the gel properties?
- Mobile, flexible --> elastic gel (not too strong gel)
- Less mobile, not very flexible (due to more junction zones) --> strong, brittle gel
What is a type I gel?
What is a type II gel? Which kind of type II gels are there?
- coupled network
- interpenetrating network
- phase separated network
What is a coupled network?
What is an interpenetrating network?
What is a phase separated network?
- For concentrated solutions/dispersions. High amount of molecules present --> depletion interactions --> phase separation of the system
- System containing 2 different polysaccharides --> 1 large, 1 small, both phases should be liquid
- When gelation occurs simultaneously, structure is 'frozen'. Polymer A will form a gel in the continuous phase, polymer B is dispersed phase.
What is the difference between a type I gel and a type II interpenetrating network?
Type II interpenetrating --> both form a gel, no interaction between polymers
What is a filled gel?
Does the enclosing of particles in a gel make the gel stronger?
- Yes
- No --> particles interfere with the aggregation behaviour of the gelling agent
Why is homogenisation of pasteurised milk, the first step in the production of yoghurt?
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