Particle and polymer gels

Question 1

What is a gel?

Answer 1

*Gels consist of a SPACE-SPANNING NETWORK, enclosing a continuous
phase.
*Gel is an intermediate state between a liquid-like rheological behavior and a solid-like rheological behaviour.
- Liquid like properties originate from the fact that the major component is a solvent (water)
- Solid like properties are due to the presence of a network that prevents the system from flowing
Properties are between elastic and viscous material

Question 2

Rheological definition: tan delta:

Answer 2

G´´( Loss modulus–>viscous part: liquid) / G´ (Storage modulus–>elastic part: solid).
G´´>G´–> more liquid like viscous behaviour. LARGE DEFORMATIONS.
G´>G´´–> more solid like elastic behaviour. SMALL DEFORMATIONS.
G´=G´´ becomes a gel.

Question 3

Two types of gel

Answer 3

1. Particle gel.

2. Polymer gel.

Question 4

1. Particle gel, explain everything you know about this gel.

Answer 4

Formed by aggregation of particles (proteins: globular). Examples:
• Acid induced aggregation of casein micelles (yoghurt)
• Enzyme induced aggregation of casein micelles (cheese)
• Heat induced aggregation of globular proteins (whey, soy, hen egg)
• Ca2+ induced aggregation of globular proteins (cold set gelling of whey protein aggregates).
A) in solution B) less repulsion and more attractive interactions C) aggregation

Question 5

To obtain a PARTICLE GEL network, bonds need to be formed between the particles, which ones?

Answer 5

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

Question 6

Structure of the network depends on: pH, salt concentration, T. PARTICLE GLES

Answer 6

1. Effect of pH during aggregation: (B-lactoglobulin), the revulsive interaction are of electrostatic nature and depend on pH:
   • close to IEP: low charge, random aggregates (low electrostatic repulsion). GELS ARE OPAQUE AND BRITTLE
   • far from IEP: higher charge, more strand-like or even rod-like aggregates (high electrostatic repulsion) –>fibrillar structures. GELS ARE TRASNPARENT AND ELASTIC

Question 7

1. Polymer gel, explain everything you know about this gel.

Answer 7

Consisting of polymer chains held together by entanglements or junction zones.
Examples of polymer gels in food:
• most polysaccharides (pectins, alginates, galactomannans, …)
• gelatin (flexible proteins)

Question 8

POLYMER GELS: the types of junction zones are due to specific attractive interactions:

Answer 8

1. H-bonds:
• Gelatin (triple helix)
• Xanthan (double helix)
• Carrageenan
2. Salt bridges (divalent ions):
• pectins
• alginates
3. Hydrophobic interactions: block copolymers in pharma
and controlled release applications. Not often present.
4. Covalent bonds: S-S bonds

Question 9

POLYMER GELS: the network strength is mostly dependent on the amount of junction zones present in gel. Example: pectin? Elastic /brittle

Answer 9

Pectin: Salt ions in each position, there are many junction zones. Gels are much stronger, not very flexible (more brittle), but when are loosely interacting, the flexibility will be high and therefore the network will be elastic.