Improving stability of foams

Question 1

How to improve the stability of the foams:

Answer 1

1. Dense films (close packing, barrier to coalescence and disproportionation).
2. Strong interfacial films (proteins and polysaccharides, slows coalescence).
3. Increasing viscosity and thickness of aqueous films (thicker film, slower drainage).

Question 2

Formation of dense films. How can be created?

Answer 2

1. Using globular proteins (whey proteins, ovalbumin): when the surface is not completely covered the air can diffuse from one bubble to another one–> Ostwald ripening and coalescence. Has to be high surface coverage.
2. Flexible proteins (caseins/gelatin): has a hydrophilic and hydrophobic side. Additional layer in the aqueous phase (hydrophilic). Stability is determined by the size of the layer. If there is repulsion between chains, chains become more straight. Larger degree of steric repulsion.

Question 3

Effect of gravity in the case of foams?

Answer 3

Drainage
• Very large molecules (protein aggregates, complexes, etc.) drain down through aqueous films due to gravity. There is going to be less surface coverage. These leads to more diffusion of air, lower stability. They also drag the liquid down with it, the amount of water in the foam decreases and a liquid layer is formed–>serum.

Question 4

Strong interfacial films:

Answer 4

Proteins + polysaccharides
• Difficult to add polysaccharide after formation
• Need to be small enough to diffuse to surface
• Solution needs to be liquid to mix in air
→ preformation of complexes, requires electrostatic attraction: complexes small enough to diffuse to the interface or surface. Coacervates, larger, difficult to diffuse to surface, very heavy and drain down.

Question 5

Proteins + polysaccharides: complexes present at the interface, they provide stability due two reasons:

Answer 5

1. Network formation: stronger film (elastic).
2. Increase in size of interfacial layer.
   - ->limited coalescence and disproportionation, minimum air diffusion prevents air from escaping.

Question 6

Viscosity of aqueous phase: Increase thickness and viscosity of aqueous film.

Answer 6

Addition of polysaccharides
• Polysaccharides need to stay in aqueous film (no interaction with the surfactant/protein, requires similar charge)
• Need to be stuck in the aqueous film (to prevent drainage)
CONCENTRATION/ CHARGE DENSITY / WATER HOLDING CAPACITY.
Stability ?
→ Increase in stability
When polysaccharides stay in
aqueous film