Emulsions - Improving stability of emulsions

6 important questions on Emulsions - Improving stability of emulsions

What are 5 ways to improve the stability of an emulsion?

  1. Increase viscosity of continuous phase or give it a yield stress (slower creaming, slower coalescence) --> adding thickener
  2. Increase repulsion between droplets (electrostractic repulsion: pH, salt, or steric repulsion (polymer addition); slows aggregation, coalescence)
  3. Formation of a strong interfacial film (add proteins: slow coalescence)
  4. Smaller droplets (less creaming, but still coalescence)
  5. Reduce density difference between oil and water (less creaming) --> increase density oil phase, or decrease density aqueous phase

Emulsion stability is a combination of:

  • Emulsion formation, droplet size determined by
    • Low interfacial tension
    • Fast diffusion to interfaces
  • Interfacial thickness needs to prevent coalescence, determined by size of surfactants
    • Diffusion
      • Small proteins --> fast diffusion --> good emulsion formation
      • Large proteins --> slow diffusion --> may prevent formation
    • Thickness of interface
      • Small proteins -> thin interface --> may not prevent coalescence
      • Large proteins --> more space between oil droplets --> prevent coalescence

Effect of pH on droplets

  • Low charge (close to pI)
    • No repulsion between proteins
    • Closer packing at interface
    • Lot of proteins needed
    • No repulsion between oil droplets
    • Flocculation/aggregation
    • Dependent on strength of the interface, interface may also rupture and coalescence may take place
  • High charge (low pH or high pH)
    • More repulsion between proteins
    • Less protein coverage
    • Due to charge of proteins --> electrostatic repulsion between oil droplets
    • Droplets cannot approach each other, less aggregation/flocculation, prevents coalescence and slows down creaming

Electrostatic repulsion causes stabilisation --> electrostatic stabilization.

What does increasing the interfacial thickness lead to?

  • Increase repulsion between droplets (steric repulsion)
  • Stronger interfacial film

Will prevent coalescence, as droplets cannot merge.

How can a the interfacial thickness be increased?

  • Proteins as emulsifier --> aggregated proteins is thicker film
  • Combination of proteins and polysaccharides --> polysaccharides are larger, so thicker interface together with proteins. The PS are often not surface active themselves, so need proteins.

How can the interfacial thickness be increased by protein/polysaccharide co-absorption?

  • Layer-by-layer --> first proteins on interface, then PS added (secondary layer). Attractive interaction needed between proteins and PS. Steric stabilization to prevent coalescence.
  • Preformed protein/polysaccharide mixtures --> proteins and PS missed together before emulsification. Attractive interaction proteins and PS --> complex formation
    • Complexes --> 50-100 nm --> good size to diffuse to the interface and stabilise
    • Coacervates --> >100 nm --> too large to diffuse to the interface and stabilize
    • Then oil added --> complexes/coacervates have to diffuse to the interface and absorb there --> slow diffusion due to large size --> difficult to form an emulsion

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