Emulsions_csm

= EMULSIONS =

Food Emulsion

 * Emulsion---consists of 2 immiscible liquids (oil & water), one of the liquid dispersed as small spherical droplets in the other.
 * Water in oil emulsion (w/o)---margerine, butter, spreads.
 * Oil in water emulsion (o/w)---mayonnaise, salad dressing, milk, beverages, cream, soups, sauces.
 * Homogenization---dispersed phase is broken down into small droplets by high pressure homogenizer (10-100MPa).

Mechanism of Emulsion Instability

 * Emulsion stability---ability to resist changes in physiochemical properties with time.

__Mechanism of Emulsion Breakdown__

 * 1) Creaming---the process in which droplets move upwards (droplets densitydensity of continuous phase).
 * 3) Flocculation---the process in which 2 or more droplets “stick” together to form an aggregate (but the droplets still retain their individual integrity).
 * 4) Coalescence---the process in which 2 or more droplets merge together to form a single larger droplet.
 * 5) Phase Inversion---the process in which o/w emulsion changes to w/o emulsion, or vice versa.

Emulsion Stabilization

 * Emulsion stability---achive using emulsifiers (surfactants) and/or stabilizers.
 * Emulsifiers---facilate formation of emulsion by lowering o/w interfacial tension & impart short-term stability by forming a protective film around the droplets.
 * Stabilizers---slightly or not surface active but impart long-term stability to emulsions by restricting interfacial interactions.

__Functions of Emulsifiers__
- Adsorption at Interface
 * Due to amphiphilic nature, adsorb at interface between o/w, form interfacial film, causes reduction of interfacial tension.
 * Reduction of interfacial tension allows emulsion formation with considerably less energy input.



- Liquid Crystal Stabilization

 * Mixture of emulsifier+water form liquid crystals or mesophase structures.
 * Liquid crystalline phases---form on oil droplets surface in o/w emulsions, reduces rate of coalescence, even if flocculation occurs.

- Ionic Stabilization

 * Ionic emulsifiers form an electrically charged double layer in aqueous solution surrounding each oil droplets.
 * Thickness of electrical double layer affected by ionic strength.
 * Droplets remain suspended when ionic strength is low, electrical repulsion>van der waals attraction.
 * With ionic emulsifiers, low salt concentration enhances stability, high salt concentration increases flocculation and/or coalescence.

- Stabilization via Steric Hindrance

 * Hydrocolloids (xanthan, arabic, guar gum, CMC, etc) function as a stabilizer, increases emulsion stability.
 * Macromolecules act by either increasing the viscosity or partitioning into the o/w interface as a physical barrier to coalescence.

- Gravitational Seperation

 * Droplets in an emulsion have a different density to liquid which surrounds thema net gravitational force acts upon them---causes creaming/sedimentation.
 * Droplets in o/w emulsion tends to cream (e.g. milk), those in a w/o emulsion tend to sediment.
 * Creaming rate of an isolated spherical particle can be predicted by Stokes’s equation.

- Methods of controlling Gravitational Seperation
1. Minimize density difference
 * matching the densities of the oil & aqueous phases.
 * density matching---mixing natural oils with brominated vegetable oils (with a higher density than water).

2. Reduce droplet size
 * stability of an emulsion to gravitational separation will increase by reducing the size of droplets (using high pressure homogenizer).



3. Modify rheology of continuous phase
 * increasing the viscosity of continuous phase surrounding the droplets decreases the velocity at which droplet moves.
 * add thickening agent, e.g. hydrocolloids such as arabic gum, xanthan gum, etc.