Dispersed system

Roles of dispersed system in food

• A dispersion is a system in which particles are dispersed in a continuous phase of a different composition (or state). Each phase can exist in solid, liquid or gaseous state.
• In dispersed phase, main types of colloids usually are sol, gel, emulsion and foam.
• The substance which is dispersed is known as the disperse phase and is suspended in the continuous phase.  Egg white foam is an example of this.  Air bubbles (disperse phase) are trapped in the egg white (continuous phase) resulting in a foam. The mechanical action causes albumen proteins to unfold and form a network, trapping the air.
• Large diameters, combined with large density difference, causing foam bubbles to cream faster than emulsion droplets by several orders of magnitude.
• An emulsion may be oil-in-water (o/w) in which case small oil droplets are dispersed through water, e.g. milk, or water-in-oil (w/o) in which case small water droplets are dispersed through oil, example butter.
• Very small to the naked eyes 
• Affected by both gravitational forces and thermal diffusion

Chemical composition of emulsion in dispersed system in food

Interfacial layer – essential to stabilizing the emulsion

Oil phase – limited effects on the properties of the emulsion.

Aqueous phase – aqueous chemical reactions affect the interface and hence emulsion stability.

• Most colloids are stable, but the two phases may separate over a period of time because of an increase in temperature or by physical force. They may also become unstable when frozen or heated, especially if they contain an emulsion of fat and water.
• Stabilisers are often added to emulsions to increase the viscosity of the product.  These help improve the stability of the emulsion, as over time the emulsion may separate.  Stabilisers also increase shelf life
• Laplace pressure pulls inwards increasing pressure on dispersed phase pressure. 

 Types of Emulsion

Emulsion

• When water and oil are shaken together, they form an emulsion.  This emulsion is unstable.  If left to stand, the oil will form a separate layer on top of the water.
• In foods, the liquid are inevitably oil and an aqueous solution with the aid of emulsifier
• Emulsify agent is made up of two parts which is one is hydrophilic (attract water) and hydrophobic (fear water). The emulsifier holds the dispersed phase within the continuous phase which results in the emulsion becoming stable.
• When emulsifier is added to the system, they arrange on the interface, anchoring the hydrophilic part into water and its lipophilic part into oil.
• Reduces surface tnesion and making them easy to be mixed.

* Viscosity- force required to achieve unit flow rate

Methods to examine the structure of emulsion

• Microscopic examination- applied to small particles (<10ppm), image analysis, calculate ave. diameter of the particles. 
• Laser distraction technique- fast, easy, and fully automated, result reproducible, measurement of dry powder and emulsion samples (1um to 1mm), measures the intensity and angle of light scattered by particles to determine the size. 
• Zatasizer- determine the particle size based on zeta potential (measures the magnitude of repulse and attraction between particles that affect the stability of the medium.) 

There are types of emulsion instability;

1. Creaming – less dense phase rises.
2. Inversion – internal phase becomes external phase.
3. Ostwald ripening – small droplets get bigger.
4. Flocculation – droplets stick together
5. Coalesence – droplets combine into larger ones.

Sols and gels

1. Sols and gels are both liquid loving (lyophilic) colloids.  A sol is a liquid colloid or mixture in which solid particles are dispersed in a liquid phase.  The disperse phase is attracted to molecules of the continuous phase. 
2. Sometimes the mixture needs to be heated and stirred.  When this solution cools, the sol changes into a gel, which resembles a solid rather than a liquid.  Both protein and starch can be used in the formation of a sol or gel.
3. The loss of liquid is known as Syneresis.

 Foams

• Foams are composed of small bubbles of gas (usually air) dispersed in a liquid, e.g. egg white foam.  As liquid egg white is whisked, air bubbles are incorporated.
• Foams are much like o/w emulsions; both are dispersions of a hydrophobic fluid in a hydrophilic liquid.
• It has large quantitative differences, their properties are also qualitatively different. 
• Bubbles diameters are large as to exclude foam bubbles from the realm of colloids.
• Large diameter + large density difference, causing foam bubbles to cream faster than emulsion droplets.
• The relatively high solubility of air in water causes rapid Ostwald ripening (often called disproportionation in foams).
• If the gas phase is CO2, as it is in some foods (bread, carbonated beverages), the solubility is even higher
• If egg white is heated, protein coagulates and moisture is driven off.  This forms a solid foam, e.g. a meringue.  Ice cream, bread, and cake are other examples of solid foams.

3 main types of instability of foam

-Ostwald ripening, which is the diffusion of gas from small to larger bubbles (or to the atmosphere). This occurs because the pressure in a small bubble is greater than in larger ones.

-Drainage of liquid from and through the foam layer, due to gravity..

-Coalescence of bubbles due to the instability of the film between them.

Tutorial 

1. Explain the properties of dispersed system.

 •It is very small to naked eyes.

•It is affected by both gravitational force and thermal diffusion 

•It has large interfacial area. 

2. What are the major components in emulsion system.

The major components in an emulsion system are:

a) Interfacial layer: needed to stabilise the emulsion. 

b) Oil phase: has limited effects on the properties of the emulsion. 

c) Aqueous phase: aqueous chemical reactions affect the interface and the stability of the emulsion.

3. Explain 3 types of emulsion instability with the aid of schematic diagram.

* Creaming – less dense phase rises. 

* Ostwald ripening – small droplets get bigger. 

* Coalesence – droplets combine into larger ones. 

4. Define HLB value.

HLB (Hydrophile-Lipophile Balance) is an empirical expression for the relationship of the hydrophilic ("water-loving") and hydrophobic ("water-hating") groups of a surfactant. The higher the HLB value, the more water-soluble the surfactant. It can be used to identify surfactants for oil and water emulsification. Water-in-oil emulsions (w/o) require low HLB surfactants. Oil-in-water (o/w) emulsions often require higher HLB surfactants.

5. Explain why foam bubbles generally cream faster than the emulsion droplets.

Foams are oil in water emulsion with large bubble diameter that exclude foam bubbles from the realm of colloids. With large diameters and large density difference causes high solubility of air in water causing rapid Ostwald ripening, thus foam bubbles can cream faster than emulsion droplet.