UNIT 8 - COLLOIDS

INTRODUCTION

A colloid is a material composed of tiny particles of one substance that are dispersed, but not dissolved, in another substance.
The mixture of the two substances is called a colloidal dispersion or a colloidal system.
All prepared food dishes are examples of a mixture known as a colloid.

There are many types of colloidal systems depending on the state of the two substances mixed together.
Gels, sols, foams (e.g. egg white foam) and emulsions (e.g. butter) are all types of colloids.
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.

Functions of colloidal systems in food products

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.

Sols and gels

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.
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.

When a jelly is made, gelatine is dispersed into a liquid and heated to form a sol.
As the sol cools, protein molecules unwind forming a network that traps water and forms a gel.
 If cornflour is mixed with water and heated, the starch granules absorb water until they rupture, the starch then disperses in the water and the mixture becomes more viscous and forms a gel on cooling.

Other types of gel 

Other types of gel are formed with pectin and agar.
Pectin, a form of carbohydrate found in fruits, is used in the production of jam to help it set.
 However, for it to gel there must be at least 50% sugar and conditions should be acidic.
Agar is a polysaccharide extracted from seaweed which is capable of forming gels.
If a gel is allowed to stand for a time, it starts to ‘weep’.
This loss of liquid is known as syneresis.

Emulsions  

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, e.g. traditional French dressing. The two liquids are immiscible (they will not mix together). A stable emulsion is formed when two immiscible liquids are held stable by a third substance, called an emulsifying agent.
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, e.g. butter.

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.
The mechanical action causes albumen proteins to unfold and form a network, trapping the air.
 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.

Properties of Colloidal Solutions

Following are the important physical properties of colloidal solutions:

1. Heterogeneity: Colloidal solutions are heterogeneous in nature. These consist of two phases-dispersed phase and dispersion medium. 

2. Visibility of dispersed particles: Although colloidal solutions are heterogeneous in nature, yet the dispersed particles present in them are not visible to the naked eye and they appear homogeneous. This is because colloidal particles are too small to be visible to the naked eye. 

3. Filterability: Due to very small size, the colloidal particles pass through an ordinary filter paper. However, they can be retained by animal membranes, cellophane membrane and ultrafilters. 

4. Stability: Lyophilic sols in general and lyophobic sols in the absence of substantial concentrations of electrolytes are quite stable and the dispersed particles present in them do not settle down even on keeping. However, on standing for a long time, a few colloidal particles of comparatively larger size may get sedimented slowly. 

5. Colour: The colour of a colloidal solution depends upon the size of colloidal particles present in it. Larger particles absorb the light of longer wavelength and therefore transmit light of shorter wavelength.