Lesson 4. EMULSIONS

Module 2. Colloidal chemistry

Lesson 4
EMULSIONS

4.1 Introduction

The characteristic and most useful feature of milk is the presence of a very important constituent namely the fat. This constituent being hydrophobic has to exist in emulsion form. Knowledge about the behavior and characteristics of an emulsion will help in effective and economic utilization of this component in the preparation of different milk and food products.

4.2 Definition

An 'emulsion' is a system in which the dispersion medium and dispersed phase are liquids. For an emulsion to be stable, the two liquids must practically be insoluble in each other or only slightly soluble. One liquid (the dispersed phase) is dispersed in the other (the continuous phase). Many emulsions are oil/water emulsions, with dietary fats being one common type of emulsion encountered in everyday life. Examples of emulsions include butter and margarine, milk and cream. In butter and margarine, fat surrounds droplets of water (a water-in-oil emulsion). In milk and cream, water surrounds droplets of fat (an oil-in-water emulsion). Emulsification is the process by which emulsions are prepared.

The two liquids that involve in the formation of emulsion must be insoluble in each other. The formation of the interface always requires work to be done and as such greater is the interfacial tension, greater is the amount of work required. Addition of emulsifying agents will reduce the inter facial tension by being adsorbed on the inter phase. Due to the stabilizing effect of these emulsifiers, substances with much larger size than that of ordinary colloids can also be stabilized.

4.3 Properties of Emulsions

Several factors influence the properties of emulsions. Some of the important factors are

  • The continuous phase
  • Temperature
  • Average droplet size
  • Distribution of the droplets
  • Volume fraction of the oil dispersed in water
  • The oil that is dispersed
  • Various additives which help in the stability of the emulsion

The two important properties of emulsions are the stability against creaming, and the flow properties, in other words their rheology. Stability against creaming relates to shelf life of commercial products. In food, the rheological property contributes to the mouth feel apart from the ease with which the product pours out of the containers.

4.4 Types of Emulsions

There are two types of emulsions. A system consisting of water and non-polar organic liquid, is known as oil- in- water and is denoted by O/W. The other type of emulsion is water in-oil type of emulsion and is denoted as W/O. In oil-in-water emulsion the lipid droplets are dispersed in water whereas in water in oil emulsion, the water droplets are dispersed in continuous oil phase. Types_of_emulsions.swf

Food emulsions may possess solid particles (coagulated protein clumps) and gas bubbles in addition to the two liquid phases. Example for food emulsions are Butter (W/O), margarine (W/O), mayonnaise (O/W), salad dressing (O/W), milk (O/W), cream (O/W), non dairy creamer (O/W) and ice cream (O/W).

4.5 Functional Attributes of Food Emulsions

Acceptability of food emulsions is dependent on attributes such as appearance (colour and opacity), texture (viscosity, plasticity, and oiliness) and flavor. Certain features of food emulsions prove to be advantageous for modifying some these attributes.

It is possible to incorporate oil, soluble flavoring and coloring matter or vitamins in an oil- in- water emulsion. Similarly by dispersing droplets of particles with a diameter of 0.05 to 1.0µm into the continuous phase, it is possible to create opacity in the fluid product. By increasing the concentration of the dispersed liquid to about 60% or more it is possible to introduce the oil into an emulsion without imparting any oily sensation e.g. mayonnaise.

4.6 Physical Properties of Emulsions

The physical properties of an emulsion are discussed here under

4.6.1 Droplet size distribution

In coarse low concentration of emulsions, with differences in their densities of the dispersed and dispersion phases, the phases either tend to rise to the surface or settle at the bottom of the vessel. In highly concentrated emulsions the dispersion medium remains as a thin film between the particles of the emulsion. In such systems, it is the strength of this film that prevents the coalescence of the particles. The geometrical form of the particles in highly concentrated emulsions may not be spherical in shape.

4.6.2 Optical properties

The light scattering property of the emulsion systems is the greatest for the shorter wave lengths i.e. for the blue and violet rays. Light scattering property is effectively studied by using the spectrophotometer.

4.6.3 Rheology

The rheological properties of emulsions play an important role in the practical utility of the emulsions. This property helps in removing the product from the containers. The pouring out property of the emulsions depends on the size distribution of the suspended particles. Butter being an emulsion of water- in- oil type has to be modified to get a product, butter spread by modifying its rheological property.


Last modified: Wednesday, 14 November 2012, 4:56 AM