Emulsions might be a description for paint but the most common type we might encounter in food is milk, butter, cream liqueurs, soups and mayonnaise.
A food emulsion like any other emulsion is a dispersion of two immiscible liquids. These two liquids may be an oil and water. the key feature is that one of those liquids is dispersed in the medium of the other as small spherical droplets. The medium containing the dispersed liquid is the continuous phase.
A key feature of any emulsion is droplet size. For a food the range for optimal stability is between 0.1 and 100 microns in diameter.
If you think about the two simplest types of emulsion, they are either water-in-oil (w/o) or oil-in-water (o/w).
Emulsion Formation
To form an emulsion needs the dispersion of one phase into another. The result is an enormous increase in the interfacial surface area between the dispersed and the continuous phase.
Dispersion is best achieved by homogenization. in this process the dispersed phase is broken into small droplets using a a very high pressure homogenizer. Most operate with a pressure of between 10 and 100MPa.
The large interfacial area is critical for stability as it helps prevent the droplets coalescing. To improve stability requires the use of emulsifiers such as lecithin, starch (modified or otherwise) and gum arabic. Other good examples include stearic acid which is often used with palmitic acid too. Both fatty acids are used as lipophilic thickening agents and so effective for stabilising oil in water emulsions.
Emulsion clouds usually contain orange terpenes and flavourless oils such as vegetable oils and edible waxes.
Flavour Emulsions
Flavour emulsions for beverages usually consist of flavour oils dispersed in water. Generally, a wide range of emulsifiers differing in their chemical nature and properties may be used to stabilise flavour oils in beverage emulsions (Tan, 2004).
A good example of a flavour compound which is highly susceptible to breaking down in citral. It is a component of citrus flavour.
Beverage flavour emulsions offer cloudiness, colour as well as flavour. The alternative is the beverage cloud emulsion which offers only cloudiness but no flavour.
In some cases weighting agents are needed to alter the density of the flavour oil by increase the density of the oil-phase.
Emulsion Instability
Emulsions breakdown into two phases of oil and water if they are disrupted. The cause of the disruption is often an external force such as vigorous mixing or shaking. Using a knife on butter or margarine causes the spread to separate as does chewing in the mouth or by pumping the emulsion about.
Multiple Emulsions
As well as simple emulsions, there are multiple emulsions with the double emulsion being the simplest of these types of more complex structure. The best description is they are “complex polydispersed systems where both oil in water and water in oil emulsion exist simultaneously which are stabilized by lipophillic and hydrophilic surfactants respectively” (Khan et al., 2006). In other words it is an emulsion system where the dispersed phase contains smaller droplets with the same composition as the external phase .
There are two distinct types: water-in-oil-in-water (w/o/w) and oil-in-water-in-oil (o/w/o) type multiple emulsions.
This type of emulsion is finding lots of applications in sustained drug delivery, taste masking, imrpoved enzyme immobilization and microencapsulation.
References
(2004). Beverage emulsions. In: Food Emulsions (edited by S. Friberg, K. Larsson & J. Sjoblom). (4th ed.). New York: Marcel Decker. pp. 485–524
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