The interaction between oil and water results in high energy at the common surface and hence they are immiscible. One can break up the oil into fine droplets by physical action of mixing and the droplets can be dispersed/distributed into water phase to form a dispersion which can be called as an emulsion. An emulsion is an unstable multi-phase system which contains minimum two immiscible liquid phases.
Once the physical mixing is stopped the oil droplets will coalesce and the oil and water will again separate into two different layers. To stabilize an emulsion, the droplets of the dispersed phase must be as small as possible and as widely distributed as possible in the continuous phase. In order to retard coalescence, the viscosity of the continuous phase must be high.
In order to prevent such coalescing of oil droplets and subsequent layer separation, certain type of chemicals can be used which are known as emulsifiers.
Emulsifiers are made up of the below two molecules:
- A non-polar (fatty acid) end which carries no charge and has an affinity for oil
- A polar (glycerol) end which carries a charge and has an affinity for water
Emulsifier can thus situate itself at the interface oil and water. The polar end will immerse itself in the aqueous phase and the non-polar end will immerse itself in the lipid phase and thus coalescence of the oil droplets will be prevented. This helps the two phases to stay intimately mixed and form a stable emulsion.
Emulsions are characterized in two ways:
- Oil droplets dispersed in the water leading to an oil in water emulsion.
- Water droplets dispersed in the oil, then it is known as a water in oil emulsion. A foam is also a type of an emulsion where a gas is dispersed in a liquid phase or sometimes in a solid phase.
The affinity of emulsifiers for either oil or water is measured by the HLB scale. If the HLB of an emulsifier system is between 3-6, it has an oil affinity and such an emulsifier will optimally stabilize a water in oil emulsion. An emulsifier with a HLB value of 9-18 has a preference for water and thus will optimally stabilize an oil in water emulsion. Hydrophilic–lipophilic balance (HLB) is the balance of the size and strength of the hydrophilic and lipophilic moieties of a surfactant molecule. The HLB scale ranges from 0 to 20.
Examples of emulsifier used in the food industry include:
- DMG: Distilled Monoglyceride
- PGMS: Propylene Glycerol Mono Stearate
- PGE: Polyglycerol esters
- DATEM: Di-Acetyl Tartaric acid Ester of Mono and diglycerides
In our next blog, we will discuss emulsifiers used for cake applications.
