Emulsifying Agent is a substance that helps an emulsion become more stable. They are added to an emulsion to prevent the coalescence of the globules of the dispersed phase. An emulsion is usually a mixture of two products such as oil and water that do not mix together or that are also referred to as immiscible. By adding an emulsifying agent to the mixture, they cause the oil to be broken down into smaller pieces that can then be dispersed throughout then water.
This is then what becomes known as an emulsion. They are also known as emulgents or emulsifiers. They act by reducing the interfacial tension between the two phases and forming a stable interfacial film. The choice of selection of emulsifying agent plays a very important role in the formulation of a stable emulsion. Substances that are soluble in both fat and water and enable fat to be uniformly dispersed in water as an emulsion.
Emulsions are stabilized by adding an emulsifier or emulsifying agents. These agents have both a hydrophilic and a lipophilic part in their chemical structure. All emulsifying agents concentrate at and are adsorbed onto the oil-water interface to provide a protective barrier around the dispersed droplets. In addition to this protective barrier, emulsifiers stabilize the emulsion by reducing the interfacial tension of the system. Some agents enhance stability by imparting a charge on the droplet surface thus reducing the physical contact between the droplets and decreasing the potential for coalescence. Some commonly used emulsifying agents include tragacanth, sodium lauryl sulfate, sodium dioctyl sulfosuccinate, and polymers known as the Spans® and Tweens®.
Types of Emulsifying Agents:
Emulsifying agents can be classified according to: 1) chemical structure; or 2) mechanism of action. Classes according to chemical structure are synthetic, natural, finely dispersed solids, and auxiliary agents. Classes according to mechanism of action are mono-molecular, multi-molecular, and solid particle films. Regardless of their classification, all emulsifying agents must be chemically stable in the system, inert and chemically non-reactive with other emulsion components, and nontoxic and nonirritant. They should also be reasonably odorless and not cost prohibitive.
They may be classified as follows, the basis being upon the type of emulsifier: (1) those stabilized by an electric charge, (2) those stabilized by colloids, and (3) those stabilized by powders.
- Stabilization of emulsion by an electric charge. Mineral oil emulsions in which the oil is present in very small amounts belong in the group stabilized by an electric charge. The oil particles are negatively charged. Hydrophilic sols are stabilized by hydration and an electric charge. Ghosh and Dhār suggest that emulsions are similar to sols and that the stability, the separation or coagulation, and the reversal of the emulsion are markedly influenced by its electric charge.
- Emulsions stabilized by colloids. So many of the emulsifiers are colloidal in nature that this group is the most important in food preparation. The following are commonly used: eggs, gelatin, flour, starch, and milk. Gum arabic, gum tragacanth, Irish moss, and other substances are used less frequently. Pectin is being used to some extent. Gum arabic and gum tragacanth are used principally in cookery of diabetic foods, the Irish moss in puddings.
- Stabilization by powders. Finely ground particles or powders such as lampblack, mustard, and paprika are a third class of substances used for emulsifying agents. The best examples of this type of emulsifier in prepared foods are the French dressings. At the present time several brands are on the market that are fairly stable. All are deep red in color, so that the emulsion must be partially and probably wholly stabilized with particles of paprika and mustard. This kind of emulsion is formed by the powder film around the drops of oil which keeps them from coalescing. Clayton reports the results of using finely divided solids as emulsifiers by Bechhold, Dede, and Reiner. ‘They found that the formation of emulsion depends upon: (1) the grain size of the powder. The smaller the grain the better the emulsion, until an optimum is reached, after which smaller grains have inferior emulsifying properties. (2) The quantity of powder. The more powder there is available the more globules there can be covered, providing the powder is sufficiently fine.” They report that zinc dust, iron powder, clay, Kieselguhr, and yeast made very efficient emulsifiers.
Working Mechanism of Emulsifying agents:
A number of different chemical and physical processes and mechanisms can be involved in the process of emulsification.
- Surface tension theory – according to this theory, emulsification takes place by reduction of interfacial tension between two phases.
- Repulsion theory – the emulsifying agent creates a film over one phase that forms globules, which repel each other. This repulsive force causes them to remain suspended in the dispersion medium.
- Viscosity modification – emulgents like acacia and tragacanth, which are hydrocolloids, as well as PEG (or polyethylene glycol), glycerin, and other polymers like CMC (carboxy methyl cellulose), all increase the viscosity of the medium, which helps create and maintain the suspension of globules of dispersed phase.
A theory to explain the formation of oil-in-water and water-in-oil Emulsions. It is found experimentally that alkali- metal soaps when used as emulsifying agents tend to give oil-in-water emulsions while heavy metal soaps give water-in-oil emulsions.
“According to Bancroft the emulsifying film may be considered as three molecules thick, consisting of an oil molecule, an emulsifying agent molecule and a water molecule.”
When the interfacial tension at the water emulsifying agent interface is less than at the oil-emulsifying agent interface, the film tends to bend and becomes convex on the water side forming an oil-in-water emulsions, and vice versa. Harkins proposed an “oriented wedge” theory of emulsions. This stated that for soaps such as the monovalent soaps for which the cross-section of the metal ion is smaller than the paraffin chain, the emulsion would be of the oil-in-water type so as to form an interfacial film of the greatest density, and vice versa. There are however a number of exceptions to this theory.
When solid powders are used as emulsifying agents, if the solid is preferentially wetted by one phase, then since the powder tends to be taken up at the interface, more particles can peak into the interface if this is curved and convex to that phase. Thus if the powder is wetted more strongly by water then oil-in-water emulsions will tend to be formed. This may be regarded as an extension of the Bancroft Theory.