Stability+of+Food+Emulsions

 **Stability of Food Emulsions **

The stability requirements of different food emulsions vary; both short-term and long-term stability are needed. Short-term stability is usually provided by using small surfactants that reduce surface energy and facilitate emulsion formation. Long-term stability is usually provided by employing macromolecules, such as proteins and polysaccharides.

__1. Increase in total interfacial area __

Reduction in interfacial tension via addition of emulsifiers allows emulsion formation with considerably less energy input than it would required without emulsifier. The disperse phase is broken up into small droplets through homogenization process which results in a massive increase in interfacial area. As a result, molecules tend to migrate from the interface into the bulk phase and reduce the actual contact area between dissimilar molecules.

__2. Electrostatic Stabilization: the DLVO Theory __

The DLVO (Derjaguin-Landau-Verwey-Overbeek) theory of colloid stability proposes a balance of the repulsive electric double layer forces (positive by convention) and the attractive van der Waals’ forces (negative by convention) that exist between all matters. These additive forces maybe expressed as a potential energy versus separation curve.

A positive resultant corresponds to an energy barrier and repulsion



A negative resultant corresponds to attraction and hence aggregation



__3. Fine Particles __

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<span style="font-family: 'Times New Roman',serif; font-size: 13px;">Basic salts, plant cell fragments (spices) can stabilize an emulsion by adsorbing at the interface to form a physical barrier around the emulsion droplets. A balance must be achieved between the interfacial tension of the solid and oil and that of the solid and water. Fine particles are usually used in o/w emulsion such as in salad dressing and mayonnaise.

<span style="font-family: arial,helvetica,sans-serif; font-size: 13px;">__<span style="font-family: 'Times New Roman',serif;">4. Steric stabilization __

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<span style="font-family: arial,helvetica,sans-serif; font-size: 13px;">Macromolecules such as proteins and gums (e.g. xanthan and Arabic gum) form film around an emulsion droplet to establish a physical barrier to coalescence. Such molecules can extend several nanometers into solution and can prevent the droplets from approaching closely by a mixture of mechanical and thermodynamic effects. When a protein is used as an emulsifier agent, it unfolds and re-orients its amino acid chain.

<span style="font-family: arial,helvetica,sans-serif; font-size: 13px;">__<span style="font-family: 'Times New Roman',serif;">5. Liquid crystal stabilization __

<span style="font-family: arial,helvetica,sans-serif; font-size: 13px;"><span style="font-family: 'Times New Roman',serif;">Micelles formation occurs when the excess molecules are positioned face to face gather and there is no change in the surface tension.

<span style="font-family: 'Times New Roman',serif; font-size: 13px;">The concentration to start micelle formation is called critical micelle concentration (cmc) and the properties of the solution change greatly with a change of this concentration.

<span style="font-family: 'Times New Roman',serif; font-size: 13px;">When the conc. exceeds cmc, spherical micelles (a) appear at first and disperse into water. Further increase in the conc. cause rod-shape micelles (b). Finally lamellar micelles with higher structures called liquid crystal are produced.

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<span style="font-family: 'Times New Roman',serif; font-size: 13px;">Under certain conditions, emulsifier can form a protecting multilayer around the emulsion droplets. The sudden increase in emulsion stability with enhanced emulsifier conc. a liquid crystalline phase could be separated from the emulsion. It has been shown that the presence of a liquid crystalline state reduces the rate of coalescence, even if the droplet flocculation occurs.

<span style="font-family: 'Times New Roman',serif; font-size: 13px;">__6. Stabilization against Gravitational Separation__

<span style="font-family: arial,helvetica,sans-serif; font-size: 13px;">**<span style="font-family: 'Times New Roman',serif;">Minimize density difference **<span style="font-family: 'Times New Roman',serif;">—it is possible to prevent gravitational separation by “matching” the densities of the oil and aqueous phases. This is commonly used for stabilizing beverage emulsion. Density matching can be achieved by mixing natural oils with brominated vegetable oils, so that the overall density of the oil droplets is similar to that of aqueous phase.

<span style="font-family: arial,helvetica,sans-serif; font-size: 13px;">**<span style="font-family: 'Times New Roman',serif;">Reduce droplet size **<span style="font-family: 'Times New Roman',serif;">—can enhance the stability against gravitational separation. For example, homogenization of raw milk retards the creaming in food emulsion by droplet size reduction.

<span style="font-family: arial,helvetica,sans-serif; font-size: 13px;">**<span style="font-family: 'Times New Roman',serif;">Modify rheology of continuous phase **<span style="font-family: 'Times New Roman',serif;">—Increasing the viscosity of the liquid surrounding a droplet decreases the velocity at which the droplet moves (Stoke’s law). E.g. by adding a thickening agent such as Arabic gum, xanthan gum, etc <span style="font-family: 'Times New Roman',serif;">