Droplet aggregation stability

The size of the droplets in an emulsion has a strong influence on many of its physicochemical and sensory properties, e.g., shelf life, appearance, texture, and flavor (1,2, 4). For example, the stability of an emulsion to gravitational separation or droplet aggregation can be greatly improved by decreasing the droplet size. This is because the velocity of sedimentation is proportional to the square of the droplet size. The size of the droplets in an emulsion is largely determined by the emulsifier type and concentration, the physicochemical properties of the component phases, and the homogenization conditions (4). A food manufacturer normally specifies a preestablished desirable droplet size distribution for a particular product. If the product does not meet this specification, it typically must be reprocessed or even discarded. 

The coalescence rate, i.e. formation of larger droplets after collision of two droplets, depends on the number of collisions and on the properties of the adsorption layers. For dilute emulsions, as well as emulsions having V2 = 0.3—0.74, coalescence is the main process leading to the disturbance of their aggregative stability. Hence, prerequisite for the production of appreciable volumes of any type of emulsions suitable for practical application is provision of their coalescence stability. In case of o/w emulsions, maximum stability against coalescence is achieved through the formation, on the surface of the disperse phase particles, of structured adsorption layers, a structure-mechanical barrier defined by Rehbinder [8]. Such layers are... 

The main process disturbing the aggregative stability of w/o emulsions is flocculation, i.e. formation of large compact aggregates which rapidly sediment in the gravity field [50, 58]. Two variants are proposed to increase the aggregative stability of w/o emulsions a) use of oil-soluble surfactants and their mixtures with water-soluble surfactants b) introduction of stabilizing additives which ai e able to enhance the electrostatic repulsion of the droplets. 

CkiUoidal interactions govern whether emulsion droplets aggregate or remain as separate entities thereby impacting the characteristics of any aggregates formed (e.g. their size, shape, porosity, and deformability) [3]. The bulk physicochemical properties and stability of many emulsions depend on the extent of droplet aggregation and the characteristics of any aggregates formed. The interactions between two emulsion droplets can be described... 

Emulsion Formation. An oil-in-water emulsion is produced by homogenizing an oil phase and an aqueous phase together (see above). This emulsion is usually stabilized by an emulsifier that adsorbs to the oil-water interface, thereby facilitating droplet formation and retarding droplet aggregation. Typically, this emulsifier is either a small molecule surfactant (e.g. lecithin. Tween, Spans, etc) or a biopolymer (e.g. protein, polysaccharide, or protein-polysaccharide complex). In addition, a material that will... 

Chanasattru, W., Decker, E.A., McClements, D.J., 2009. Influence of glycerol and sorbitol on thermally induced droplet aggregation in oil-in-water emulsions stabilized by p-lactoglobulin. Food HydrocoUoids 23,253-261. 

Structured laundry liquids are currently available in Europe and were recently introduced in the United States [50,51]. These products typically contain high levels of surfactants and builder salts, as well as enzymes and other additives. In the presence of high ionic strength, the combination of certain anionic and nonionic surfactants form lamellar liquid crystals. Under the microscope (electron microscope, freeze fracturing) these appear as round droplets with an onion-like, multilayered structure. Formation of these droplets or sperulites permits the incorporation of high levels of surfactants and builders in a pourable liquid form. Stability of the dispersion is enhanced by the addition of polymers that absorb onto the droplet surface to reduce aggregation. 

GA is well recognized as emulsifier used in essential oil and flavor industries. Randall et al., 1998, reported that the AGP complex is the main component responsible for GA ability to stabilize emulsions, by the association of the AGP amphiphilic protein component with the surface of oil droplets, while the hydrophilic carbohydrate fraction is oriented toward the aqueous phase, preventing aggregation of the droplets by electrostatic repulsion. However, only 1-2% of the gum is absorbed into the oil-water interface and participates in the emulsification thus, over 12% of GA content is required to stabilize emulsions with 20%... 

Similar investigations have been carried out on water in oil microemulsions. A microemulsion is a clear, transparent, and stable system consisting of essentially monodisperse oil in water (OAV) or water in oU (W/O) droplets with diameters generally in the range of 10-200 nm. Microemulsions are transparent because of their small particle size, they are spherical aggregates of oil or water dispersed in the other liquid, and they are stabilized by an interfacial film of one or more surfactants. 

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