Lecithins emulsion stability

Stability in plasma is an important requirement for IV emulsions as flocculated droplets may result in lung embolism. It was found that tocol-based emulsions stabilized by sodium deoxycholate/lecithins flocculated strongly when mixed with mouse, rat, and sheep plasma and serum, whereas soya oil-based emulsions with the same emulsifiers did not [123], It was hypothesized that this effect was caused by the adsorption of plasma proteins onto the tocol droplets (opsonization). Indeed, the steric stabilization of emulsions by incorporation of emulsifiers like poloxamer 188 or PEGylated phospholipids such as PEG5000PE proved to be effective in the stabilization of tocol-based emulsions in plasma. Conversely, in vitro studies were... 

Swietlikowska, D. W., and Sznitowska, M. (2006), Partitioning of parabens between phases of submicron emulsions stabilized with egg lecithin, Int. J. Pharm., 312, 174-178. 

If the relationship between the film thicknesses of the three surfactant system (fig.11) and potential emulsion stability is considered, perhaps the most significant feature is the widening of region D (the unstable film region) with increasing lecithin mol fraction. This means that lecithin has a destabilising effect and it would be predicted (bearing in mind the instability of the... 

Lecithin use in low fat spreads may decrease the emulsion stability and increase the tendency to oil off however, it also functions to slow the emulsion breakdown in the mouth. Therefore, the use of lecithin and the level of use must be evaluated for each formulation. 

The manner in which lecithin is modified to achieve increased hydrophilicity will greatly affect its emulsification properties. Different modifications will create lecithin products with different apparent HLB (hydrophile-lipophile balance) values, a term used to convey the approximate degree of water dispersibility (hydrophilicity) of lecithin products (31). The higher its HLB value, the more water dispersible the lecithin product. In o/w emulsions, the type of fat to be emulsified may require a specific type of hydrophilic lecithin for optimum emulsion stability. Dashiell (31) provides a short listing of fat types, and the corresponding class of lecithin found to give the most stable emulsion in model systems of water/fat/ emulsifier. 

Milk replacer fat emulsion. One important application of lecithin in animal feeds is as an emulsifier (314). As an example, milk replacer fat is emulsified to improve product wetting and dispersion upon mixing, to help minimize fat separation (i.e., emulsion stability), and to significantly reduce fat particle size, allowing improved fat digestibility. As an emulsifier in milk replacers, lecithin is used at 3-12% of the added fat. The type of lecithin used is affected by factors such as fat type, protein type, and the presence of stabilizers. 

Lipid emulsion formulations of prostaglandin Ej have been used in the treatment of vascular disorders. They exhibit a reduced incidence of side-effects at the site of injection, as does the diazepam lipid emulsion. A soybean emulsion stabilized by egg-yolk lecithin releases the PGEj over a period of 4-16 hours depending on pH (Fig. 7.24). The partition coefficients of PCEj between soybean emulsion and aqueous buffers at 20°C are shovm in Fig. 7.25, explaining why... 

Dzygiel, P. and Wieezorek, P. (2000). Extraction of amino acids with emulsion liquid membranes using industrial surfactants and lecithin as stabilizers. J. Membr. Sci., 172, 223-32. 

Garcia-Fuentes et al. described a modified method based on a w/o/w multiple emulsion technique to encapsulate hydrophilic macromolecules in the SLN matrix. The aqueous drug solution is dispersed by e.g. probe sonication in a 1 10 volume ratio in the water-immiscible organic solvent (e.g. dichloromethane) containing the matrix lipid and lecithin as stabilizer. To this w/o emulsion different volumes of an aqueous phase containing e.g. poloxamer as emulsifier is added and double w/o/w multiple emulsions are prepared by further sonication. Upon evaporation of the organic solvent, the lipid nanoparticles are formed. 

Process catalysed by phospholipases. Soybean lecithin is a widely used natural emulsifier. Compared to synthetic emulsifiers, lecithin is inferior with respect to its emulsion stabilization potency. One of the ways to improve the stabilization properties of lecithin is to convert it to lysolecithin. By the loss of one of the fatty acids on the molecule, the hydrophilicity of the compound is generally improved, thus enhancing the stabilization of oil in water (OAV) types of emulsions. Such a transformation is performed industrially using phospholipases (Yamane, 1991). 

W/O/W emulsions stabilized with soy lecithin-Span 80 mixtures have been used as the basis for the preparation of phospholipid vesicles [158]. A water-in-n-hexane emulsion was first prepared and the bulk of the hexane removed, the concentrate being dispersed in aqueous solution using a low concentration of hydrophilic surfactant which itself could then be removed leaving the phospholipid vesicles. 

Glycerophospholipids are important components of biological membranes and are thus widely spread in nature. Partially purified products are used for a variety of applications, with soya lecithin as a typical example. Enzymes can be used to modify glycerophospholipids in various ways and in the surfactant area removal of one of the fatty acids to make lysophospholipids is the most important example. Sometimes this reaction is carried out only to make it easier to remove the phospholipids fraction from the neutral fat, such as in the processing of vegetable oils. This enzymatic de-gumming is an important industrial process [20]. In other applications, lysophospholipids are produced in order to improve the emulsifying properties of the lipids. One such example is in the preparation of mayonnaise, with improved emulsion stability [21]. In this application, phospholipase A2 is used selectively to remove the fatty acid in the sn-2 position. 

Emulsion stability. Protein or other ingredients are incorporated in the lecithin/oil/water system and the stability of the emulsion (fat creaming, protein sedimentation) is measured over a period of time. When preparing an oil-in-water emulsion, the modified lecithins with enhanced hydrophilic properties usually give a specific stability performance. 

Lecithins, found in egg yolk, are natural products biocompatible with blood. The storage stability of emulsions lecithin has formed with some fluorochemicals is considerable [78,113]. Lecithin-PFOB emulsions, once sterilized, can be stored at room temperature for several months [53,78]. It has been speculated that the bromine atom of PFOB contributes to emulsion stabilization by lecithin. Mukherji and Sloviter [114] prepared stable emulsion of perfluorodecalin with egg-yolk lecithin by sonication in Tyrode s buffer (pH 7.4). The viscosity of the emulsion did not change when stored for 60 weeks at 5°C. At 21 C, viscosity increased after 20 weeks of storage. The viscosity increase was related to slow oxidation of lecithin on storage of the emulsion at either 5°C or 21 C. 

Another associated issue was the possibility of inactivating the LRES (lym-phoreticuloendothelial system). By analogy with other injectable systems, it could also be deduced that the injectable emulsion system needed to be sterile and apy-rogenic and free of acute or chronic toxicities from components or their associated degradation products. It also followed that the injectable system required to be stable, although how stability was to be determined and, more to the point, measured, has remained an issue to the present day. This is mainly because emulsions are thermodynamically unstable although their stability can be extended by formulation. As a result emulsion products are now available that are submicron in diameter, sterile, and stable for several years after preparation. In major part this has been due to the use of phospholipids as stabilizers and emulsifiers, in particular the mixed products identified as the lecithin of commerce. 

Approximate Constitution of Purified Lecithin Used to Stabilize Injectable Emulsions... 

PE), usually present in the ratio of approximately 3 1 and making up about 90% of the total weight of the lecithin phospholipids (Table 9.1). It is known that the two main phospholipids account for most of the stabilization and emulsification activity of the lecithin, but it is thought that minor components such as sphingomyelin and phosphatidic acid also play some as yet undefined role in the process. It might be emphasized here that the natural mixture of components is more effective at stabilizing emulsions than any of the major components in either purified or synthetic form, alone or in artificial admixtures. 

Ogawa, S., Decker, E.A., McClements, D.J. (2003). Influence of environmental conditions on the stability of oil-in-water emulsions containing droplets stabilized by lecithin-chitosan membranes. Journal of Agricultural and Food Chemistry, 51, 5522-5527. 

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