Lecithin emulsions

Table 9.2. Effect of iron and EDTA on the oxidation of an oil-in-water lecithin emulsion prepared with mackerel oil containing long-chain polyunsaturated fatty acids at 40°C=...

FIGURE 15.15 The calculated enc sulation of PFOB in the emulsion system by phase region. (a) DOPA emulsion system demonstrated Regions a and c encapsulate PFOB more than 50%, and (b) lecithin emulsion system mostly encapsulates PFOB except Regions d and f. 

FIGURE 15.17 The cryoTEM images of the lecithin emulsion system for each phase region from a to f. 

Commercial lecithin is a mixture of phos-phatides and glycerides obtained in the manufacture of soya bean oil. It gives a thick yellow emulsion with water, and is widely used in the food and other industries. 

Permeation enhancers are used to improve absorption through the gastric mucosa. Eor example, oral dehvery of insulin (mol wt = 6000) has been reported from a water-in-oH- emulsion containing lecithin, nonesterified fatty acids, cholesterol [57-88-5], and the protease inhibitor aprotinin [9087-70-1] (23). 

Nutritional considerations Contains soy bean oil, egg lecithin, and glycerol. Provides 1.1 kcal/mL of emulsion may need to adjust nutritional regimen. One formulation contains EDTA. Prolonged therapy with the EDTA-containing product may decrease serum zinc levels. May need to monitor serum zinc levels and supplement. 

Emulsifiers. Natural lecithin is one of the most widely used emulsifiers because it is metabolized in the body. However, type I allergic reaction to soybean lecithin emulsified in lipid solutions has been observed [195], Among the synthetic emulsifying agents, block copolymers of polyoxyethylene-polyoxypropylene (poloxamer) have attracted increasing interest for parenteral emulsions. Other examples of emulsifiers commonly found in parenteral formulations are given in Table 9 [190]. 

A further example of radical asymmetric polymerization is the copolymerization between maleic anhydride and styrene (357-359). The reaction takes place in an emulsion in the presence of lecithin and the asymmetric induction... 

Highly concentrated emulsions are also evident in everyday applications. A classic example is mayonnaise, in which a large volume of vegetable oil is emulsified in a small amount of vinegar, using lecithin from egg-yolk as the emulsifier. In addition, HIPEs are most probably found in many cosmetic products, especially gels and creams. However, little information is available on products of commercial importance, so one can only speculate on their exact nature and composition. 

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. 

As noted, the key to successful preparation of an injectable emulsion product proved to be the selection of a suitable commercial source of lecithin, in this case a purified material obtained from the yolk of hen eggs. Lecithin can also be obtained from a number of natural sources such as soy beans and comprises a number of different phospholipids. These include phosphatidylcholine (PC) andphosphatidylethanolamine... 

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. 

Figure 6.17 Microscopy images of 1 wt% soybean oil-in-water emulsion (0.25 wt% modified lecithin, 0 or 0.25 wt% chitosan, pH = 3.0) following heat treatment (30-90 °C, 30 min). The scale bars correspond to 100 pm. Reproduced from Chuah et al. (2009) with permission.

Chuah, A.M., Kuroiwa, T., Kobayashi, I., Nakajima, M. (2009). Effect of chitosan on the stability and properties of modified lecithin stabilized oil-in-water monodisperse emulsion prepared by microchannel emulsification. Food Hydrocolloids, 23, 600-610. 

Dispersed systems, such as emulsions, have also been employed to achieve high drug loading for parenteral administration. Emulsions generally consist of a vegetable oil (e.g., soybean), a phospholipid surfactant (e.g., lecithin), and glycerol added for isotonicity. The surfactant (emulsifier) is necessary to provide a barrier to agglomeration of the emulsion droplets. Unlike micellar solutions that are thermodynamically stable,... 

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