Amphoteric Emulsifiers

Lecithin. During the purification of crude oil extracted from soybean, safflower, or corn germ a phosphatide-rich gum is obtained that is treated and purified to give the various commercial lecithin products available to the food processor today (12-14). The dark crude material is bleached to give a more acceptable light brown color. Treatment with up to 1.5% hydrogen peroxide gives the product known as single-bleached lecithin, and further addition of up to 0.5% benzoyl peroxide yields double-bleached lecithin (14). Reaction with even higher levels of hydrogen peroxide plus lactic acid hydroxylates unsaturated fatty acid side chains at the double bond (yielding, eg; dihydroxystearic acid from oleic acid) giving hydroxylated lecithin, which is more dispersible in cold water than the other types and is more effective as an emulsifier for oil-in-water emulsions

(15). Normally some vegetable oil is added to the lecithin to reduce the viscosity to 7.5 to 10 Pa-s; this is called standardized fluid lecithin and contains roughly one-third oil and two-thirds phosphatides.

Figure 8 shows the structure of the main surface active components of lecithin. The phosphatidyl group is a phosphate ester of diglyceride. The fatty acid composition of the diglyceride is similar to that of the basic oil (16) so a number of different fatty acids are found, not just the stearic and oleic acids depicted. There is little phosphatidylserine present in soybean lecithin, and the other three derivatives are found in approximately equal amounts. Phosphatidyl ethanolamine (PE) and phosphatidyl choline (PC) are amphoteric surfactants, whereas phosphatidyl inositol (PI) is anionic. The HLB values of the three species are varied, with PC having a high, PE an intermediate, and PI a low value. The HLB of the natural blend is around 9 to 10, and emulsifier mixtures having this value will tend to form either oil-in-water or water-in-oil emulsions, although neither one is highly stable. On the other hand, intermediate HLB emulsifiers are excellent wetting agents and this is a principal application for lecithin.

The emulsifying properties of lecithin can be improved by ethanol fractionation (15). PC is soluble in ethanol, PI is rather insoluble, and PE is partially soluble. Adding lecithin to ethanol gives a soluble and an insoluble fraction. The phosphatide compositions of the two are (1) ethanol soluble—60%, PC; 30%, PE; 2%, PI; and (2) ethanol insoluble—4%, PC; 29%, PE; 55%, PI (17). The soluble fraction is effective in promoting and stabilizing oil-in-water emulsions, whereas the insoluble portion promotes and stabilizes water-in-oil emulsions. At least one company (Lucas Meyer) is using this process to produce industrial food-grade emulsifiers.

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