Lecithin in egg yolk

Besides phospholipid composition, the main difference between plant/legume lecithin (e.g., soy) and lecithin in egg yolk is that the former has a higher unsaturated fatty acid content and no cholesterol. Egg lecithin as a commercial ingredient, with the exception of some medical feeding programs, is too expensive for routine use in food (10). In some infant formulas, egg yolk lipids and egg lecithin are used (22). 

With egg lecithin, a different manufacturing process must be used since the lecithin in egg yolks is more tightly bound to proteins than in vegetable sources. Egg lecithin is thus obtained by solvent extraction from liquid egg yolks using acetone or from freeze-dried egg yolks using ethanol (95%). ... 

Milk is an emulsion ofbutterfat droplets in an aqueous mixture of sugars, mineral salts and proteins. Ice cream is another emulsion with a similar composition. The emulsifying agents in both emulsions are proteins with smaller amounts of phospholipids, mainly lecithin. In egg yolk, the emulsifying agents are phospholipids and cholesterol. 

Phosphorus, like nitrogen, is an essential constituent of living matter where it may be partly in combination (as phosphate groups) with organic groups, for example in lecithin and egg yolk, or mainly in inorganic form, as calcium phosphate(V), in bones and teeth. 

Choline occurs widely in nature and, prepared synthetically, it is available as an article of commerce. Soybean lecithin and egg-yolk lecithin have been used as natural sources of choline for supplementing the diet. Other important natural-food sources include Hver and certain legumes (18—22). 

Lecithin is widely used as an emulsifying agent, allowing oil and water to mix. It is used in ice creams, salad dressings, and cosmetics, and it is the main ingredient in nonstick cooking sprays. Lecithin is the emulsifier in egg yolks that allows the oil and water to mix to make mayonnaise. 

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. 

TMA is normally formed from dietary choline and lecithin, but also from TMA N-oxide by intestinal bacteria (Fig. 7.4.2). Choline bound to lecithin is present most abundantly in egg yolk, liver, kidney legumes, soy beans and peas. TMA N-oxide is present in considerable amounts in marine fish, amounting to approximately... 

In initial work, L- a-phosphatidylcholine (lecithin) from egg yolk was selected as the phospholipid, and later studies compared other phospholipids and lipid extracts from meat. As the study originated from investigations of cooked meat flavor, the model system reactions were carried out in aqueous solution buffered with phosphate at an initial pH of 5.7 and concentrations of the reactants were selected to approximate their relative compositions in mammalian muscle. The reactions were carried out under pressure... 

The use of surfactants in the food industry has been known for centuries. Naturally occurring surfactants such as lecithin from egg yolk or soybean and various proteins from milk are used for the preparation of many food products, such as mayonnaise, salad creams, dressing, and desserts. Polar lipids such as monoglycerides have been introduced as emulsifiers for food products. More recently, synthetic surfactants such as sorbitan esters (Spans) and their ethoxylates (Tweens), sucrose esters, have been used in food emulsions. It should be mentioned that the structures of many food emulsions is complex, and in... 

Traditional mayonnaise is an 80% oil-in-water emulsion, which may rely on mustard seeds for solid particle stabilization. Two constituents of egg yolk, lecithin and cholesterol, are surfactants, which promote the formation of oil-in-water and water-in-oil emulsions, respectively. The ratio of lecithin to cholesterol in egg yolk favors the water-in-oil type but the final emulsion type formed is due to the action of mustard seed, which favors an oil-in-water emulsion (Petrowski, 1976). 

Compositional data on commercial egg products and various lipid extracts from egg yolk have been compiled by Gornall and Kuksis (13), Kuksis (11), Schneider (14), and Satirhos et al. (15). Tables 1 and 2 (14, 16) compare the composition of soy and egg lecithins and their fatty acids, and Table 3 (16-21) shows the distribution of phospholipid classes in egg yolk. 

Isolation of Lecithin from Egg-yolk (section 217).— Grind the yolk of one hard-boiled egg with 50 cc. of ether. Filter and wash the solid residue twice with 10 cc. of ether. Evaporate off the ether on the steam-bath, or distil it off from a small flask. Extract the residue twice with hot alcohol, using 10 cc. each time. Pour off the alcohol from the heavy oil through a small filter. Evaporate off the alcohol, dissolve the residue in 10 cc. of cold ether, and add 10 cc. of acetone. Stir until the particles of the precipitated lecithin adhere together and form a ball. Place the latter on a filter-paper. Describe its properties. Boil about one-fourth of the lecithin with about 10 cc. of a 10 per cent solution of sodium hydroxide. Note the odor of the gas evolved. What is it Cool the solution. Is there any evidence of the formation of a soap Filter, dissolve the precipitate in warm water and add dilute hydrochloric acid to the solution. What is precipitated Test a part of the lecithin for nitrogen and for phosphorus ( 58, 59, 61, page 39). 

The naturally occurring anti-inflammatory agent in egg-yolk was later identified as iV-(2-hydroxyethyl)palmitamide , and shown to be identical with a crystalline fraction prepared from arachis oil and soya bean lecithin which is active at 0-3 g/kg in the guinea-pig anaphylactic arthritis test . It is not active, however, in a cotton-pellet test or in a capillary permeability test in rats. When the compound is degraded, anti-allergic activity resides in... 

In addition to the milk proteins, ice cream also contains other surface active molecules, namely emulsifiers, such as mono- and diglycerides or lecithin (from egg yolk). Despite their name, emulsifiers, as we will see in Chapter 4, are actually used in ice cream to de-emulsify some of the fat. 

Lecithin is a natural surfactant and has been used as an emulsifier in foods since the last century. The first application of egg lecithin in an industrial food product was in margarine in 1896. In 1922 soybean lecithin become commercially available and it is still extensively used in margarine together with monoglycerides. Lecithin from egg yolk is the primary emulsifier in mayonnaise. 

Lecithin, (10.47a), is fonnd in egg yolk, brain tissue and skin. The ethanolamine (10.47b) and serine (10.47c) derivatives, known as cephalins, exist as zwitterions in their physiological environment of pH 7. 

Vitellin and vitellenin (Table 12.20) are present in egg yolk as the lipoproteins lipovitellin and lipovitellenin, respectively, the lipid portions of which are mainly the phospholipid lecithin with smaller amounts of cephalin (Chapter 10.3). 

Figure 4.4 NMR spectra quantification of cholesterol in egg yolk lecithin with use of internal standard (I.S.) (a) pure sample (detail) (b) pure sample plus I.S. (c) CH3 signals of the cholesterol reference. Instrument details are given in text. Section 4.10. (Source Spectral Service, unpublished material.)...

Macro- and miniemulsions are thermodynamically unstable. If not stabilized, the droplets tend to fiocculate, coalesce, sediment or cream [2-4]. Other instabilities, such as Ostwald ripening and phase inversion, are also known. At worst, an emulsion will break, i.e. the two phases will separate completely. A product becoming unstable will lose its quality within a short period of time and thus cannot be commercialized. Therefore, even in natural emulsion-based products, amphiphilic molecules are found (e.g. lecithin and proteins in egg yolk and milk and artificial surfactants and emulsifiers in cosmetics and chemical products. They adsorb at the droplets interfaces and stabilize them against flocculation and coalescence. Adsorption and stabilization mechanisms depend on the molecular structure of a surfactant or an emulsifier as depicted in Figure 20.1. Stabilization mechanisms are summarized in... 

Lecithins can act as emulsifying agents. The lecithin of egg yolk, for example, is used to make mayonnaise. Identify the hydrophobic part(s) and the hydrophilic part(s) of a lecithin. Which parts interact with the oils used in making mayonnaise Which parts interact with the water ... 

Nonpolar oils and greases are not soluble in water, but they are attracted to the uncharged ends of the soap or detergent ions. As a result of this attraction, the soap or detergent forms a charged layer around the oil droplets, which keeps them separated and suspended (see > Figure 7.16). Certain compounds (lecithins) of egg yolk act in much the same way to stabilize the mayonnaise-like dressing discussed earlier. 

Lecithin, a common name for PC, is a natural emulsifier found in egg yolk. It is present in bile and helps prevent the formation of gallstones by stabilizing bile cholesterol. In addition, lecithin present in bile excreted into the intestine aids in fat digestion. Lecithin is widely used in foods such as margarines and salad dressings to stabilize the fat components, thus preventing the separation of the fat and water components. In addition, lecithin is used in non-food products such as soaps, paints and cosmetics (Brown, 1990). 

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