Understanding the Structure of Lecithin

Published: 22nd September 2009
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There are actually two meanings to the term "lecithin." Commercially, the structure of lecithin consists of a mixture of neural and polar lipids that act as emulsifiers and/or lubricants. In biochemistry, it refers to phosphatidylcholine, a phospholipid composed of glycerol, two fatty acids, a phosphate group and choline.

Lecithin was first discovered in 1846 when French scientist Maurice Gobley separated the nutrient from egg yolk. In 1850, he gave it the name lekithos, which is Greek for "egg yolk." But by the 1930s, majority of the lecithin sold commercially was derived from soybean oil. The nutrient was found as a by product of the degumming process of soybean oils.

The Structure of Lecithin: Egg Yolk
There are differences in the structure of lecithin derived from egg yolk and those derived from plants, such as soy lecithin. First, the structure of lecithin derived from egg yolk is essentially composed of triglycerides and phospholipids, which differs considerably from plant lipid mixtures.

Egg yolk lecithin contains long chain polyunsaturated fatty acids known as LC-PUFAs or Arachidonic Acid and Docosahexaenoic Acid. Both are bound phospholipids, the former occurring exclusively in cell membranes while the latter is found in high amounts in optic nerve membranes.

In the structure of lecithin derived from egg yolk, arachidonic acid is involved in signal transduction and ensures high membrane fluidity. It is important for body weight development. On the other hand, docosahexaenoic acid is required for normal development of the brain, nervous system, and visual acuity.

Consequently, both these important components of the structure of lecithin play a vital role during pregnancy and infancy. Since babies have only limited capacities, they need arachidonic acid and docosahexaenoic acid to synthesize fatty acids from their precursors.

Structure of Lecithin: Soy
Markedly different from egg yolk lecithin, the structure of lecithin derived from soy actually consists of three types of phospholipids: phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylinositol (PI). Since soy lecithin is obtained in the process of degumming crude soy oil, it contains minimal amounts of hydratable compounds (1.8%), comprised primarily of phosphatides. After going through the synthesizing process, the resulting structure of lecithin is 30-35% crude soy oil and 65-70% phosphatides. Oil can then also be removed completely with the addition of acetone.

In the structure of lecithin derived from soy, the phosphatides consist of glycerides, which are the basic components of soy oil. However, instead of one fatty acid radical, this has been replaced with phosphoric acid. Now, this is different in the case of pure or chemical lecithin or phosphatidylcholine, where the phosphoric acid is further esterified with choline. But commercially, the structure of lecithin is mostly fatty acids, with roughly the same proportion as that of soy oil, which is 50-57% linoleic acid and 5% linolenic.

There are different types of lecithin derived from soy: unrefined or natural (including bleach varieties), refined, and chemically modified. All of these have different chemical structures and compositions. For instance, unrefined lecithin contains 17.5% phosphatidylcholine, 15% phosphatidylethanolamine, 10% phosphatidylinositol, 14-18% other phospholipids, 31-34% unrefined soy oil, 13-16% glycolipids, and 2-4% neutral lipids (mostly triglycerides).

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