Lecithin. Polarized light micrograph of crystals of lecithin. Lecithin is a group of substances containing fatty acid residues, in which usually one acid is saturated and one unsaturated. Lecithins occur in every animal and vegetable cell as an essential constituent, especially in cell membranes. They swell up with water to produce slimy yellow emulsions, and melt at about 60 degrees Celsius. Commercial lecithin is obtained from the manufacture of soya bean oil, and is widely used in the food industry. Magnification: x20 at 35mm size.