Lecithin is the popular and commercial name for a naturally occurring mixture of phosphatides (also called phospholipids or, more recently by biochemists, phosphoglycerides), which varies in color from light tan to dark reddish brown and in consistency from a fluid to a plastic solid. Lecithin is the gummy material contained in crude vegetable oils and removed by degumming. Soybeans are by far the most important source of commercial lecithin and lecithin is the most important by-product of the soy oil processing industry because of its many applications in foods and industrial products. The three main phosphatides in this complex mixture called "commercial soy lecithin" are phosphatidyl choline (also called "pure" or "chemical" lecithin to distinguish it from the natural mixture), phosphatidyl ethanolamine (popularly called "cephalin"), and phosphatidyl inositols (also called inositol phosphatides). Commercial soy lecithin also typically contains roughly 30-35% unrefined soy oil. Indeed lecithin is one of the most complex and versatile substances derived from the soybean.

Etymology and Nomenclature. The word "lecithin" is derived from the Greek term lekithos meaning "egg yolk." In 1846 Gobley isolated lecithin from egg yolk and in 1850 gave it its present name (Maclean and Maclean 1927). In the late 1800s it was also spelled "lecithine" in English, a spelling that is still used (conveniently) in German to refer to the pure or chemical lecithin (Kunze 1941). In present-day English, the term "lecithin" has two different meanings, which can be confusing. To most food processors and chemists it refers to the natural complex mixture of phosphatides, but to most regular chemists, biochemists, and pharmacists it is a trivial term for the chemically pure phosphatide, phosphatidyl choline. In this chapter we will consistently use the term "lecithin" in its broader sense, to refer to the natural complex. The commercial term "soybean phosphatides" may be used to denote the oil-free lecithin complex.

Manufacture. Lecithin is obtained in the process of degumming crude soy oil, usually at the refinery of the company making commercial lecithin rather than at the oil mill. Crude soy oil contains an average of 1.8% (range 1.2-3.2%; Bailey 1951) hydratable compounds, primarily lecithin phosphatides. Roughly 1% of live steam or warm water is added to the crude soy oil at about 70*C, in a batch or continuous process. The emulsion is then agitated or stirred for 10-60 minutes as the phosphatides hydrate and agglomerate, forming a heavy oil-insoluble sludge, which is separated from the oil by use of a centrifuge. The sludge coming from the degumming centrifuge, a lecithin and water emulsion containing 25-50% water, may then be bleached once or twice, typically with hydrogen peroxide, to reduce its color from brown or beige to light yellow. Fluidizing additives such as soy oil, fatty acids, or calcium chloride can then be added?? to reduce the viscosity to that of honey and prevent the end product, on cooling, from being a highly plastic solid. Finally the product is film or batch dried to reduce the moisture to about 1% (Szuhaj 1980). Whether bleached or not, the finished commercial product is called "unrefined lecithin" or "natural lecithin;" it contains 65-70% phosphatides and 30-35% crude soy oil. The oil in unrefined lecithin can be removed by extraction with acetone (phosphatides are insoluble in acetone) to give a dry granular product called "refined lecithin."

Varieties of Lecithin and Their Composition. All varieties of soy lecithin can be classified into three broad types: unrefined or natural (including bleached varieties), refined, and chemically modified. Unrefined or natural lecithin comes in six basic varieties, long defined by specifications of the National Soybean Processors Association: plastic or fluid, each either unbleached, bleached, or double bleached. (Because fluid lecithins are easier to handle and dissolve more rapidly in various solvents, only small amounts of plastic grades are now produced.) Refined lecithin (which has had the oil removed using acetone) comes in three basic varieties: custom blended natural, oil free phosphatides (as is or custom blended), and alcohol-fractionated oil-free phosphatides (as is or custom blended). These latter special refined grades, which may contain 60-99.7% phosphatidyl choline, are used mostly for pharmaceutical applications and research (Brekke 1980). Chemically modified lecithin products, altered through selective chemical treatment, improve lecithin's compatibility to certain systems. Szuhaj (1983), using another method of classification, has noted that in addition to the six basic types of natural or unrefined lecithin, there are six types of upgraded lecithin products, including clarified lecithins (filtered), fluidized lecithins, compounded lecithins, hydroxylated lecithin, deoiled lecithin (granular), and fractionated lecithin. Recent composition figures for both unrefined (natural) and refined (deoiled) lecithin are given in Figure 28.1. Interestingly, earlier publications (Erdahl 1973 in Wood and Allison 1981; Brian 1976) showed these two products to contain significantly higher percentages of the three major phosphatides and no glycolipids.

Lecithin is also available as a dietary supplement in two forms: as granular lecithin (oil-free refined lecithin with calcium phosphate as a flow agent) and as capsules, containing a dispersion in oil (Wood and Allison 1981).

Composition by Weight of Unrefined and Refined Soy Lecithin

Phosphatides

Source: B.F. Szuhaj (1982), Central Soya Co., Inc. Lecithin Div. Fort Wayne, Indiana. Personal communication.

Structurally, the phosphatides in soy lecithin consist of glycerides (the basic component of soy oil; see Chapter 40) in which one fatty acid radical has been replaced with phosphoric acid. In the case of pure or chemical lecithin (phosphatidyl choline), the phosphoric acid is further esterified with choline; in cephalin it is similarly esterified with cholamine. Lecithin is composed mostly of fatty acids, and they are in roughly the same proportion as in soy oil; 50-57% linoleic and 5% linolenic.

Natural Sources of Lecithin. The most concentrated natural and unrefined sources of lecithin are soybeans (1.48 to 3.08% lecithin), peanuts (1.11%), calf liver (0.85%), wheat (0.61%), oatmeal (0.65%), and eggs (0.39%) (Wood and Allison 1981). The human spinal cord contains 6-10% lecithin and the human brain 4-6% lecithin in fresh substance. Among refined substances, especially concentrated sources of lecithin include dehydrated (powdered) egg yolk (14-20%), natural egg yolk (7-10%), wheat germ 2.82%, soy oil (1.8% but 2.65% including the 30-35% entrained soy oil??), and butterfat (1.4%). Soy oil has the highest lecithin and phosphatide content of any known oil; other vegetable oils average 0.5% lecithin. Unlike animal phosphatides, soybean phosphatides contain no cholesterol. In plant seeds the phosphatides are largely associated with oil, but strangely their content varies roughly with the protein rather than the oil content (Stanley 1950). Moreover, all of the above indicates that phosphatides and lecithin appear to be closely connected with the most important vital and reproductive organs and processes. In addition to the spinal cord, brain, eggs, and seeds, they are also concentrated in the nerves, liver, kidneys, and sperm. Actually, lecithin is found in the cell membranes of all human cells, and they tend to be most concentrated where membrane functions are specialized. Lecithin compounds are also closely associated with fatty acids in the body.

Functional Properties. Lecithin is a multi-functional surface-active agent. Each molecule has, like Janus, two faces. The fatty-acid portion of the molecule is attracted to fats (it is lipotrophic) and the phosphoric acid?? portion is attracted to water (it is hydrotrophic). Because of this dual nature, lecithin molecules tend to position themselves at the boundary between immiscible materials, such as oil and water. There they serve many useful functions through a surface modifying effect. According to Szuhaj (1980, 1983) lecithin serves the following major functions: (1) Emulsifying allows the mixing of otherwise immiscible substances, especially in water-in-oil systems, such as margarine and chocolate. This is the most widespread of its various uses; (2) Solubilization makes it possible to dissolve oils (such as flavor oils and oil-soluble colors) in water; (3) Suspension, for example, keeps pigments dispersed in paints, preventing agglomeration; (4) Wetting/instantizing helps powers to dissolve quickly in water; (5) Lubrication and Release; when lecithin is applied in a thin film to a cooking utensil or a mold, it promotes release of food or other materials from that surface; (6) Crystallization Control is used especially to control the crystallization of sugar in fat systems, as in chocolate; (7) Complexing tends to retard crystallization of starch associated with staling in baked goods. (8) Anti-spatter, as in margarine; (9) Viscosity Modifying; and (10) Therapeutical. It also serves as a stabilizer in ice creams and shortenings and an antioxidant in oils and fats.

Food Uses. Lecithin is used in a surprisingly large array of our daily foods. Perhaps most widely used in margarine (for anti-spatter and as an emulsifier), it is also used in chocolates, caramels and coatings (to control viscosity, crystallization, weepage, and sticking), in chewing gum (for its softening, plasticizing, and release effects), in instant foods such as cocoa powders, coffee creamer and instant breakfast (for wetting, dispersing, and emulsifying), in calf milk replacers (to add energy and aid digestibility and emulsification). It is also found in baked goods, cheeses, meat and poultry products, dairy and imitation dairy products, and still other products (Stanley 1950; Brekke 1980; Szuhaj 1980, 1983).

Therapeutic Uses. Much research has been done and is being done on the therapeutic use of lecithin, especially in the prevention or treatment of neurochemical and cardiovascular orders. Although the results are not conclusive, many health food consumers use lecithin for benefits they believe it will bring in these areas.

Nonfood and Industrial Uses. In this realm there are at least as many applications as in the food industry. Lecithin is used in cosmetics, pharmaceuticals, coatings (paints, magnetic tape coatings, waxes, polishes, wood coatings), plastic and rubber industry, glass and ceramic processing, paper and printing, masonry and asphalt products, petroleum industry, metal processing, pesticides, adhesives, textiles, and leathers (Stanley 1950; Brekke 1980; Szuhaj 1980, 1983).

World Production. The major countries refining soy oil (USA, Western Europe, Japan) are also the major producers of soy lecithin. Stanley (1950) estimated that in the year 1936-37 the world produced 1,787,000 tonnes (metric tons) of soy oil. From this it recovered 1,814 tonnes of lecithin and left 47,174 tonnes unrecovered, thereby utilizing only about 4% of potential production. The main producers were the USA, Germany, Japan, Denmark, and Norway. In 1948 world soy lecithin recovery was estimated at 4,535 tonnes, and plant derived lecithin other than soy was estimated at one-fifth this amount. Recovery and utilization of soy lecithin was thought to be less than 10% of potential production. In 1976 Van Nieuwenhuyzen (in Brekke 1980) estimated world recovery of soy lecithin to be 90,700 tonnes a year, from 8.8 million tonnes of soy oil produced containing 233,200 tonnes of soy lecithin. Thus roughly 39% of the total lecithin was recovered and used. Clearly the percentage used has been increasing, but the majority that could be recovered is not yet used directly. The unsold portion is mixed back into defatted soybean meal, which is used for livestock fodder. Although this practice is not widely discussed by the industry, the lecithin is not considered a negative nor a positive factor.