Composition of Soybean Lecithin
ABSTRACT
Commercial soybean lecithin is a complex mixture containing ca.65-75% phospholipids together with triglycerides and smaller amounts of other substances. The major phospholipids include phosphatidylcholine, phosphatidylethanolamine, and inositol-containing phosphatides. Other substances reported include carbohydrates, pigments, sterols, and sterol glycosides. This paper reviews the nature of the compounds found in soybean lecithin and our present knowledge of its composition.
INTRODUCTION
The"lecithin" as used today refers to the material obtained by degumming crude vegetable oils and drying the hydrated gums. In the U.S., commercial lecithin is predominantly from soybean oil. The specific phospholipid formerly called lecithin is now referred to as phosphatidylcholine. Lecithin, then, consists not only of a mixture of phospholipids but also of triglycerides and other non phospholipid compounds removed from the oil in the degumming process. The composition of soybean lecithin has been considered in many reviews, and substances commonly reported included triglycerides, fatty acids, pigments, sterols, sterol glycosides, and esters, tocopherols, and carbohydrates 0-6). Ranges in composition from some of these reviews are in Table I, and values for some other minor components (7) are in Table II. The composition of commercial lecithin is further complicated by the production of six grades of plastic and fluidized lecithin in unbleached, single-bleached, and double-bleached fonts (2,4,6). In addition, various other refined and modified materials are produced (5,6,8). Little has been published about the chemical nature of changes produced by peroxide bleaching or other modifications. This paper reviews the composition of the phospholipids and other materials extracted with the oil and removed by degumming as determined by studies of commercial and laboratory prepared materials. Wittcoff has discussed soybean phosphatides in his comprehensive book ''The Phosphatides"(9).
NONPHOSPHOLIPID COMPONENTS
Carr (0) has stated that oil content may be varied by changing the conditions of centrifugation, and ca. 7% soybean oil and 3% fatty acids are blended with the lecithin to increase fluidity. The color of soybean lecithin depends on processing and bleaching conditions. At our laboratory (11), we showed that xanthophylls are preferentially removed with the gums and that carotene remains with the oil. Lutein made up about 75% of the carotenoid pigments in the gums. These carotenoid pigments are largely destroyed by peroxide bleaching, leaving a variable amount of brown color with no characteristic absorption bands. Studies by Zuev et al. 02-14) have' confined the foundation of brown-colored substances as well as the destruction of carotenoids by heating. They suggest oxidation is involved. Although Burkhardt (5) found in safflower oil that phosphatidylethanolamine contributed most to the color foundation, Tomioka and Kaneda 06,17) found the brown products similar to those fonned in an aldol condensation reaction rather than in a Maillard reaction.
Sterols and sterol glycosides have long been known in soybean lecithin. In addition, Lepage (18) has isolated an esterified for or acylated steryl glucoside. In a sample of crude soybean lecithin, Kiribuchi et al. (19) reported free sterols 0.5%, steryl glucoside 2.1%, and acylated steryl glucoside 2.6%. Sheryl ester was present in negligible amounts. In all three fonts, campesterol, stigmasterol, and l3-sitosterol were present in a ratio of ca. 20:30:50. All fatty acids Connally in soybean oil were found in the acylated steryl glucosides with increased amounts of saturated acids. Popov et al. (20) found 0.74% free sterol and 2.68% combined steryl glycoside and esterified steryl glycosides.
Although tocopherols are present in soybean lecithin, no recent values were found for their amount. Review articles (2,4,7) list a concentration of ca. 0.1%. One might expect that the proportions of individual tocopherols would be similar to soybean oil; a recent study of soybean oil (21) shows a ratio of a:1:a tocopherol of ca. 5:68:27. These oil-soluble non-phosphatide materials are largely separated from the phospholipids by extracting with acetone. Some sterol glycosides and free carbohydrates, as well as sugars bound to lipid constituents, remain with the acetone-insoluble phospholipids. At our laboratory (22), one sample of oil-free phosphatides was found to contain 7% free sugars made up of 45% sucrose, 9% raffinose, and 47% stachyose.
PHOSPHOLIPID COMPONENTS
Each of the phospholipid classes is a mixture of any individual compounds with different fatty acids. Reported fatty acid compositions of total acetone-insoluble phosphatides vary considerably. Some values are listed in Table III. Although the fatty acids of phosphatide classes differ, little has been published on pure materials isolated so as to preclude fractionation. Another complexity is added to phosphatide composition by the different cations associated with acidic groups. Potassium, sodium, magnesium, and calcium have been reported (TABLE II).
TABLE I
Reported Range of Components of Deoiled Soybean Lecithin (1-6)
%
Phosphatidylcholine (PC) 19-24%
Phosphatidylethanolamine (PE) 15-20%
Inositol phosphatides (PI) 15-16%
Phosphatidic acid (PA) 6-8%
Other phosphatides 5-11%
Glycolipids 15%
Complexed sugars: 8%
Triglycerides: 3-5%
Moisture 1%
TABLE II
Some Minor Components of Soybean Lecithin (7)
Tocopherol 1.3 mg/g
Biotin 0.42 ~g/g
Folic acid 0.60 ~glg
Thiamin 0.115 ~glg
Riboflavin 0.33 ~glg
Pantothenic acid 5.59 ~glg
Pyridoxine 0.29 ~glg
Niacin 0.12 ~g/g
Reported Range of Components of Soybean Lecithin Oil
%
Phosphatidylcholine (PC) 12-16%
Phosphatidylethanolamine (PE) 12-15%
Inositol phosphatides (PI) 4-6%
Phosphatidic acid (PA) 6-8%
Other phosphatides 2-3%
Soybean oil 40%
Moisture 1%
Of the phospholipids in soybean lecithin, the best known is phosphatidylcholine (PC), for which the structure is shown in Figure 1. This formula represents the isomer. Hydrolysis of PC results in both a and {3 glycerophosphoric acid, and this was formerly considered evidence for both a and {3 PC. However, Baer (26) has shown that hydrolysis of the synthetic compounds L-a-glyceryl phosphorylcholine, L-a-glyceryl phosphorylethanolamine, L-a-PC, or L-a-phosphatidylethanolamine
(PE) all yield mixtures of a-and {3-glycerophosphoric acid that resemble closely those reported from natural PC. He states that the concept of the occurrence of {3-PC or {3-PE in nature is no longer tenable. Privett and Blank (27) and Mangold (28), by ozonolysis of PC and thin layer chromatography of the resulting aldehyde "core," have shown four farry acid groups in PC: a-saturated-{ 3-unsaturated, a-unsaturated-{3-saturated, a-{3-unsaturated, and a small amount of a-~-saturated. Kimura et al. (29) by AgN03-TLC separated soybean PC into four fractions, including one containing 64% arachidonic acid. Recently, Crawford et al. (30) by high-pressure reverse-phase chromatography have isolated fractions from soybean PC ranging in unsaturation from dilinolenyl to stearyl, linalyl pc.
In older literature, the term "cephalin" has been used for PE (Fig. 1) and as "cephalin fraction" for material insoluble in alcohol. Actually, soybean PE has been found both in the alcohol and alcohol insoluble fractions (31-33), and a 30 transfer countercurrent distribution of the alcohol-soluble fraction between hexane and 90% methanol resulted in only partial separation of PE from the completely alcohol-soluble PC. Kimura et al. (34), by methods like they used with PC, also found arachidonic acid in soybean PE.
Closely related to PE is phosphatidylserine (PS), found in soybean lecithin by Van Handel (35). Negishi et al' (36) reported 5.9% PS in commercial soybean lecithin and Nielson (37) found PS in difficultly extractable soybean phosphatides.
The structure of N-acylphosphatidylethanolamine (NPE) in soybean lecithin was determined by Aneja et al. (38). Wilson and Rinne (39) found it to be more abundant in developing seeds and to decrease at maturity.
Phosphatidic acid (PA), the moiety common to all the glycerophosphate, is present in soybean lecithin (1,2,5). It, also, has been found in greater amounts of lipids from developing than from mature seeds (39, 40). Both PA and NPE are decreased by extraction procedures that inactivate enzymes (41, 42). The amount of NPE and PA in lecithin, therefore, may be dependent on enzymatic reactions during storage and processing of the beans, and PA as calcium and magnesium salts also remains in the degummed oil as nonhydratable phosphatides (43, 44).
Inositol was first found in soybean phosphatides by Klenk and Sakai (45), and early work was done by Woolley (46) and Folch (47). At our laboratory, the inositol lipids in soybean phosphatides were separated into two fractions by countercurrent distribution between hexane and 95% methanol (31). The fraction of lower partition coefficient more soluble in methanol was found to contain PE and an inositol-containing phosphatidic acid (32). This inositol compound corresponds to phosphatidylinositol (PI) for which the structure was determined by Okuhara and Nakayama (48). Its absolute configuration was later shown to be as in Figure 1 (49). The fraction of higher partition coefficient-more soluble in hexane-contained, in addition to inositol, the sugars galactose, mannose, and arabinose (22).
These complex glycolipids and similar materials from other sources have been very difficult to purify and characterize. They were the object of much work by Carter and his group, and a list of his papers on this and related subjects have been published (50). Van Handel (35, 51) found evidence for a long chain base in soybean phosphatides, and Carter et al. (52,53) identified phytosphingosine and de-hydro-phytosphingosine (1, 3,4-trihydroxy-2-amino-octadecane and 1,3,4,-trihydroxy-2-amino-8 trans-octadecene) in soybean phosphatides in the ratio saturated:de-hydro of 20:80.
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