Chapter 10 Degumming and Lecithin Processing and Utilization David R. Erickson Consultant American Soybean Association St. Louis, MO Introduction Degumming is the process for removal of phosphatides from crude soybean and other vegetable oils. The phosphatides are also called gums and lecithin. The latter term is also the common name for phosphatidyl choline, but common usage refers to the array of phosphatides present in all crude vegetable oils. Although all crude vegetable oils contain gums, soybean oil is currently the major source of commercial lecithin, because it contains the largest amount of gums and is also the world's leading vegetable oil (see Chapter 1). The relative composition of commercial soybean lecithin from conventionally solvent-extracted crude soybean oil is shown in Table 10.1. Changes from this composition, as affected by different extraction practices will be discussed in the following section, followed by a discussion of lecithin production and utilization. Degumming Processes Simply stated, degumming is the process used to remove phosphatides, or gums, from crude soybean oil. As shown in Table 10.1, the gums obtained from degumming consist of a mixture of soybean oil and phosphatidyl compounds (1). The chemical structures of the three major phosphatides are shown in Fig. 10.1. In conventional solvent extraction, only half of the phosphatides present in soybeans are extracted with the composition shown in Table 10.1. Use of preparation processes such as the Alcon process or expanders will change the array of phosphaTABLE 10.1 Approximate Composition of Natural Commercial Soybean Lecithin (1) % Soybean oil Phosphatidylcholine Phosphatidyl ethanolamine Phosphatidyl inositol Phytoglycolipids, minor phosphatides Carbohydrates Moisture 174 35 16 14 10 17 7 1 D.R. Erickson 175 tides in the crude oil by increasing the phosphatidyl choline content by about 30 to 40% and will increase the total extracted phosphatides (2). In the Alcon process double the normal amount of gums is extracted, and use of the expander may approach that level depending on the expander conditions of temperature, time, and moisture addition (for details of these processes, see Chapter 6). Degumming of soybean oil is done for one of the following reasons: 1. To produce lecithin 2. To prepare degummed oil for long-term storage or transport 3. To prepare a degummed oil for caustic or physical refining The first reason is obvious and the second is necessary to prevent development of troublesome sludges in storage or transport. Such sludges form because the phosphatides are hygroscopic and become hydrated by moisture from the air. Phosphatides are soluble in dry crude oil, but when hydrated, they become more dense than the triglycerides and precipitate, or settle out, from the crude oil, causing the troublesome sludges. Although this phenomenon is unwanted in storage or transport of crude oils, it is the basis for the process of degumming. Water is added to the crude oil to hydrate the phosphatides and thus prepare them for removal by gravitational forces or centrifugation. The latter is the modern process. The process of degumming is simple, but the quality of the crude soybean oil has an influence on the efficacy of degumming. The phosphatides in crude soybean oil exist in either the hydratable or nonhydratable forms. The hydratable phosphatides (HP) are readily removed by the addition of water, whereas the nonhydrat- 9 I9 I 9 9 I 9 I9 CHJJOCR! CH2OCR1 CHOCR CHOCR 2 2 CH2OP-OCH2CH2N(CH3)3 CH20-P-0-CH2CH2NH3 Phosphatidyl choline Phosphatidyl ethanolamine Phosphatidyl inositol Fig. 10.1. Structures of the three major phosphatides in soybean lecithin. 176 Degumming and Lecithin Processing able phosphatides (NHP) are relatively unaffected by water and tend to be more oilsoluble (i.e., remain in the oil phase). The NHPs are generally considered to be the calcium and magnesium salts of phosphatidic acids that arise from the enzymatic action of phospholipases released by damage to the cellular structure of the soybean (3). Such damage may occur with handling, processing practices, or both. The formation of NHPs is shown in Fig. 11.3. Poor-quality soybean oils are defined by a higher FFA content (>1.0%) indicating a higher than normal NHP content. Other indicators of poor-quality crude soybean oils are a lower phosphatide content (<1.0%) in the extracted oil and a high content (>0.79%) of phosphatides in degummed oils (4). Normal-quality soybean oil from conventional solvent extraction will have about 90% HP and 10% NHP, and their total phosphatide content will range from 1.1 to 3.2% (5). The FFA of goodquality crude soybean oil will be in the range of 0.5 to 1.0%, which will be reduced by 20 to 40% in the degummed oil (6,7). Phosphatides also form complexes with metals (iron and copper), and their content in degummed and refined oils is reduced as discussed in Chapter 11. Recognition of the role of calcium and magnesium has led to the use of demineralized (soft) water for degumming and the use of phosphoric or citric acids as aids in degumming. These latter two acids are food-grade and combine with the calcium and magnesium salts, allowing transfer of the phosphatidic acids from the oil to the aqueous phase, thus removing them from the crude oil. The use of acids in degumming is not recommended for gums intended for use as lecithins, because their presence will cause darkening of the lecithin. The effect of the NHPs on caustic refining is discussed in Chapter 11. Some plants use steam condensates for degumming rather than deionized water; however, the absence of iron in the condensate should be ensured. Some plants also use excess stripping steam in the last stages of miscella desolventizing as a means for hydration of phosphatides, but this is more difficult to control than simple water addition. Water Degumming In degumming of soybean oil for lecithin production, it is standard practice to filter the crude oil first to remove meal fines, which affect the clarity of lecithin and contribute to the hexane-insoluble content (8). If the degumming centrifuge is self-clean¬ ing and the gums are destined for return to the meal stream, it may not be necessary to filter the crude oil; however, some plants filter the crude oil routinely to prevent the possible increase of free fatty acids during storage and to reduce refining losses (7). In the batch water degumming process, soft water at a level of 1 to 2%, depending on the phosphatide content, is added to warm (70°C, 158°F) oil and mixed thoroughly for 30 to 60 minutes, followed by settling or centrifugation. The amount of water to add is about 75% of the phosphatide content (9), but some plants routinely add a fixed percentage based on experience. A diagram of the process of continuous water degumming is shown in Fig. 10.2 (10). In this case, the oil is heated to about 70 to 80°C (158 to 176°F), soft D.R. Erickson 177 water is added by an in-line proportioning system, the oil and water are mixed inline, and then the mixture flows to a retention vessel, where it is held for 15 to 30 minutes and then sent to a centrifuge. For best results in both batch and continuous degumming, it is absolutely essential to provide sufficient time for the water and the phosphatides to react. Hydration of phosphatides is not an instantaneous reaction and requires initial intensive mixing, followed by sufficient time to allow hydration and coalescence of the hydrated micelles. An excellent general discussion of the chemistry of degumming has been published by Dijkstra (11). Experience with degumming and caustic refining systems for soybean oils with little or no retention times ("short-mix") has shown that such systems are not adequate for production of optimum-quality soybean oils (12,13,14). With good-quality crude soybean oil, simple water degumming will reduce the phosphorus content to less than 50 ppm (0.005%), which is well below the 200 ppm (0.02%) level specified in the National Oilseed Processors Association (NOPA) trading rules for crude degummed soybean oil (15). The relation between phosphorus and phosphatide content is % Phosphatides = 30.0 x % Phosphorus content Filtered, warm, crude oil Water _J Flow meter L_ Flow meter T Pipeline dwell agitator Centrifuge Gums Oil Heat Vacuum dryer Bleaching agent Fluidity agent Cooler Mixing tank Agitated-film evaporator Vacuum Condenser Cooler Condensate receiver Dry lecithin Degummed dry soybean oil To storage Condensate +> To packaging Fig. 10.2. Flowsheet for degumming soybean oil and crude lecithin production. Source: Brian, R., /. Amer. Oil Chem. Soc. 53: 27, (1976). 178 Degumming and Lecithin Processing Degumming of Nonconventional Solvent-Extracted Oils As just mentioned, only about 50% of the phosphatides present in soybeans are extracted by conventional extraction processes. However, it has long been recognized that there is an increase in hexane-extractables of soybean meal after the heat and moisture treatment in the desolventizer-toaster. Subsequently, it was discovered and patented by Koch (16) that a heat and moisture treatment of flaked soybeans before extraction essentially doubled the amount of phosphatides in the extracted crude soybean oil. It was also found that there was a different array of phosphatides and that the hydratability was increased, resulting in degummed oils of 0.03 to 0.09% phosphatides (10-30 ppm P). These findings have been commercialized and called the Alcon process (Lurgi Ol. Gas. Chemie Gmbh, Frankfurt, Germany). A description of this process, and plant results, have been published by Penk (17). The lecithin produced by this is of a different composition than lecithin from conventional extraction processes, being enriched 30 to 40% in phosphatidyl choline. The use of expanders in preparation for extraction gives results approaching those found with the Alcon process, depending on the degree of moist heat treatment occurring in the expander. Degumming Agents In addition to water, other additives have been studied and or used to facilitate degumming. The use of phosphoric and citric acid for removal of NHPs has already been mentioned. Other acids studied have been acetic, oxalic, nitric, boric, and tannic (18), but to our knowledge they have never been commercialized. The use of acetic anhydride for degumming of soybean oil has been done on a commercial scale in the United States (19) and is in use in some plants. The advantage is a higher yield of lecithin (i.e., lower gum content of the degummed oil) and ready removal of the volatile acetic acid in drying of the lecithin. Degumming to Prepare an Oil for Caustic or Physical Refining Caustic refining of soybean oil may be done on either crude or crude degummed oil. Since the market for soybean lecithins is much less than the potential supply as will be shown in this chapter, it is a practice in the United States simply to caustic refine crude soybean oil, thereby disposing of lecithin into the soapstock, when not making lecithin. The conventional thinking has been that the loss of neutral oil in refining crude oil is less than the combined losses of degumming followed by caustic refining of the degummed oil. When a refinery is adjacent to an extraction plant, it has the possibility of adding the wet gums to the meal stream and getting at least a meal price for the gums. Such practice will then reduce the refining loss by caustic-refining degummed oil. In addition, there would be less soapstock, and it would give fewer problems in acidulation. Although it is not necessary to degum and refine soybean oil sequentially, the decision as whether to do so or simply to refine crude oil may be more involved than just considering refining losses, due to the effects just mentioned. D.R. Erickson 179 Degumming of soybean oil, as a preparatory step for physical refining, has received considerable attention in the last few years and has recently been well reviewed by Dijkstra (11). The principal impetus for physical refining of soybean oil is to avoid the environmental problems associated with soapstocks from caustic refining. For a discussion of physical refining, see Chapter 14. In the published results of degumming practices for physical refining, the degummed oils have a P content of 5 to 10 ppm for "good" oils and >30 for "poor" oils (20). To date, even with the use of special degumming techniques, physically refined soybean oils do not meet the flavor and flavor stability requirements of the U.S. market. They have met market requirements in other countries whose consumers are less demanding. Production of Lecithin The potential supply of lecithin from soybean oil is about 374,000 metric tons (802 million lbs) on a worldwide basis, but the market for lecithin is estimated at 100,000 to 150,000 metric tons. The other routes for use or disposal of the excess lecithin are to return it to the meal stream at the extraction plant or to caustic-refine crude oil, which disposes of the excess lecithin into the soapstock. The process for lecithin production is shown in Fig. 10.2, and the important aspects up to and including the centrifugation have been discussed in the foregoing section on degumming. The wet gums coming from the centrifugation will contain about 50% water, with the nonaqueous portion having the composition shown in Table 10.1. Wet gums are susceptible to microbial fermentation and require immediate drying or treatment, for brief storage, with a preservative such as a dilute solution of hydrogen peroxide. Required dosage will depend on expected storage time, ambient temperature, and sanitary conditions (microbial types and load). Any storage of wet gums is not recommended, and the brief storage just mentioned is for necessary accumulation for batch drying systems. In the process flow shown in Fig. 10.2, the wet gums from the centrifuges are transferred to a mixing tank, where bleaching agents, fluidity agents, or both can be added. With or without additives, the wet gums, containing about 50% water, must then be dried down to a maximum of 1% moisture. The drying of lecithins is a critical step, because the gums tend to darken with heat, and during drying there is a large viscosity increase as the moisture is reduced. This phenomenon is shown in Fig. 10.3, where the increase in viscosity begins at about 20% moisture, peaks at about 8% moisture, and then falls rapidly between 7 and 4% reduction (21). Batch-type dryers operate under vacuum and are equipped with rotating coils circulating water at about 60 to 70°C (140 to 158°F). More modern continuous dryers utilize agitated-film dryers for moisture removal. A comparison of the conditions used in these two types of dryers is shown in Table 10.2 (21). From the process flow shown in Fig. 10.2 a variety of lecithins may be produced, and the National Oilseed Processors Association (NOPA) publishes sped- 180 Degumming and Lecithin Processing TABLE 10.2 Average Process Conditions for Drying Lecithin Sludge (Wet Gums) (21)a Continuous agitated-film Process variable Batch dryer dryer 6 Temperature °F 140-176 176-203 °C 60- 80 80- 95 Residence time, min 180-240 1- 2 Absolute pressure, mm Hg 20- 60 50-300 Starting product: Wet gums with 50% moisture. End product: Lecithin with less than 1 % moisture. ^Vacuum dryer with rotating, ball-shaped coils heated with warm water. fications for six lecithin commercial grades, as shown in Table 2.14. Sullivan and Szuhaj (23) created a useful classification of soybean lecithins as shown in Table 10.3. In their classification, the NOPA products are considered "natural", followed by "refined" lecithins, from custom blending and solvent treatments, and ending with "Chemically modified" lecithins. 10,000 r 8,000 co $ 8 & 6,000 4,000 2,000 I 10 Moisture, % 15 20 Fig. 10.3. Viscosity of lecithin sludge at 158°F (70°C) in relation to moisture content. Source: Van Nieuwenhuyzen, W., J. Amer. Oil Chem. Soc. 53:425, (1976). D.R. Erickson 181 TABLE 10.3 Classification of Soybean Lecithins (23) I. Natural A. Plastic 1. Unbleached 2. Single-bleached 3. Double-bleached B. Fluid 1. Unbleached 2. Single-bleached 3. Double-bleached II. Refined A. Custom blended natural B. Oil-free phosphatides 1. As is 2. Custom blended C. Fractionated oil-free phophatides 1. Alcohol-soluble a. As is b. Custom blended III. Chemically modified Oil-free lecithins are produced by extracting the soybean oil from the natural lecithins with acetone. This is done by both batch and continuous processes and requires high-quality crude lecithin for best results. In turn, alcohol fractionation of an oil-free lecithin can be employed to give an alcohol-soluble fraction high in phosphatidyl choline, and an alcohol-insoluble fraction, enriched in phosphatidyl inositol. The composition of oil-free lecithin and the alcohol fractionation products is shown in Table 10.4. As shown in the table, the lecithin products vary in their emulsifying properties. Chemically modified lecithins include hydrogenated, hydroxylated, acetylated, sulfonated, and halogenated products. All of the chemical modifications are designed to modify the emulsifying properties of the lecithins and improve their dispersibility in aqueous systems (21). TABLE 10.4 Approximate Composition of Commercially Refined Lecithin Fractions (23) Fraction Phosphatidyl choline Cephalin Inositol and other phosphatides, including glycolipids Soybean oil Other constituents Emulsion type favored 3 Oil-free lecithin (%) Alcohol-soluble lecithin (%) Alcohol-insoluble lecithin (%) 29 29 60 30 4 29 32 3 7 Either oil-in-water or water-in-oil 2 4 4 Oil-in-water 55 4 8 Water-in-oil includes sucrose, raffinose, stachyose, and about 1 % moisture. Degumming and Lecithin Processing 182 Soybean Lecithin Utilization A comprehensive listing of the uses and functions of phospholipids has been published by Schneider (24) and is shown in Table 10.5. Lecithin also has unique release properties and as such has been used in pan frying formulations and in pan greases for baking (25). In addition, it is also used industrially as a release agent for ready removal of both wooden and metal concrete casting forms. A more detailed description of lecithin utilization, including those from other oilseeds can be found in the recent AOCS monograph by Szuhaj (26). TABLE 10.5 Uses and Functions of Phospholipids (24) Product Food Instant food Baked goods Chocolate Margarine Dietetics Feedstuffs Calf milk replacers Industry Insecticides Paints Magnetic tapes Leather Textiles Cosmetics Hair care Skin care Pharmaceuticals Parental nutrition Suppositories Cremes, lotions References Function Wetting and dispersing agent; emulsifier Modification of baking properties; emulsifier; antioxidant Viscosity reduction; antioxidant Emulsifier; antispattering agent; antioxidant Nutritional supplement Emulsifier; wetting and dispersing agent Emulsifier; dispersing agent; active substance Dispersing agent; stabilizer Dispersing agent; emulsifier Softening agent; oil penetrant Softening; lubricant Foam stabilizer; emollient Emulsifier; emollient, refatting, wetting agent Emulsifier Softening agent; carrier Emulsifier; penetration improver 1. Brekke, O.L., in Handbook of Soy Oil Processing and Utilization, edited by D.R. Erickson et al., American Oil Chemists' Society, Champaign, IL, 1978, p. 77. 2. Kock, M., J. Amer. Oil Chem. Soc. 60: 210 (1983). 3. Hvolby, A., J. Amer. Oil Chem. Soc. 48: 503 (1971). 4. List, G.R., in Handbook of Soy Oil Processing and Utilization, edited by D.R. Erickson et al., American Oil Chemists' Society, Champaign, IL, 1978, pp. 355-376. 5. Swem, D., in Bailey's Industrial Oil and Fat Products, edited by D. Swem, Vol. I, 4th edn., John Wiley and Sons, New York, 1979, p. 49. 6. Sleeter, R.T., J. Amer. Oil Chem. Soc. 58: 239 (1981). 7. Charpentier, R., INFORM 2: 208 (1991). D.R. Erickson 183 8. Official Methods and Recommended Practices, 4th edn., American Oil Chemists' Society, Champaign, IL, 1993. Method Ja 3-87 (1993) 9. Brekke, O.L., in Handbook of Soy Oil Processing and Utilization, edited by D.R. Erickson et al., American Oil Chemists' Society, Champaign, IL, 1978, p. 73. 10. Brian, R., J. Amer. Oil Chem. Soc. 53: 27 (1976). 11. Dijkstra, A.J., in Proceedings of the World Conference on Oilseed Technology and Utilization, edited by T.A. Applewhite, American Oil Chemists' Society, Champaign, IL, 1992, pp. 138-151. 12. Wiedermann, L.H., J. Amer. Oil Chem. Soc. 58: 159 (1981). 13. Erickson, D.R., J. Amer. Oil Chem. Soc. 60: 351 (1983). 14. Erickson, D.R., and L.H. Wiedermann, INFORM 2: 201 (1991). 15. Yearbook and Trading Rules, National Oilseed Processors Association, Washington, DC, 1993-1994, p. 86. 16. Kock, M., U.S. Patent 4,255,346 (1981). 17. Penk, G., in Proceedings: World Conference on Emerging Technologies in the Fats and Oils Industry, edited by A. R. Baldwin, American Oil Chemists' Society, Champaign, IL, 1986, pp. 38^5. 18. Ohlson, R., and C. Svenson, J. Amer. Oil. Chem. Soc. 53: 8 (1976). 19. Meyers, N.W., J. Amer. Oil Chem. Soc. 34: 93 (1957). 20. Seger, J.C., and R. van de Sande, in World Conference Proceedings Edible Fats and Oils Processing, edited by D.R. Erickson, American Oil Chemists' Society, Champaign, IL, 1990, pp. 88-93. 21. Van Nieuwenhuyzen, W, J. Amer. Oil. Chem. Soc. 53: 425 (1976). 22. Yearbook and Trading Rules, National Oilseed Processors Association, Washington, DC, 1993-1994, p. 98. 23. Sullivan, D.R., and B.F. Szuhaj, J. Amer. Oil Chem. Soc. 52: 152A (1975). 24. Schneider, M., in Proceedings World Conference on Emerging Technologies in the Fats and Oils Industry, edited by A.R. Baldwin, American Oil Chemists' Society, Champaign, EL, 1986, pp. 160-164. 25. Dashiell, G., in World Conference Proceedings Edible Fats and Oil Processing, edited by D.R. Erickson, American Oil Chemists' Society, Champaign, IL, 1990, pp. 396-401. 26. Szuhaj, B.F., Lecithins: Sources, Manufacture, and Uses, American Oil Chemists' Society, Champaign, IL, 1989.
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