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First published in 1945, Bailey's has become the standard reference on the food chemistry and processing technology related to edible oils and the nonedible byproducts derived from oils. This Sixth Edition features new coverage of edible fats and oils and is enhanced by a second volume on oils and oilseeds. This Sixth Edition consists of six volumes: five volumes on edible oils and fats, with still one volume (as in the fifth edition) devoted to nonedible products from oils and fats. Some brand new topics in the sixth edition include: fungal and algal oils, conjugated linoleic acid, coco butter, phytosterols, and plant biotechnology as related to oil production. Now with 75 accessible chapters, each volume contains a self-contained index for that particular volume.
Scottish Crop Research Institute Dundee, Scotland
Fatty acids, esterified to glycerol, are the main constituents of oils and fats. The industrial exploitation of oils and fats, both for food and oleochemical products, is based on chemical modification of both the carboxyl and unsaturated groups present in fatty acids. Although the most reactive sites in fatty acids are the carboxyl group and double bonds, methylenes adjacent to them are activated, increasing their reactivity. Only rarely do saturated chains show reactivity. Carboxyl groups and unsaturated centers usually react independently, but when in close proximity, both may react through neighboring group participation. In enzymatic reactions, the reactivity of the carboxyl group can be influenced by the presence of a nearby double bond.
The industrial chemistry of oils and fats is a mature technology, with decades of experience and refinement behind current practices. It is not, however, static. Environmental pressures demand cleaner processes, and there is a market for new products. Current developments are in three areas: "green" chemistry, using cleaner processes, less energy, and renewable resources;enzyme catalyzed reactions, used both as environmentally friendly processes and to produce tailor-made products; and novel chemistry to functionalize the carbon chain, leading to new compounds. Changing perceptions of what is nutritionally desirable in fat-based products also drives changing technology; interesterification is more widely used and may replace partial hydrogenation in the formulation of some modified fats.
The coverage in this chapter is necessarily selective, focusing on aspects of fatty acid and lipid chemistry relevant to the analysis and industrial exploitation of oils and fats. The emphasis is on fatty acids and acylglycerols found in commodity oils and the reactions used in the food and oleochemical industries. The practical application of this chemistry is dealt with in detail in other chapters. Current areas of research, either to improve existing processes or to develop new ones, are also covered, a common theme being the use of chemical and enzyme catalysts. Compounds of second-row transition metals rhodium and ruthenium and the oxides of rhenium and tungsten have attracted particular interest as catalysts for diverse reactions at double bonds. Recent interest in developing novel compounds by functionalizing the fatty acid chain is also mentioned. To date, few of these developments have found industrial use, but they suggest where future developments are likely. A number of recent reviews and books cover and expand on topics discussed here.
2. COMPOSITION AND STRUCTURE
2.1. Fatty Acids
Fatty acids are almost entirely straight chain aliphatic carboxylic acids. The broadest definition includes all chain lengths, but most natural fatty acids are [C.sub.4] to [C.sub.22], with [C.sub.18] most common. Naturally occurring fatty acids share a common biosynthesis. The chain is built from two carbon units, and cis double bonds are inserted by desaturase enzymes at specific positions relative to the carboxyl group. This results in even-chain-length fatty acids with a characteristic pattern of methylene interrupted cis double bonds. A large number of fatty acids varying in chain length and unsaturation result from this pathway.
Systematic names for fatty acids are too cumbersome for general use, and shorter alternatives are widely used. Two numbers separated by a colon give, respectively, the chain length and number of double bonds: octadecenoic acid with 18 carbons and 1 double bond is therefore 18:1. The position of double bonds is indicated in a number of ways: explicitly, defining the position and configuration; or locating double bonds relative to the methyl or carboxyl ends of the chain. Double-bond position relative to the methyl end is shown as n-x or [Omega]x, where x is the number of carbons from the methyl end. The n-system is now preferred, but both are widely used. The position of the first double bond from the carboxyl end is designated [Delta]x. Common names (Table 1) may be historical, often conveying no structural information, or abbreviations of systematic names. Alternative representations of linoleic acid (1) are 9Z,12Z-octadecadienoic acid; 18:2 9c12c; 18:2 n-6; 18:2 [Omega]6; 18:2 [Delta]9,12; or C[H.sub.3](C[H.sub.2])[4.sup.C]H=CH[CH.sub.2]CH=CH(C[H.sub.2])[C.sub.7]OOH.
The terms cis and trans, abbreviated c and t, are used widely for double-bond geometry; as with only two substituents, there is no ambiguity that requires the systematic Z/E convention. An expansive discussion of fatty acid and lipid nomenclature and structure appears in Akoh and Min.
Over 1000 fatty acids are known, but 20 or less are encountered in significant amounts in the oils and fats of commercial importance (Table 1). The most common acids are [C.sub.16] and [C.sub.18]. Below this range, they are characterized as short or medium chain and above it as long-chain acids.
Fatty acids with trans or non-methylene-interrupted unsaturation occur naturally or are formed during processing; for example, vaccenic acid (18:1 11t) and the conjugated linoleic acid (CLA) rumenic acid (18:2 9t11c) are found in dairy fats. Hydroxy, epoxy, cyclopropane, cyclopropene acetylenic, and methyl branched fatty acids are known, but only ricinoleic acid (12(R)-hydroxy-9Z-octadecenoic acid) (2) from castor oil is used for oleochemical production. Oils containing vernolic acid (12(S),13(R)-epoxy-9Z-octadecenoic acid) (3) have potential for industrial use.
Typical fatty acid composition of the most widely traded commodity oils is shown in Table 2.
Most commodity oils contain fatty acids with chain lengths between [C.sub.16] and [C.sub.22], with [C.sub.18] fatty acids dominating in most plant oils. Palm kernel and coconut, sources of medium-chain fatty acids, are referred to as lauric oils. Animal fats have wider range of chain length, and high erucic varieties of rape are rich in this [C.sub.22] monoene acid. Potential new oil crops with unusual unsaturation or additional functionality are under development. Compilations of the fatty acid composition of oils and fats and less-common fatty acids are available.
The basic structure, a hydrophobic hydrocarbon chain with a hydrophilic polar group at one end, endows fatty acids and their derivatives with distinctive properties, reflected in both their food and industrial use. Saturated fatty acids have a straight hydrocarbon chain. A trans-double bond is accommodated with little change in shape, but a cis bond introduces a pronounced bend in the chain (Fig. 1).
In the solid phase, fatty acids and related compounds pack with the hydrocarbon chains aligned and, usually, the polar groups together. The details of the packing, such as the unit cell angles and head-to-tail or head-to-head arrangement depend on the fatty acid structure (Fig. 2).
The melting point increases with chain length and decreases with increased unsaturation (Table 3). Among saturated acids, odd chain acids are lower melting than adjacent even chain acids. The presence of cis-double bonds markedly lowers the melting point, the bent chains packing less well. Trans-acids have melting points much closer to those of the corresponding saturates. Polymorphism results in two or more solid phases with different melting points. Methyl esters are lower melting than fatty acids but follow similar trends.
Fatty acid salts and many polar derivatives of fatty acids are amphiphilic, possessing both hydrophobic and hydrophilic areas within the one molecule. These are surface-active compounds that form monolayers at water/air and water/surface interfaces and micelles in solution. Their surface-active properties are highly dependent on the nature of the polar head group and, to a lesser extent, on the length of the alkyl chain. Most oleochemical processes are modifications of the carboxyl group to produce specific surfactants.
Fatty acids in oils and fats are found esterified to glycerol. Glycerol (1,2,3-trihydroxypropane) is a prochiral molecule. It has a plane of symmetry, but if the primary hydroxyls are esterified to different groups, the resulting molecule is chiral and exists as two enantiomers. The stereospecific numbering system is used to distinguish between enantiomers. The Fischer projection of glycerol is drawn with the backbone bonds going into the paper and the hydroxyl on the middle carbon to the left. The carbons are then numbered 1 to 3 from the top (Figure 3). The prefix sn- (for stereospecific numbering) denotes a particular enantiomer, rac- an equal mixture of enantiomers, and x- an unknown stereochemistry. In an asymmetric environment such as an enzyme binding site, the sn-1 and sn-3 groups are not interchangeable and reaction will only occur at one position. Simplified structures are often used; e.g., 1-palmitoyl-2-linoleoyl-3-oleoyl-sn-glycerol is abbreviated to PLO or drawn as shown in Figure 3.
Storage fats (seed oils and animal adipose tissue) consist chiefly (~98%) of triacylglycerols, with the fatty acids distributed among different molecular species. With only two fatty acids, a total of eight triacylglycerol isomers are possible, including enantiomers (Table 4). A full analysis of triacylglycerol molecular species is a major undertaking, and for some oils, there are still technical difficulties to be resolved. More commonly, triacylglycerols are distinguished by carbon number (the sum of the fatty acid chain lengths) or unsaturation, using GC or HPLC for analysis. The number of isomers increases as the cube of the number of fatty acids; hence, even in oils with a simple fatty acid composition, many molecular species of triacylglycerol may be present.
Most natural triacylglycerols do not have a random distribution of fatty acids on the glycerol backbone. In plant oils, unsaturated acids predominate at the sn-2 position, with more saturated acids at sn-1 and sn-3. The distribution of fatty acids at the sn-1 and sn-3 positions is often similar, although not identical. However, a random distribution between these two positions is often assumed as full stereospecific analysis is a time-consuming specialist procedure. In animal fats, the type of fatty acid predominating at the sn-2 position is more variable; for example, palmitate may be selectively incorporated as well as unsaturated acids (Table 5).
Only oils that are rich in one fatty acid contain much monoacid triacylglycerol, for example, olive (Table 5), sunflower, and linseed oils containing OOO, LLL, and LnLnLn, respectively. Compilations of the triacylglycerol composition of commodity and other oils are available.
The melting behavior of triacylglycerols generally reflects that expected from the fatty acid composition; triacylglycerols rich in long-chain and saturated acids are high melting, and those rich in polyunsaturated acids are lower melting. However, the situation is complicated by the possibility that the fatty acids can be distributed in different molecular species with different melting points. Oils with similar fatty acid composition may have different solid fat content, polymorphic forms, and melting behavior as a result of a different triacylglycerol composition.
Mono- and diacylglycerols (Figure 3) are not significant components of good quality oils, but elevated levels may be found in badly stored seeds, resulting from the activity of lipolytic enzymes. These compounds are produced industrially by partial hydrolysis or glycerolysis of triacylglycerols for use as food grade emulsifiers. Mono- and diacylglycerols readily isomerize under acid or base catalysis and are normally produced as an equilibrium mixture in which 1(3)-monoacylglycerols or 1,3-diacylglycerols predominate.
Phospholipids (Figure 3) are constituents of membranes and are only minor components of oils and fats, sometimes responsible for cloudiness. They are usually removed during degumming, the residue from soybean oil processing being a source of phospholipids used as food emulsifiers. The term "lecithin" is used very loosely for such material, and it may variously mean phosphatidylcholine, mixed glycerophospholipids, or crude phospholipid extracts from various sources. Where possible, more specific nomenclature or the source and purity should be used.
2.3. Bulk Properties
Saponification value and iodine value. Oils and fats are now characterized mainly by their fatty acid composition determined by gas chromatography, replacing the titrimetric and gravimetric assays used previously. However, the saponification value (SV) or equivalent (SE) and iodine value (IV) are still used in specifications and to monitor processes. SE, expressed as grams of fat saponified by one mole of potassium hydroxide, is an indication of the average molecular weight and hence chain length, whereas the IV, expressed as the weight percent of iodine consumed by the fat in a reaction with iodine monochloride, is an index of unsaturation (Table 6). Standard analytical methods are available, but these parameters are now often calculated from the fatty acid composition, assuming that the sample is all triacylglycerol. Indirect measurement of IV and SV (as well as peroxide and trans-content) using FT-NIR spectroscopy have been developed for real-time process monitoring.
Unsaponifiable matter. Oils and fats contain variable amounts of sterols, hydrocarbons, tocopherols, carotenoids, and other compounds, collectively referred to as unsaponifiable matter because they do not produce soaps upon hydrolysis (Table 6). The sterol and tocopherol composition of commodity oils is discussed in another chapter. Some of these minor components are removed during refining, and the resulting concentrates may be useful byproducts, for example, tocopherol antioxidants. Characteristic fingerprints of minor components, particularly phytosterols and tocopherols, are also used to authenticate oils and detect adulteration.
3. HYDROLYSIS, ESTERIFICATION, AND ESTER EXCHANGE
Reactions converting acids to esters or vice versa and the exchange of ester groups are among the most widely used in fatty acid and lipid chemistry (Figure 4). They find applications from microscale preparation of methyl esters for GC analysis to the industrial production of oleochemicals and biodiesel. The exchange of groups attached to the fatty acid carboxyl is usually an equilibrium process driven to one product by an excess of one reactant or the removal of one product, and it is usually carried out with the aid of a catalyst. The catalyst may be an acid, a base, or a lipolytic enzyme. These reactions produce the fatty acids and methyl esters that are the starting point for most oleochemical production. As the primary feedstocks are oils and fats, glycerol is produced as a valuable byproduct. Reaction routes and conditions with efficient glycerol recovery are required to maximize the economics of large-scale production.
There is increasing interest in the use of lipase enzymes for large-scale reactions. Enzyme reactions require milder conditions, less solvent, and give cleaner products-attributes of "green chemistry." Enzymes can exert regio- or stereospecific control over reactions and may also offer a degree of selectivity for particular fatty acids, not observed with acid or base catalysts. Although the reactions of the carboxyl group are normally independent of those of the double bonds in the fatty acid molecule, the presence of a double bond at the [Delta]4, [Delta]5, or [Delta]6 position often results in slower reaction when a reaction is catalyzed by a lipase. Lipase catalyzed reactions are considered in detail below, following a brief description of the reactions involved.
Excerpted from Bailey's Industrial Oil and Fat Products, Edible Oil and Fat Products Copyright © 2005 by John Wiley & Sons, Inc.. Excerpted by permission.
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VOLUME 1: EDIBLE OIL AND FAT PRODUCTS: CHEMISTRY, PROPERTIES, AND HEALTH EFFECTS.
1. Chemistry of Fatty Acids (Charlie Scrimgeour).
2. Crystallization of Fats and Oils (Serpil Metin and Richard W. Hartel).
3. Polymorphism in Fats and Oils (Kiyotaka Sato and Satoru Ueno).
4. Fat Crystal Networks (Geoffrey G. Rye, Jerrold W. Litwinenko, and Alejandro G. Marangoni).
5. Animal Fats (Michael J. Haas).
6. Vegetable Oils (Frank D. Gunstone).
7. Lipid Oxidation: Theoretical Aspects (K. M. Schaich).
8. Lipid Oxidation: Measurement Methods (Fereidoon Shahidi and Ying Zhong).
9. Flavor Components of Fats and Oils (Chi-Tang Ho and Fereidoon Shahidi).
10. Flavor and Sensory Aspects (Linda J. Malcolmson).
11. Antioxidants: Science, Technology, and Applications (P. K. J. P. D. Wanasundara and F. Shahidi).
12. Antioxidants: Regulatory Status (Fereidoon Shahidi and Ying Zhong).
13. Toxicity and Safety of Fats and Oils (David D. Kitts).
14. Quality Assurance of Fats and Oils (Fereidoon Shahidi).
15. Dietary Lipids and Health (Bruce A. Watkins, Yong Li, Bernhard Hennig, and Michal Toborek).
VOLUME 2: EDIBLE OIL AND FAT PRODUCTS: EDIBLE OILS.
1. Butter (David Hettinga).
2. Canola Oil (R. Przybylski, T. Mag, N.A.M. Eskin, and B.E. McDonald).
3. Coconut Oil (Elias C. Canapi, Yvonne T. V. Agustin, Evangekube A. Moro, Economico Pedrosa, Jr., Mar&ıacute;a J. Bendaño).
4. Corn Oil (Robert A. Moreau).
5. Cottonseed Oil (Richard D. O’Brien, Lynn A. Jones, C. Clay King, Phillip J. Wakelyn, and Peter J. Wan).
6. Flax Oil and High Linolenic Oils (Roman Przybylski).
7. Olive Oil (David Firestone).
8. Palm Oil (Yusof Basiron).
9. Peanut Oil (Harold E. Pattee).
10. Rice Bran Oil (Frank T. Orthoefer).
11. Safflower Oil (Joseph Smith).
12. Sesame Oil (Lucy Sun Hwang).
13. Soybean Oil (Earl G. Hammond, Lawrence A. Johnson, Caiping Su, Tong Wang, and Pamela J. White).
14. Sunflower Oil (Maria A. Grompone).
VOLUME 3: EDIBLE OIL AND FAT PRODUCTS: SPECIALTY OILS AND OIL PRODUCTS.
1. Conjugated Linoleic Acid Oils (Rakesh Kapoor, Martin Reaney, and Neil D. Westcott).
2. Diacylglycerols (Brent D. Flickinger and Noboru Matsuo).
3. Citrus Oils and Essences (Fereidoon Shahidi and Ying Zhong).
4. Gamma Linolenic Acid Oils (Rakesh Kapoor and Harikumar Nair).
5. Oils from Microorganisms (James P. Wynn and Colin Ratledge).
6. Transgenic Oils (Thomas A. McKeon).
7. Tree Nut Oils (Fereidoon Shahidi and Homan Miraliakbari).
8. Germ Oils from Different Sources (Nurhan Turgut Dunford).
9. Oils from Herbs, Spices, and Fruit Seeds (Liangli (Lucy) Yu, John W. Parry, and Kequan Zhou).
10. Marine Mammal Oils (Fereidoon Shahidi and Ying Zhong).
11. Fish Oils (R. G. Ackman).
12. Minor Components of Fats and Oils (Afaf Kamal-Eldin).
13. Lecithins (Bernard F. Szuhaj).
14. Lipid Emulsions (D. Julian McClements and Jochen Weiss).
15. Dietary Fat Substitutes (S. P. J. Namal Senanayake and Fereidoon Shahidi).
16. Structural Effects on Absorption, Metabolism, and Health Effects of Lipids (Armand B. Christophe).
17. Modification of Fats and Oils via Chemical and Enzymatic Methods (S. P. J. Namal Senanayake and Fereidoon Shahidi).
18. Novel Separation Techniques for Isolation and Purification of Fatty Acids and Oil By-Products (Udaya N. Wanasundara, P. K. J. P. D. Wanasundara, and Fereidoon Shahidi).
VOLUME 4: EDIBLE OIL AND FAT PRODUCTS: PRODUCTS AND APPLICATIONS.
1. Frying Oils (Monoj K. Gupta).
2. Margarines and Spreads (Michael M. Chrysan).
3. Shortenings: Science and Technology (Douglas J. Metzroth).
4. Shortenings: Types and Formulations (Richard D. O’Brien).
5. Confectionery Lipids (Vijai K.S. Shukla).
6. Cooking Oils, Salad Oils, and Dressings (Steven E. Hill and R. G. Krishnamurthy).
7. Fats and Oils in Bakery Products (Clyde E. Stauffer).
8. Emulsifiers for the Food Industry (Clyde E. Stauffer).
9. Frying of Foods and Snack Food Production (Monoj K. Gupta).
10. Fats and Oils in Feedstuffs and Pet Foods (Edmund E. Lusas and Mian N. Riaz).
11. By-Product Utilization (M. D. Pickard).
12. Environmental Impact and Waste Management (Michael J. Boyer).
VOLUME 5: EDIBLE OIL AND FAT PRODUCTS: PROCESSING TECHNOLOGIES.
1. A Primer on Oils Processing Technology (Dan Anderson).
2. Oil Extraction (Timothy G. Kemper).
3. Recovery of Oils and Fats from Oilseeds and Fatty Materials (Maurice A. Williams).
4. Storage, Handling, and Transport of Oils and Fats (Gary R. List, Tong Wang, and Vijai K.S. Shukla).
5. Packaging (Vance Caudill).
6. Adsorptive Separation of Oils (A. Proctor and D. D. Brooks).
7. Bleaching (Dennis R. Taylor).
8. Deodorization (W. De Greyt and M. Kellens).
9. Hydrogenation: Processing Technologies (Walter E. Farr).
10. Supercritical Technologies for Further Processing of Edible Oils (Feral Temelli and Özlem Güçlü-Üstünda&gcaron;).
11. Membrane Processing of Fats and Oils (Lan Lin and S. Sefa Koseoglu).
12. Margarine Processing Plants and Equipment (Klaus A. Alexandersen).
13. Extrusion Processing of Oilseed Meals for Food and Feed Production (Mian N. Riaz).
VOLUME 6: INDUSTRIAL AND NONEDIBLE PRODUCTS FROM OILS AND FATS.
1. Fatty Acids and Derivatives from Coconut Oil (Gregorio C. Gervajio).
2. Rendering (Anthony P. Bimbo).
3. Soaps (Michael R. Burke).
4. Detergents and Detergency (Jesse L. Lynn, Jr.).
5. Glycerine (Keith Schroeder).
6. Vegetable Oils as Biodiesel (M. J. T. Reaney, P. B. Hertz, and W. W. McCalley).
7. Vegetable Oils as Lubricants, Hydraulic Fluids, and Inks (Sevim Z. Erhan).
8. Vegetable Oils in Production of Polymers and Plastics (Suresh S. Narine and Xiaohua Kong).
9. Paints, Varnishes, and Related Products (K. F. Lin).
10. Leather and Textile Uses of Fats and Oils (Paul Kronick and Y.K. Kamath).
11. Edible Films and Coatings from Soybean and Other Protein Sources (Navam S. Hettiarachchy and S. Eswaranandam).
12. Pharmaceutical and Cosmetic Use of Lipids (Ernesto Hernandez).