Chapter
4 Edible Oils, Fats, and Waxes
Mohammad Farhat AIi 4.1
Introduction
86
4.2
Fatty Acids
88
4.3
Glycerides
92
4.4
Physical Properties of Triglycerides
94
4.4.1
Melting point
94
4.4.2
Specific heat
94
4.4.3
Viscosity
94
4.4.4
Density
96
4.4.5
Refractive index
96
4.4.6
Polymorphism
96
4.4.7
Other physical properties
96
4.5
Chemical Properties of Triglycerides
98
4.5.1
Hydrolysis
98
4.5.2
Methanolysis
98
4.5.3
lnteresterlfication
98
4.5.4
Hydrogenation
99
4.5.5
Isomerization
100
4.5.6
Polymerization
100
4.5.7
Autoxidation
100
4.6
Sources of Edible Oils and Main Fats
102
4.7
Oils and Fats: Processing and Refining
103
4.8
Fats and Oils Stability and Antioxidants
115
4.9
Methods of Analysis and Testing of Fats and Oils
118
4.9.1
Identification and compositional analysis
118
4.9.2
Quality control tests
120
References
121
4.1
Introduction
Fats, oils, and waxes are naturally occurring esters of long straight-chain carboxylic acids. They belong to the saponifiable group of lipids. Lipids are biologically produced materials that are relatively insoluble in water but soluble in organic solvents (benzene, chloroform, acetone, ether, and the like). The saponifiable lipids contain an ester group and react with hot sodium hydroxide solution undergoing hydrolysis (saponification): No reaction (unsaponifiable) no ester group
Includes steroids, prostaglandins, leukotrienes
Lipids (hot NaOH solution) Hydrolysis reaction (saponifiable) contains ester group
Includes oils, fats, waxes, phospholipids
Saponification is a chemical process in which an ester is heated with aqueous alkali (sodium hydroxide) to form an alcohol and the sodium salt of the acid corresponding to the ester. The sodium salt formed is called soap.
Fats and oils are esters of glycerol, the simplest triol (tri-alcohol), in which each of the three hydroxyl groups has been converted to an ester. The acid portion of the ester linkage (fatty acids) usually contains an even number of carbon atoms in an unbranched chain of 12 to 24 carbon atoms. The triesters of glycerol fats and oils are also known as triglycerides. Typical fat-triester of glycerol
glycerol
The difference between fats and oils is merely one of melting point: fats are solid at room temperature (200C) whereas oils are liquids. Both classes of compounds are triglycerides.
As glycerol is common to all fats and oils, whether animal or vegetable, it is the fatty acid part of the fat (oil) that is of interest. The differences among triglycerides (fats and oils) are because of the length of the hydrocarbon chains of the acids and the number of position of double bonds (unsaturation). The hydrocarbon chains of the fatty acids may be completely saturated (saturated fat) or may contain one or more double bonds. The geometric configuration of the double bond in fats and oils is normally cis, If the chain includes more than one double bond, the fat is called polyunsaturated. The presence of a double bond puts a kink in the regular zigzag arrangement characteristic of saturated carbons. Because of this kink in the chains, the molecules cannot form a neat, compact lattice and tend to coil, so unsaturated triglycerides often melt below room temperature and are thus classified as oils.
Unsaturated fat (oil)—includes cis double bond
Kink Poor packing
Fats and oils are the most concentrated source of energy. They provide approximately 9 kcal of energy per gram, compared to 4 kcal/g for proteins and carbohydrates. They are carriers of fat-soluble vitamins and essential fatty acids. They also contribute to food flavor and mouth-feel as well as to the sensation of product richness. They are used as frying fats or cooking oils where their role is to provide a controlled heatexchange medium as well as to contribute to color and flavor. They are also used in many other commercial applications, including soaps, detergents, and emulsifiers, printing inks, protective coatings, and feeds for domesticated animals. Waxes are monoesters of long-chain fatty acids, usually containing 24 to 28 carbon atoms, with long-chain fatty primary alcohols. A fatty alcohol has a primary alcohol group (—CH2OH) attached to a
16-to-36 carbon atoms long chain. Waxes are normally saturated and are solids at room temperature. Wax-ester of fatty acid and fatty alcohol
Plant waxes are usually found on leaves or seeds. Thus, cabbage leaf wax consists of the primary alcohols C12 and C18—C28 esterified with palmitic acid and other acids. The dominant components are stearyl and ceryl alcohol (C26H53OH). In addition to primary alcohols, esters of secondary alcohols, for example, esters of nonacosane-15-ol, are present:
Waxes can be classified according to their origins as naturally occurring or synthetic. The naturally occurring waxes can be classified into animal, vegetable, and mineral waxes. Beeswax, spermaceti, wool grease, and lanolin are important animal waxes. The vegetable waxes include carnauba, ouricouri, and candelilla. Petroleum waxes are the most prominent mineral waxes. Paraffin wax is petroleum wax consisting mainly of normal alkanes with molecular weights usually less than 450. Microcrystalline wax is another type of petroleum wax containing substantial amounts of hydrocarbons other than normal alkanes, and the components have higher molecular weights than paraffin wax components. Compositions of significant commercial waxes from natural sources are given in Table 4.1 [I]. 4.2
Fatty Acids
The carboxylic acids obtained from the hydrolysis of fat or oil are called fatty acids. They are the building blocks of the triglycerides and the fats and oils are often named as derivatives of these fatty acids. For example, the tristearate of glycerol is named tristearin and the tripalmitate of glycerol is named tripalmitin. Normal saturated fatty acids have a long, unbranched hydrocarbon chain having a general formula CH3(CH2)nCOOH, where n is usually even and varies from 2 to 24. The unsaturated fatty acids may have one double bond (monosaturated) or have more than one ds-methylene interrupted double bond (polyunsaturated) as illustrated in Fig. 4.1.
TABLE 4.1
Sources and Compositions of Normal Waxes
Type Animal waxes Beeswax Chinese Shellac Spermaceti Wool (anhydrous lanolin) Mineral waxes Montan Petroleum waxes Microcrystalline Paraffin Vegetable waxes Bayberry Candelilla Carnauba Esparto Japan Jojoba (a liquid wax) Ouricury Sugarcane
Melting point 0C 64
Main components
36-42
Myricyl palmitate Isoheptacosyl isoheptacosanoate, ceryl lignocerate Ceryl lignocerate, ceryl cerotate Cetyl palmitate Cholesteryl estolidic esters, alcohol esters of iso- and anteiso acids
86
Tricontanyl esters of C28-3o acids
71-88 54-57
Hydrocarbons (490-800 molecular weights) Hydrocarbons (350-420 molecular weights)
43-48 70-80 80-85 69-81 51-62 11-12 79-85 79-81
Trimyristin, tristearin C29_33 hydrocarbons, simple esters and lactones Esters of C26-3o alcohols and C26-3o co-hydroxy acids Hydrocarbons, esters of C26_32 acids and alcohols Tripalmitin Docosenyl eicosanoate Myricyl cerotate and hydroxycerotate Myricyl palmitate stigmasteryl palmitate
82-84 81-82
SOURCE: Riegel's Handbook of Industrial Chemistry, 9th ed., 1992.
The following three systems of nomenclature are in use for naming fatty acids. a. The common (trivial) names b. The number of carbon atoms in the chain c. The International Union of Pure and Applied Chemistry (IUPAC) system The trivial names that indicate the initial source of fatty acids are used more often than the IUPAC names in the industry. For example, butyric acid is a major component of butter flavor, palmitic acid comes from palm kernel, and oleic acid from olives. The number of carbon atoms in the chain, followed by a colon and additional numbers indicating the number of double bonds, are used as an abbreviation to designate fatty acids. Thus in the 18-carbon series, C18:0, C18:l, C18:2, and C18:3 represent stearic, oleic, linoleic, and linolenic acids, respectively. One or two letter abbreviations are also used, and these four acids sometimes are designated by St, O, L, and Ln, respectively.
Saturated
Stearic acid
Mono-unsaturated
Oleic acid
Linolenic acid
Polyunsaturated
Linolenic acid
y - Linolenic acid
Arachidonic acid-important fatty acid for animals Figure 4.1 The structure of some fatty acids.
The IUPAC name of fatty acid is that of the alkane parent with the -e changed to -oic acid. The carboxyl carbon is carbon 1: CH3.CH2.CH2.CH2.CH2.CH2.CH2.CH3 (octane) 8
7
6
5
4
3
2
1
CH3.CH2.CH2.CH2.CH2.CH2.CH2.COOH (octanoic acid) The suffix dioic is used if the acid contains two carboxyl groups. The double bonds in fatty acids differ in (a) number, (b) location, (c) geometrical configuration, and (d) conjugation. Conjugation is a special case of location in which two double bonds are separated only by a single carbon-carbon bond. Unsaturated fatty acids may have one or as many as six double bonds. Those containing multiple double bonds usually have a methylene (CH2) group between the double-bond sequence, so the system is not conjugated.
When double bonds are present, the suffix anoic is changed to enoic, dienoic, or trienoic to indicate the number of bonds present. The location of the first carbon in the double bond is indicated by a number preceding the IUPAC systematic name. The geometric configuration of double bonds is indicated by the Latin prefixes cis- (both hydrogens on one side) and trans- (hydrogen across from each other). Unsaturation between the 9 and 10 carbons with cis orientation is most common in polyunsaturated fatty acids. Accordingly, oleic, linoleic, and linolenic acids are called 9-octadecenoic, 9,12-octadecadienoic and 9,12,15-octadecatrienoic acids, respectively. Table 4.2 lists some common examples of fatty acids, their sources, common names, and systematic names [I]. Many additional terms are used to distinguish unsaturated fatty acids by the location of the first double bond relative to the omega (co) or —CH3 carbon. Thus oleic acid is both A9 and a C18:l co-9 acid. Linoleic acid is a A9'12 and C18:2 co-6 acid. Linolenic acid is both A9'12'15 and a C18:3 co-3 acid.
TABLE 4.2 Some Important Fatty Acids, Their Names and Common Sources
Carbon atoms
Common name
Saturated fatty acids 3 Propionic 4 Butyric 5 Valeric 5 Isovaleric 6 Caproic 8 Caprylic 10 Capric
Systematic name
Common sources
12 14 16 18 19 20 22
Laurie Myristic Palmitic Stearic Tuberculostearic Arachidic Behenic
Dodecanoic Tetradecanoic Hexadecanoic Octadecanoic 10-Methylstearic Eicosanoic Docosanoic
24
Lignoceric
Tetracosanoic
26 28 30
Cerotic Montanic Mellisic
Hexacosanoic Octacosanoic Triacontanoic
Bacterial fermentation Milk fats Bacterial fermentation Dolphin and porpoise fats Milk fats, some seed oils Milk fats, Palmae seed oils Sheep and goat milk, palm seed oils, sperm head oil Coconut oil Palm and coconut oils Palm oil Animal fats Tubercle bacillus lipids Some animal fats Peanut and various other seed oils Minor amounts in some seed oils Plant waxes Beeswax and other waxes Beeswax and other waxes
9-Decenoic 2,4-Decadienoic 2-Dodecenoic 9,12,15-Octadecatrienoic
Milk fats Stillingia oil Butterfat Linseed oil
Unsaturated fatty acids 10 Caproleic 10 Stillingic 12 Lauroleic 18 Linolenic
Propanoic Butanoic Pentanoic 3-Methylbutanoic Hexanoic Octanoic Decanoic
SOURCE: Riegel's Handbook of Industrial Chemistry, 9th ed., 1992.
4.3
Glycerides
Glycerol can be esterified commercially with one, two, or three fatty acids to produce mono-, di-, or triglycerides. Fats and oils are naturally occurring triglycerides, the distribution of which varies in different plants and animals.
Fatty acids
(Glycerol) (A fat with three different carboxylic acid-triglyceride)
The properties of triglycerides depend on the fatty acid composition and on the relative location of fatty acids on the glycerol. As accurate methods for determining the composition are available, several conventions have been developed to specify arrangements of fatty acids on the glycerol molecule. Natural fats and oils are designated as the triglyceride type in terms of saturated and unsaturated acids and isomeric forms. Table 4.3 illustrates the triglyceride types and isomeric forms of some natural fats. GS3 in the table refers to a fully saturated glyceride and GS2U refers to a glyceride composed of two saturated acids and one unsaturated acid. Distinguishing between the 1, 3, and 2 positions permits identification of the SUS and SSU isomers of GS2U and the USU and UUS isomers of GSU2. A stereospecific numbering system (Sn) is used to indicate the location of specific fatty acids in triglyceride molecules such as in 1-stearoyl2-oleoyl-3-myristoyl-Sn-glycerol; the respective fatty acids are indicated in the 1, 2, and 3 positions. This kind of information is very valuable in relating properties of certain fats to compositional data. Table 4.4
TABLE 4.3 Triglyceride Types and Isomeric Forms of Natural Fats
Types (% wt)
Pig fat (lard) Peanut oil Beef fat (tallow) Cocoa butter Soybean oil
Isomers (% wt)
GS3
GS2U
GSU2
GU3
SUS
SSU
USU
UUS
2.5 0.1 12.6 7.1 0
22.4 9.9 43.7 67.5 3.7
55.7 42.5 35.3 23.3 31.0
19.4 47.5 8.4 2.1 65.3
1.0 9.3 30.6 65.0 3.7
21.4 0.6 13.1 2.5 0
46.9 0.7 3.4 0.2 0
8.8 41.8 31.9 23.1 31.0
TABLE 4.4 Fatty Acid Composition of Some Edible Oils and Fats Source Almond oil Avocado oil Butter fat Canola oil Cocoa butter Coconut oil Corn oil Cotton seed oil Fish (manhaden) oil Lard Mustard seed oil Olive oil Palm oil Palm kernel oil Peanut oil Rapeseed oil Rice bran oil Safflower oil Sesame oil Soybean oil Sunflower oil Tallow Walnut oil
COR
CH2OH • } RCOOCH3 + CHOH +Na0H
CH2OH
3RCOONa + CH3OH 4.5.3
Interesterification
Interesterification causes a fatty acid redistribution within and among triglyceride molecules, which can lead to substantial changes in the
physical properties of fats and oils or their mixtures without altering the chemical structure of the fatty acids. Intermolecular acyl groups exchange triglycerides in the reaction until an equilibrium is reached, which depends on the structure and composition of the triglyceride molecules. The reaction is very slow even at 200 to 3000C, but the rate of reaction can be accelerated by using sodium methylate.
Interesterification may be either random or directed. In random interesterification the acyl groups are randomly distributed as demonstrated by the following example where equal proportions of tristearin (S—S—S) and triolein (O—O—O) are allowed to interesterify.
In directed interesterification, the reaction temperature is lowered until the higher-melting and least-soluble triglyceride molecule in the mixture crystallizes. In this way, a fat can be separated into higher and lower melting fractions.
The directed interesterification is of much industrial significance because it can be used to convert oils into more and/or less saturated fractions of original fat or oil or blend of two oils. 4.5.4
Hydrogenation
The unsaturated double bonds in a fatty-acid chain are converted to saturated bonds by addition of hydrogen. The reaction between the liquid oil and hydrogen gas is accelerated by using a suitable solid catalyst such as nickel, platinum, copper, or palladium. Hydrogenation is exothermic,
and leads to an increase in melting point and drop in iodine value. Partial hydrogenation can lead to isomerization of cis double bonds (geometrical isomerization). Polyunsaturated fatty acids such as linolenic acid (C18:3) are hydrogenated more quickly to linoleic (C18:2) or oleic acid (C18:1) than linoleic to oleic acid or oleic acid to stearic acid (C18:0). The conversion steps can be represented as follows:
4.5.5
Isomerization
The configuration of the double bond in naturally occurring fatty acids, present in oils and fats, is predominantly in the cis form. Isomerization can occur if oils and fats are heated at temperatures above 1000C in the presence of bleaching earths or catalysts such as nickel, selenium, sulfur, or iodine. Two types of isomerization spontaneously occur during hydrogenation: geometrical and positional. The extent to which isomerization occurs can be affected by processing conditions and catalyst selection. 4.5.6
Polymerization
Under deep-frying conditions (200 to 3000C) the unsaturated fatty acids undergo polymerization reactions forming dimeric, oligomeric, and polymeric compounds. The rate of polymerization increases with increasing degree of unsaturation: saturated fatty acids are not polymerized. In thermal polymerization polyunsaturated fatty acids are first isomerized into conjugated fatty acids, which in turn interact by Diels-Alder reactions producing cyclohexene derivatives. The cyclohexene ring is readily dehydrogenated to an aromatic ring; hence compounds related to benzoic acid can be formed. On the other hand, oxidative polymerization involves formation of C —O —C bonds. Polymers with ether and peroxide linkages are formed in the presence of oxygen. They may also contain hydroxy, oxo, or epoxy groups. Such compounds are undesirable in deep-fried oil or fat because they permanently diminish the flavoring characteristics of the oil or fat and also cause a foaming problem. 4.5.7
Autoxidation
Fats and oils often contain double bonds. Autoxidation of a fat or oil yields a mixture of products that include low molecular weight carboxylic acids, aldehydes, and methyl ketones.
Drying oils such as linseed oil contain many double bonds. These oils are purposely allowed to undergo air oxidation leading to a tough polymer film on the painted surface. The autoxidation reactions involve three steps: initiation, propagation, and termination. The initiation step leads to the formation of a hydroperoxide on a methylene group adjacent to a double bond; this step proceeds via a free-radical mechanism: (H abstraction)
Hydroperoxide
The second step, which is also a reaction in the propagation cycle, is the addition of another molecule to the hydroperoxide radical to generate new free radicals. The chain length of these two radical-reaction steps is about 100. When the radical concentration has reached a certain limit, the chain reaction is gradually stopped by mutual combination of radicals, the termination step. Numerous compounds result from these reactions. For example, Table 4.7 lists a series of aldehydes and methyl ketones derived preferentially when tristearin is heated in air at 192°C [2].
TABLE 4.7
Oxidation Products of Tristearin
Class of compounds Alcohols v-Lactones Alkanes Acids Aldehydes Methyl ketones
% Portion
C-number
2.7 4.1
4—14 4-14
8.8 9.7 36.1 38.4
4-17 2-12 3-17 3-17
Major compounds n-Octanol, n-Nonanol, n-Deanol v-Butyroactone, v-Pentalactone, v-Heptalactone n-C 7 , nC- 9 , nC- 10 Caproic, Butyric n Hexanal, n Octanal 2-Heptanone, 2-Decanone
4.6
Sources of Edible Oils and Main Fats
Several hundred plants and animals produce fats and oils in sufficient quantities to warrant processing into edible oils; however, only a few sources are commercially significant. Table 4.8 summarizes the major sources in the world and the method of processing. The organ fats of domestic animals, such as cattle and hogs, and milk fat are important raw materials for fat production. Edible oils are mostly of plant origin. Olive oil and palm oil are extracted from fruits. All other oils are extracted from oilseeds. The world production of oilseeds and other crops has significantly increased in recent years to meet the growing needs for oils and fats in the world. World oilseed production in 2003 was 335.9 million tons [3]. Figure 4.2 shows world production of the different oilseeds. Soybeans constitute the largest share (56 percent) and the United States is the main crop-grower. Malaysia grows mainly palm. The Philippines grows coconut. China, Europe, India, and Canada grow rapeseed (canola). Sunflower is grown in the United States, Australia, Europe, and Argentina. The cottonseed market is dominated by the United States, China, Pakistan, India, and the former Soviet Union. World vegetable oil consumption in 2003 was 87.2 million tons. U.S. consumption was 9.91 million tons. In the U.S. market, animal fats (tallow and lard) have a relatively small share (2 percent) compared to vegetable oils. The consumption of four oils—soybean (80 percent), corn (4 percent), canola (4 percent), and cottonseed (3 percent) has grown rapidly over the past 30 years compared to the traditional oils and animal fats. Figure 4.3 shows U.S. consumption of edible fats and oils in 2003 [3].
TABLE 4.8
Major Edible Fats and Oils in the World and Methods of Processing
Source Soybean Corn (germ)
Oil content (%) 19 40
Tallow (edible tissue) Canola Coconut (dried copra) Cottonseed
70-95 42 66 19
Lard (edible tissue) Palm Palm kernel Sunflower Peanut (shelled)
70-95 47 48 40 47
Prevalent method of recovery Direct solvent extraction Wet or dry milling and prepress solvent extraction Wet or dry rendering Prepress solvent extraction Hard pressing Hard pressing or prepressing or direct solvent extraction Wet or dry rendering Hard pressing Hard pressing Prepress solvent extraction Hard pressing or prepress solvent extraction
Soybeans 56% Copra 2%
Palm kernel 2% Rapeseed Sunflowerseed 8%
12%
Cottonseed 10%
Peanut 10%
Million short tons Soybeans Rapeseed Cottonseed Peanut Sunflowerseed Palm kernel Copra Total
Million metric tons
209.5 43 38.7 35.4 28.7 8.9 5.9 370.1
190.1 39 3 2
^ 26 8.1 5.4 335 9 -
Source: USDA Figure 4.2
4.7
World oilseed production 2003.
Oils and Fats: Processing and Refining
Crude fats and oils consist primarily of glycerides. However, they also contain many other lipids in minor quantitites. Corn oil, for example, may contain glycerides plus phospholipids, glycolipids, many isomers of sitosterol and stigmasterol (plant steroids), several tocopherols (vitamins E), vitamin A, waxes, unsaturated hydrocarbons such as squalane and dozens of carotenoids and chlorophyll compounds, as well as many products of decomposition, hydrolysis, oxidation, and polymerization of any of the natural constituents. All crude oils and fats obtained after rendering, crushing, or solvent extraction, inevitably contain variable amounts of nonglyceridic coconstituents like fatty acids, partial glycerides (mono- and diglycerides), phosphatides, sterols, tocopherols, hydrocarbons, pigments (gossypol, chlorophyll), vitamins (carotene), sterol glucosides, protein fragments as well as resinous and mucilaginous materials, traces of pesticides, and
Soybeans 80%
(Other 1) 10% Peanut 1% Coconut 1% Edible tallow 1%
Canola (rapeseed) 4%
Lard 1%
Cottonseed 2% Million pounds
Soybeans Canola (rapeseed) Cottonseed Lard Edible tallow Coconut Peanut (Other 1) Total Source: USDA
Million metric tons
17,471 857 482 205 236 310 185 2,112 21,858
7.92 0.39 0.22 0.09 0.11 0.14 0.08 0.96 9.91
Figure 4.3 U.S. fats and oils edible consumption, 2003.
"heavy" metals. Table 4.9 shows some of the most important coconstituents found in some major oils [4]. Some of these materials are highly undesirable and must be removed to provide satisfactory processing characteristics and to provide desirable color, odor, flavor, and keeping qualities in the finished products.
TABLE 4.9
Minor Components in Some Major Oils
Component
Soybean oil
Canola
Palm oil
FFA Phosphatides Sterols/triterpenic alcohols Tocopherols/tocotrienols Carotenoids Chlorophyll/pheophytine Peroxides meg O2/kg Fe Cu
0.3-0.8% 1.0-3.0% 0.04-0.07% 0.06-0.2% 40-50 ppm 1-2 ppm
