Fat and Oil

Definition of lipid is “a wide variety of natural products including fatty acids and their derivatives, steroids, terpenes, carotenoids and bile acids, which have in common a ready solubility in organic solvents such as diethyl ether, hexane, benzene, chloroform or methanol.” (Christie, 1982 in O’Keefe, 2002).

The terms fats and oils are used interchangeably, and the choice of terms is usually based on the physical state of the material at ambient temperature and tradition. Generally, fats appear solid at ambient temperatures and oils appear liquid. In the final analysis, it is the chemical composition that defines the characteristics of the individual fat or oil, which, in turn, determines the suitability of this ingredient in various processes and applications. Most of the oil-bearing tree fruits and kernels provide the highest oil yields. Oil seeds are obtained from annual plants that must be replanted each year, whereas fruit oils are harvested from trees with long life spans (O’Brien, 2009).

An important contributor to the availability of fats and oils is the fact that most fats and oils production is controlled by factors other than the demand. Weather is another factor that affects both availability and demand for fats and oils. For example, a hurricane in Malaysia can decrease the oil available on the world market (O’Brien, 2009).

Climate and availability certainly influenced the eating habits of our ancestors. Inhabitants of central and northern Europe obtained their edible fats from animals, whereas people in southern Europe, Asia, and Africa acquired their edible oils from vegetable sources. The food products developed in these different regions used the available fats and oils products. Consequently, the cuisine of central and northern European countries developed around the use of solid fats, such as butter, lard, and tallow for breads, pastries, and other baked and fried products. Similarly, the diets of inhabitants from warmer climates were developed around the use of liquid oils for their food products, which include many sauces and dressings. These trends appear to continue to be the preference of their descendants (Markey, 1960).

Fats and oils consumption has been categorized into visible and invisible sources. Visible fats and oils are those isolated from animal tissues, oilseeds, or oil fruits and used for food preparation as shortening, margarine, or liquid oil or for specialty uses. Invisible fats and oils are consumed as part of meats, poultry, eggs, dairy products, fish, fruits, or vegetables and account for approximately 60% of an individual’s fat consumption (O’Brien, 2009).

Fatty acids are almost entirely straight chain aliphatic carboxylic acids. The broadest definition includes all chain lengths, but most natural fatty acids are C4 to C22, with C18 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 (Scrimgeour, 2005).

The fatty acid components are distinguished in three ways: (1) chain length, (2) number and position of the double bonds, and (3) position of the fatty acids within the glyceride molecule. Variations in these characteristics are responsible for the chemical and physical differences experienced with edible fats and oils (O’Brien, 2009).

The fatty acid compositions of natural fats and oils vary significantly depending, not only on the plant or animal species, but also within the same species. Among the factors that affect the vegetable oil fatty acid compositions are climate conditions, soil type, growing season, plant maturity, plant health, microbiological conditions, seed location within the flower, and genetic variation of the plant (Sonntag, 1979 in O’Brien, 2009). Table 2.1. demonstrates the nomenclature, structure, and occurrence of fatty acid.

The melting point of a lipid is dependent on both the degree of unsaturation and the chain length (O’Brien, 2009). The melting point increases with chain length and decreases with increased unsaturation. 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 (Scrimgeour, 2005). Vegetable oils’ saturated fatty acids are predominately even numbered carbon atoms ranging from 4 to 24 (O’Brien, 2009). Table 2.2. presents melting point of some fatty acid and methyl ester.

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 (Scrimgeour, 2005).

 

 

Table 2.1. Nomenclature, Structure, and Occurrence of Fatty Acid

Fatty acid

Common name

Formula

Chain length

Significant Sources

4:0 butyric CH3(CH2)2CO2H short butter, dairy fats
6:0 caproic CH3(CH2)4CO2H short coconut, palm kernel
8:0 caprylic CH3(CH2)6CO2H short/medium coconut, palm kernel
10:0 capric CH3(CH2)8CO2H medium coconut, palm kernel
12:0 lauric CH3(CH2)10CO2H medium coconut, palm kernel
14:0 myristic CH3(CH2)12CO2H medium coconut, palm kernel
16:0 palmitic CH3(CH2)14CO2H   cottonseed, palm
18:0 stearic CH3(CH2)16CO2H   cocoa butter, tallow
18:1 9c oleic CH3(CH2)7CH=CH(CH2)7CO2H   cottonseed, olive, palm, rape
18:2 9c12c linoleic CH3(CH2)4(CH=CHCH2)2(CH2)6CO2H   corn, sesame, soybean, sunflower
18:3 9c12c15c α-linolenic CH3CH2(CH=CHCH2)3(CH2)6CO2H   Linseed
22:1 13c erucic CH3(CH2)7CH=CH(CH2)11CO2H long high erucic rape
20:5 5c 8c11c14c17c EPA* CH3CH2(CH=CHCH2)5(CH2)2CO2H long fish and animal fats
22:6 4c7c10c13c16c19c DHA* CH3CH2(CH=CHCH2)6CH2CO2H long fish and animal fats

*Abbreviations of the systematic names eicosapentaenoic acid and docosahexaenoic acid.

(Scrimgeour, 2005)

 

 

 

Table 2.2. Melting Point of Some Fatty Acids and Methyl Ester

Fatty Acid

Melting Point (oC)

16:0 62.9 (30.7)*
17:0 61.3 (29.7)*
18:0 70.1 (37.8)*
18:1 9c 16.3, 13.4
18:1 9t 45
18:2 9c12c -5
18:2 9t12t 29
19:0 69.4 (38.5)*
20:0 76.1 (46.4)*

*Values for methyl esters in parenthesis

(Scrimgeour, 2005)

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. 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 (Gordon, 2002).

Acylglycerols are the predominant constituent in oils and fats of commercial importance. Glycerol can be esterified with one, two, or three fatty acids, and the individual fatty acids can be located on different carbons of glycerol. The terms monoacylglycerol, diacylglycerol, and triacylglycerol are preferred for these compounds over the older and confusing names mono-, di-, and triglycerides (Gunston and Padley, 1997).

The fatty acids with two hydrogen atoms bonded to each carbon atom in the chain are saturated, that is, they contain no double bonds between carbons. Saturated fatty acids generally vary in chain length from 4 to 24 carbons atoms. Saturated fatty acids, with some exceptions, have straight, even numbered carbon chains. They are the least reactive and have a higher melting point than unsaturated fatty acids of the same chain length due to the dense packing of the unbranched chain structure into the crystal lattice. The fatty acids identified without double bonds are saturated (O’Brien, 2009).

The saturated fatty acids with 2 to 6 carbon atoms are short chain fatty acid. These short-chain fatty acids have little or no effect on cholesterol, are a liquid at room temperature, and vaporize readily at high temperatures. Saturated fatty acids with 8 to 12 carbon atoms are medium chain fatty acid. Medium chain fatty acids are thought to be directed to the liver and burned as energy rather than being stored in the body as fat. They provide 8.3 calories/gram compared with 9.2 for the other fatty acids. Laboratory animal and human research revealed that medium-chain fatty acids act more like carbohydrates than saturates, that is, they do not raise serum cholesterol levels. Esters of medium-chain fatty acids with glycerol are critical ingredients in sports foods, clinical nutrition, and infant formulations (Wainwright, 2000; Grundy and Denke, 1999; Nicolosi, 1997 in O’Brien, 2009).  Saturated fatty acids with 14 to 24 carbon atoms are classified as long-chain fatty acids. The most notable long chain saturates are those within the 14 to 18 carbon atom range (O’Brien, 2009).

The fatty acids that contain double bonds between the carbon atoms are termed unsaturated. As many as seven double bonds have been reported; fatty acids with an excess of three double bonds are most likely of aquatic origin. Those containing 1, 2, and 3 double bonds and 18 carbon atoms are the most important unsaturated fatty acids of vegetable and land animal origin. Normal double bonds in the cis form cause a bend in the carbon chain, which restricts the freedom of the fatty acid. This bend becomes more pronounced as the number of double bonds increase. The presence of double bonds also makes the unsaturated fatty acids more chemically reactive than the saturated fatty acids and this activity increases as the number of double bonds increase. The notable reactions are oxidation, polymerization, and hydrogenation (O’Brien, 2009).

Monounsaturated fatty acids have only one double bond. This fatty acid class is the least reactive of the unsaturated fatty acids. Of the monounsaturated fatty acids, oleic and palmitoleic are the most widely distributed and oleic is considered the most important. Polyunsaturated fatty acids have two or more double bonds. Chemically reactivity increases as the number of double bonds increase. Polyunsaturated fatty acids with two to six double bonds are of considerable interest nutritionally. Vegetable oils are the principal source of the two essential fatty acids: linoleic and linolenic (O’Brien, 2009).

 

 

 

 

Table 3. Omega-3 and Omega-6 Polyunsaturated Fatty Acid

Omega Family Systematic Name Common Name Symbol
omega-3 cis-9,12,15-octadecatetraenoic α-linolenic C-18:3
omega-3 cis-6,9,12,15-octadecatetraenoic stearidonic C-18:4
omega-3 cis-5,8,11,14,17-eicosapentaenoic EPA C-20:5
omega-3 cis-7,10,13,16,19-docaosapentaenoic clupanodonic C-22:5
omega-3 cis-4,7,10,13,16,19-docsahexaenoic DHA C-22:6
omega-6 cis-9,12-octadecenoic linoleic C-18:2
omega-6 cis-6,9,12-octadecatrienoic γ-linolenic C-18:3
omega-6 cis-8,11,14-eicosatrienoic dihomo-γ-linolenic C-20:3
omega-6 cis-5,8,11,14-eicosatetraenoic arachidonic C-20:4

(O’Brien, 2009)

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