Triglycerides are comprised predominantly of fatty acids present in the form of esters of glycerol. One hundred grams of fat or oil will yield approximately 95 grams of fatty acids. Both the physical and chemical characteristics of fats are influenced greatly by the kinds and proportions of the component fatty acids and the way in which these are positioned on the glycerol molecule. The predominant fatty acids are saturated and unsaturated carbon chains with an even number of carbon atoms and a single carboxyl group as illustrated in the general structural formula for a saturated fatty acid given below:
Saturated carbon chain carboxyl group
Edible oils also contain minor amounts of branched chain and cyclic acids. Also straight chain acids with an odd number of carbon atoms are typically found in animal fats.
Fatty acids occurring in edible fats and oils are classified according to their degree of saturation.
1. Saturated Fatty Acids. Those containing only single carbon-to-carbon bonds are termed “saturated” and are the least reactive chemically.
The saturated fatty acids of practical interest are listed in Table II by carbon chain length and common name. The principal fat sources of the naturally occurring saturated fatty acids are also included in the table.
|Typical Fat Source|
|Tetradecanoic||Myristic||14||54.4||Butterfat, coconut oil|
|Hexadecanoic||Palmitic||16||62.9||Most fats and oils|
|Octadecanoic||Stearic||18||69.6||Most fats and oils|
*A number of saturated odd and even chain acids are present in trace quantities in many fats and oils.
The melting point of saturated fatty acids increases with chain length. Decanoic and longer chain fatty acids are solids at normal room temperatures.
2. Unsaturated Fatty Acids. Fatty acids containing one or more carbon-to-carbon double bonds are termed “unsaturated.” Some unsaturated fatty acids in food fats and oils are shown in Table III. Oleic acid (cis-9-octadecenoic acid) is the fatty acid that occurs most frequently in nature.
Saturated and unsaturated linkages are illustrated below:
When the fatty acid contains one double bond it is called “monounsaturated.” If it contains more than one double bond, it is called “polyunsaturated.” In the International Union of Pure and Applied Chemistry (IUPAC) system of nomenclature, the carbons in a fatty acid chain are numbered consecutively from the end of the chain, the carbon of the carboxyl group being considered as number 1. By convention, a specific bond in a chain is identified by the lower number of the two carbons that it joins. In oleic acid (cis-9-octadecenoic acid), for example, the double bond is between the ninth and tenth carbon atoms.
Another system of nomenclature in use for unsaturated fatty acids is the “omega” or “n minus” classification. This system is often used by biochemists to designate sites of enzyme reactivity or specificity. The terms “omega” or “n minus” refer to the position of the double bond of the fatty acid closest to the methyl end of the molecule. Thus, oleic acid, which has its double bond 9 carbons from the methyl end, is considered an omega-9 (or an n-9) fatty acid. Similarly, linoleic acid, common in vegetable oils, is an omega-6 (n-6) fatty acid because its second double bond is 6 carbons from the methyl end of the molecule (i.e., between carbons 12 and 13 from the carboxyl end). Eicosapentaenoic acid (EPA), found in many fish and algal oils, is an omega-3 (n-3)fatty acid. Alpha-linolenic acid (ALA), found in certain vegetable oils, is also an omega-3 (n-3) fatty acid.
When two fatty acids are identical except for the position of the double bond, they are referred to as positional isomers. Fatty acid isomers are discussed at greater length in subparagraph C of this chapter.
Because of the presence of double bonds, unsaturated fatty acids are more reactive chemically than are saturated fatty acids. This reactivity increases as the number of double bonds increases.
Although double bonds normally occur in a non-conjugated position, they can occur in a conjugated position (alternating with a single bond) as illustrated below:
With the bonds in a conjugated position, there is a further increase in certain types of chemical reactivity. For example, fats are much more subject to oxidation and polymerization when bonds are in the conjugated position.
3. Polyunsaturated Fatty Acids. Of the polyunsaturated fatty acids, linoleic, linolenic, arachidonic, eicosapentaenoic, and docosahexaenoic acids containing respectively two, three, four, five, and six double bonds are of most interest.
Vegetable oils are the principal sources of linoleic and linolenic acids. Arachidonic acid is found in small amounts in lard, which also contains about 10% of linoleic acid. Fish and algae based oils contain large quantities of a variety of longer chain fatty acids having three or more double bonds including eicosapentaenoic (EPA) and docosahexaenoic acids (DHA).
C. Isomerism of Unsaturated Fatty Acids
Isomers are two or more substances composed of the same elements combined in the same proportions but differing in molecular structure. The two important types of isomerism among fatty acids are (1) geometric and (2) positional.
1. Geometric Isomerism. Unsaturated fatty acids can exist in either the cis or trans form depending on the configuration of the hydrogen atoms attached to the carbon atoms joined by the double bonds. If the hydrogen atoms are on the same side of the carbon chain, the arrangement is called cis. If the hydrogen atoms are on opposite sides of the carbon chain, the arrangement is called trans, as shown by the following diagrams. Conversion of cis isomers to corresponding trans isomers result in an increase in melting points as shown in Table III.
A comparison of cis and trans molecular arrangements.
|Systematic Name||Common Name||No. of
|Typical Fat Source|
|9-Hexadecenoic||Palmitoleic||1||16||-||Some fish oils, beef fat|
|9-Octadecenoic||Oleic||1||18||16.3||Most fats and oils|
|9-Octadecenoic*||Elaidic||1||18||43.7||Partially hydrogenated oils|
|9,12-Octadecadienoic||Linoleic||2||18||-6.5||Most vegetable oils|
|9,12,15-Octadecatrienoic||Linolenic||3||18||-12.8||Soybean oil, canola oil|
|9-Eicosenoic||Gadoleic||1||20||-||Some fish oils|
|5,8,11,14,17-Eicosapentaenoic||-||5||20||-53.5||Some fish and algal oils|
|4,7,10,13,16,19-Docosahexaenoic||-||6||22||-||Some fish and algal oils|
*All double bonds are in the cis configuration except for elaidic acid and vaccenic acid which are trans.
Elaidic and oleic acids are geometric isomers; in the former, the double bond is in the trans configuration and in the latter, in the cis configuration. Generally speaking, cis isomers are those naturally occurring in food fats and oils. Trans isomers occur naturally in ruminant animals such as cows, sheep and goats and also result from the partial hydrogenation, and to a limited extent, the deodorization of fats and oils.
2. Positional Isomerism. In this case, the location of the double bond differs among the isomers. Vaccenic acid, which is a minor acid in tallow and butterfat, is trans-11-octadecenoic acid and is both a positional and geometric isomer of oleic acid.
The position of the double bonds affects the melting point of the fatty acid to a limited extent. Shifts in the location of double bonds in the fatty acid chains as well as cis-trans isomerization may occur during hydrogenation.
The number of positional and geometric isomers increases with the number of double bonds. For example, with two double bonds, the following four geometric isomers are possible: cis-cis, cis-trans, trans-cis, and trans-trans. Trans-trans dienes, however, are present in only trace amounts in partially hydrogenated fats and thus are insignificant in the human food supply.