Lipids are broadly classified in to simple, complex and derived, which are further subdivided into different groups.

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1 Paper No. 01 Paper Title: Food Chemistry Module -9: Classification of lipids Lipids are organic substances which are insoluble in water and soluble in organic solvents. Lipids are not polymers and exist as small molecules. Lipids can be classified based on physical properties, polarity, their essentiality and their structure. Lipids are broadly classified in to simple, complex and derived, which are further subdivided into different groups. Simple lipid: These are esters of fatty acids with various alcohols. Simple lipids are two types. a. Fats and oils: These are esters of glycerol and fatty acids. The structure of fatty acids determines the physical characteristics of fats and oils. They are the major form of energy storage in animals. Fats are solids at room temperature and most come from animal sources. Oils are liquids at room temperature and the most commonly used oils come from plant sources. b. Waxes: Waxes are esters of fatty acids with alcohol other than glycerol. The esterified alcohol may be aliphatic or alicyclic. Waxes are secreted by sebaceous glands in the skin of animals and perform mostly external protective functions. Cetyl alcohol is most commonly found in waxes. Complex or compound lipids: These are esters of fatty acids with alcohols containing additional groups. These important lipids are widely distributed in plants, bacteria and animals. They are the major constituents of cell membranes but are found in circulating fluids also. a. Phospholipids: Phospholipids are a family of lipids similar to tryglycerides except that one hydroxyl group of glycerol is replaced by the ester of phosphoric acid and an amino alcohol, bonded through a phosphodiester bond. Depending on the amino

2 alcohol, these can be Lecithins (containing choline) or Cephalines (containing ethanolamine or serine). These are the most abundant lipids in cell membranes. b. Glycolipids: Lipids containing a glycosidic moiety with a glycerol or an amino alcohol and with fatty chain. Glycolipids are derived from sphingosine. They differ from sphingomyelins by having a carbohydrate group at first carbon instead of a phosphate bonded to a choline. Their role is to provide energy and also serve as markers for cellular recognition. c. Llipoproteins: Lipids without phosphate group and containing one amino acid linked to long-chain alcohol and acids. Lipoproteins circulate in blood. d. Other complex lipids: Lipids such as sulfolipids, and amino lipids and lipopolysaccharides. Derived lipids: These are the derivatives obtained on the hydrolysis of simple and complex lipids. These include fatty acids, glycerol, steroids, other alcohols, fatty aldehydes, and ketone bodies, hydrocarbons, lipid-soluble vitamins and hormones. Miscellaneous lipids: These include a large number of compounds possessing the characterstics of lipids. These categories include carotenoids, terpenes, pentacosane etc. LIPID CLASSES A. Fatty Acids 1. Saturated Fatty Acids: Saturated fatty acids are fats that contain single bond between the carbon atoms, so it is saturated with hydrogen. The saturated fatty acids begin with methanoic (formic) acid. Methanoic, ethanoic, and propanoic acids are uncommon in natural fats and are often omitted from definitions of lipids. However, they are found nonesterified in many food products. Omitting these fatty acids because they are water soluble would argue for also eliminating butyric acid, which would be difficult given its importance in dairy fats.

3 2. Unsaturated Fatty Acids: Unsaturated fatty acids are fats that contains at least one double bond within the fatty acid chain. A fatty acid chain is monounsaturated if it contains one double bond and polyunsaturated if it contains more than one double bond. By far the most common monounsaturated fatty acid is oleic acid (18:1 ω9); although more than 100 monounsaturated fatty acids have been identified in nature. The most common double-bond position for monoenes is Δ9. Polyunsaturated fatty acids (PUFAs) are best described in terms of families because of the metabolism that allows interconversion within, but not among, families of PUFA. 3. Acetylenic Fatty Acids: A number of different fatty acids have been identified having triple bond. The nomenclature is similar to double bonds except that the -ane ending of the parent alkane is replaced with -ynoic acid, -diynoic acid, ethyonic acids etc. While many acetylenic fatty acids have been prepared synthetically, only some species are found in natural oils. Shorthand nomenclature uses a lowercase a to represent the acetylenic bond; 9c,12a-18:2 is an octadecynoic acid with a double bond in position 9 and the triple bond in position 12. Since the ligands attached to triple-bonded carbons are 180 from one another (the structure through the bond is linear). The acetylenic fatty acids found in nature are usually 18-carbon molecules with unsaturation starting at Δ9 consisting of conjugated double triple bonds. 4. trans Fatty Acids trans fatty acids (trans fats) are a third form of fatty acids. trans fats do occur in very small amounts in some animal origin foods. trans fat is the common name for unsaturated fat with trans-isomer (E-isomer) fatty acid(s). The term refers to the configuration of a double carbon-carbon bond, trans fats are sometimes monounsaturated or polyunsaturated, but never saturated. trans fats exist in nature but also occur during the processing of polyunsaturated fatty acids in food production. The three main origins of trans fatty acids in our diet are bacteria, deodorized oils, and partially hydrogenated oils. The preponderance of trans fatty acids in our diets are derived from the hydrogenation process. The partial hydrogenation process produces a mixture of positional and geometrical isomers. 5. Branched Fatty Acids: A large number of branched fatty acids have been identified. The fatty acids can be named according to rules for branching in hydrocarbons. Beside standard

4 nomenclature, several common terms have been retained, including iso-, with a methyl branch on the penultimate (ω 2) carbon, and anteiso, with a methyl branch on the antepenultimate (ω 3) carbon. The iso and anteiso fatty acids are thought to originate from a modification of the normal de novo biosynthesis, with acetate replaced by 2-methyl propanoate or 2-methylbutanoate, respectively. Other branched fatty acids are derived from isoprenoid biosynthesis including pristanic acid (2,6,10,14-tetramethylpentadecanoic acid) and phytanic acid (3,7,11,15- tetramethylhexadecanoic acid). 6. Cyclic Fatty Acids: Many fatty acids that exist in nature contain cyclic carbon rings. Ring structures contain either three (cyclopropyl and cyclopropenyl), five (cyclopentenyl), or six (cyclohexenyl) carbon atoms and may be saturated or unsaturated. As well, cyclic fatty acid structures resulting from heating the vegetable oils have been identified. In nomenclature of cyclic fatty acids, the parent fatty acid is the chain from the carboxyl group to the ring structure. The ring structure and additional ligands are considered a substituent of the parent fatty acid. 7. Hydroxy and Epoxy Fatty Acids: Saturated and unsaturated fatty acids containing hydroxy and epoxy functional groups have been identified. Hydroxy fatty acids are named by means of the parent fatty acid and the hydroxy group(s) numbered with its Δ location. A fatty acid with a hydroxy substituent in the Δ2 position is commonly called an α-hydroxy acid; fatty acids with hydroxy substituent in the Δ3 and Δ4 positions are called β-hydroxy acids and γ -hydroxy acids, respectively. Cutins, which are found in the outer layer of fruit skins, are composed of hydroxy acid polymers, which also may contain epoxy groups. Epoxy acids, found in some seed oils, are formed on prolonged storage of seeds. They are named similarly to cyclopropane fatty acids, with the parent acid considered to have a substituted oxirane substituent. 8. Furanoid Fatty Acids: Some fatty acids contain an unsaturated oxolane heterocyclic group. There are more commonly called furanoid fatty acids because a furan structure (diunsaturated oxolane) is present in the molecule. Furanoid fatty acids have been identified in Exocarpus seed oils. They have also been identified in plants, algae, and bacteria and are a major component in triacylglycerols from latex rubber. They are important in marine oils and may total several percentage points of the total fatty acids or more in liver and testes.

5 B. Acylglycerols: 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. Fatty acids can be esterified on the primary or secondary hydroxyl groups of glycerol. C. Sterols and Sterol Esters: The steroid class of organic compounds includes sterols of importance in lipid chemistry. Steroids are the compounds containing a cyclic steroid nucleus namely cyclopentanoperhydrophenanthrene. There are several steroids in the biological system. These include cholesterol, bile acids, vitamin D, sex hormones, adrenocortical hormones, sitosteros and alkaloids. Sterol contains one or more hydroxyl group to the steroids. The sterols may be derived from plant (Phytosterols) or animal (Cholesterols) sources. They are widely distributed and are important in cell membranes. Sterol esters exist commonly in esters form. Reference: 1- Gunstone, F. D., Harwood, J. L. and Padley F. D. (1994) The Lipid Handbook. Chapman and Hall, New York, p IUPAC (1979) Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H. Pergamon Press, London, p IUPAC-IUB (1977) Commission on Biochemical Nomenclature. The nomenclature of lipids. Lipids 12: O Keefe. S. F. (2002) Nomenclature and classification of lipids. In: Food Lipids Chemistry, Nutrition, and Biotechnology (Akoh. C. C, Min. B. D, ed.). Marcel Dekker Inc, New York, p

6 5- Satyanarayana, U. and Chakarapani, U. (2009) Lipids. In: Biochemistry, Books and Allied (P) Ltd., Kolkata, India. P