Properties of Lipids

Suaib Luqman

epgp books

 

  1. Objectives

 

  • v To understand the property of lipids
  • v What they do
  • v How they perform

 

  1. Concept Map
  1. Description

 

3.1 Properties of Lipids

 

Lipids are a dissimilar group of biological compounds, but they are poised of a chain of hydrocarbons, ending with groups of bonded oxygen and hydrogen and share the general property of being at least partially not water soluble. They are essential to all forms of life and are the key constituent of every cell membrane on Earth. Some of key properties of lipids include energy storage for animals and other organisms while others are described below:

  1. Solubility: Lipids are soluble in non-polar solvent (ether, chloroform, benzene etc) but insoluble in water.
  2. Consistency: They are colorless, odorless and tasteless. Being lighter than water they have a specific gravity of 0.86. The consistency of lipids depends upon the presence of saturated and/or unsaturated fatty acids.
  3. Hydrolysis: It is brought about in presence of acids or alkalis under the activity of enzyme lipases. Acid hydrolysis results in the formation of glycerol and long chain of fatty acid whereas alkaline hydrolysis of fats results in the formation of sodium or potassium salts of fatty acids called as Soaps and the process is called as saponification.
  4. Hydrogenation: The conversion of liquid vegetable oil into solid vegetable ghee by reacting with hydrogen gas in the presence of catalyst (Ni) at 200º C.
  5. Emulsification: When fats or oils are rubbed with water , the large molecules of lipids breaked into smaller ones forming the emulsion and the process is called emulsification.
  6. Rancidity: When lipids are exposed to atmosphere (heat, light, air, moisture) for more than 30 days, an unusual and undesirable odour is developed. Such types of lipids are called rancid lipids and the phenomenon is termed as Rancidity. Rancidity is of two types: (a) Hydrolytic and (b) Oxidative
  7. Hydrolytic Rancidity: This type of phenomenon occurs due to liberation of volatile acids during the hydrolysis of fats.
  8. Oxidative rancidity: When a part of unsaturated fatty acid oxidized to form aldehyde ketone, the phenomenon is termed as oxidative rancidity.

Fatty acids are basic units of lipids with hydrocarbon chains amid oxygen-hydrogen ends, making it hydrophilic at one end and hydrophobic at the other. That is why soaps made by processing accustomed lipids soften in water and emulsifies lipids. Cell membranes are made from phospholipids bilayer with hydrophobic ends facing the cell’s externally and internally but the hydrophilic ends in front of each other within the membrane. In most of the lipids, polar hydrophilic areas are veiled and do not dissolve in water at all. Among fats and oils, the lone unswerving difference between fats and oils is melting temperature as both the types of triglycerides contains glycerol bonded to three fatty acid chains.

 

Besides the frequent fatty acids with unsaturated or saturated acyl chains, chemically several fatty acids are customized by hydrocarbon chain substituent e.g. the tidy duck gland secretes 10 carbons long fatty acid with methyl groups on carbons 2, 4, 6, and 8 replaced with one of the hydrogens. Few bacteria make FAs with a methyl group on the last but one carbon from the acidic group. Other bacteria integrate a cyclopropane ring in the vicinity of the acyl chain. Mycobacterium tuberculosis synthesizes α-mycolic acids, a member of cyclopropane-containing fatty acids. Monohydroxyl acids are another frequent component of a hydroxyl group, present in both animals and plants in minuscule quantity, but they are ubiquitous in bacteria.

 

Physical properties of lipids

 

 

The chaste FAs form crystals set up of stacked layers of molecules. The thickness of each layer of two extended molecules arranged in such a way that the water loving (hydrophilic) carboxylic acid groups shape the two faces and the water-fearing (hydrophobic) chains outline the inner core. In case of specific FAs, the specifics of the molecular packing might differ in forming different crystal configuration termed polymorphs. The SFAs of biological significance have melting temperatures exceeding 27° C (81° F) and rise with escalating hydrocarbon chain length. Polyunsaturated and monounsaturated molecules liquefy at considerably lower temperatures than their saturated moiety. In most of the biological molecules, the melting temperatures becomes lowest when the C=C are positioned in close proximity to the centre of the hydrocarbon chain. At room temperature, these molecules form viscous liquids and the hydrophobic nature of the majority of FAs surpasses the hydrophilic character of the carboxylic acid, creating the aqueous solubility of these molecules very squat. At 77° F (25° C), the solubility of FA is 3 × 10−6 per gram of solution and the aqueous solubility lessens rampantly with the accumulation of each carbon units to the hydrocarbon chain. This association imitates the energy necessary to transmit the molecule to water from a pure hydrocarbon solvent. With every CH2 group, for example, additional energy is obligatory to organize water molecules of FA in the region of the hydrocarbon chain resulting in the hydrophobic effect.

 

The carboxylate group separates a positively charged hydrogen ion in pure water to a very small degree thus representing the below mentioned equation:     R−COOH → RCOO− + H+

 

Where R signify the hydrocarbon chain.

 

The negative charge bearing carboxylate ion is more polar than the undissociated acid. RCOOH can be changed to RCOO− with the addition of equal number of base molecules (e.g. NaOH). In fact, it reinstates the Na+ with H+ to provide the FA salt refereed as soap. The RCOO− anions in water impulsively shape stable, spherical aggregates known as micelles that dole out the useful detergent property of soaps. The micelle diameter is more or less twice the length of the unmitigated FA. Concentrated preparations of micelles-water dispersions display immense cleansing power. Usually look like pure water, these dispersions are quite stable. Foams and bubbles on the exterior surface of soap dispersions results due to spontaneous adsorption of RCOO− ions at the edge between the air and aqueous dispersion with the mechanically stretched and vigorously stabilized effect between the air-water interfaces.

 

Chemical properties

 

 

Chemically, the acidic carboxyl group (COOH) of the fatty acid is the most reactive portion. It reacts with alcohols (R′OH) to yield esters (RCOOR′) releasing water molecule. In complex lipids, the ester (principal covalent) bond link FA moieties to other groups. Ether (R′−O−R, second chemical) bond also links FAs and are chemically more stable as compared to ester bond.

 

In fatty acids, the hydrocarbon part is relatively defiant to chemical assail except C=C and different molecules respond with similar double bond. For instance, when platinum is present as a catalyst, hydrogen append to the double bond to bestow a SFA. Halogens (Iodine, Chlorine, Bromine) and their derivatives (hydroiodic acid) also retort with the double bond to structure the SFAs, but herein the halogen (one or two atoms) swap the hydrogen atoms (one or two) usually endowed in the saturated acyl chain. C=C can also respond with oxygen in either enzymatically catalyzed oxidation reactions or non-enzymatic processes. The procedure engenders a range of products, several of which add to the stale smell in vegetable products and spoiled meat. Universally, it is known that highly unsaturated fatty acid is more easily oxidized.

 

Properties of lipids explain its consistency, solubility, hydrolysis, saponification, emulsification and rancidity.

 

Based on the chemical and physical properties, lipids undergo many reactions which are described below:

 

Reactions of fatty acids

 

Alike other carboxylic acids, fatty acids demonstrate some of the key reactions i.e. they undergo acid-base reactions and esterification.

 

 

Acidity

 

The pKa of FAs do not illustrate a vast disparity in their acidities. For instance, nonanoic acid has a pKa of 4.96, to some extent weaker than acetic acid (pKa 4.76). In water, the solubility of the FAs decreases very rapidly due to increase in the chain length, so the longer chain fatty acids have least effect on aqueous solution pH. On the other hand water insoluble FAs liquefy in warm ethanol can be titrated by NaOH solution using phenolphthalein as an indicator with a pink endpoint. This testing is utilized to ascertain the FFA content of fats i.e. the hydrolyzed amount of the triglycerides.

 

 

Hardening and Hydrogenation

 

Hydrogenation is the phenomenon extensively accomplished for unsaturated fatty acids to give SFAs which are not as much of decumbent to rancidification. Since the UFAs have inferior melting than the SFAs, the procedure is called hardening. This knowledge and technology is employed to make margarine from vegetable oils. During partial hydrogenation, the isomerization of UFAs from cis to trans configuration takes place. Use of high temperature and high pressure (forcing hydrogenation) form fatty alcohols from fatty acids. However, fatty acid esters convert to fatty alcohols more simply. Certain UFAs are cleaved in molten alkali in the Varrentrapp reaction (structure elucidation from classical significance).

 

Rancidity and Auto oxidation

 

Auto oxidation is a chemical change endured by UFAs. The process gain momentum by the charisma of trace metals and requires oxygen (air). Oils of vegetable origin defy this practice as they restrain antioxidants (tocopherol). Oils and fats and oils are repeatedly are treated with citric acid (a chelating agents) to eliminate the metal catalysts.

 

 

Ozonolysis

 

UFAs are more vulnerable to dilapidation by ozone. The ozonolysis reaction is accomplished for the transformation of oleic acid to azelaic acid ((CH2)7(CO2H)2).

 

 

Analysis

 

Chemically, the methyl esters of FAs are alienated by gas chromatography and furthermore, the unsaturated isomers separation is achieved by TLC.

 

Some of the well-known examples of lipids are as under:

 

Fatty acids:

Palmitoleic, Oleic, Linoleic and Arachidonic acid.

 

Fats & Oils:

Sunflower, Peanut, Palm, Lard, Herring oil, Human fat, Coconut oil, Corn, Butter, Animal fats.

 

Waxes:Beeswax, Carnauba wax, Spermacti.

 

Phospholipids:

Cephalins, Lecithins, Plasmalogens, Phosphatidyl inositols, Sphingomyelins.

Glycolipids: Phrenosin, Oxynervon, Nervon, Kerasin.

Steroids: C 21, C 24, C 27, C 28, C 29 steroids.

 

  1. Summary

 

In this lecture we learnt about:

 

  • The Physical and Chemical Properties of Lipids
  • Their Hydrolysis
  • Saponifiable and emulsifiable nature
  • Rancidity
  • Reactions of lipids and fatty acids
  • Well-known examples of lipids

 

you can view video on General Introduction to the Course on Knowledge Society

 

Weblinks

 

 

 

Books

 

  1. Hernandez, E. M. and Kamal-Eldin, A. (2013) Chemical and Physical Properties of Lipids, in Processing and Nutrition of Fats and Oils, John Wiley & Sons, Ltd, Chichester, UK.doi: 10.1002/9781118528761.ch2 http://onlinelibrary.wiley.com/doi/10.1002/9781118528761.ch2/summary
  2. Physical Properties of Lipids by Alejandro G. Marangoni, Suresh‎ S. Narine. 2002. https://books.google.co.in/books?isbn=0203909178
  3. Spectral Properties of Lipids by Richard John Hamilton, John‎ Cast. 1999. https://books.google.co.in/books?isbn=0849397367
  4. Crystallization and Solidification Properties of Lipids by Neil Widlak, Richard‎ W. Hartel, Suresh‎ Narine. 2001. https://books.google.co.in/books?isbn=1893997219
  5. Chemical and functional properties of food lipids by Zdzisław‎E.‎Sikorski, Anna‎Kołakowska.‎2003. https://books.google.co.in/books?isbn=1587161052
  6. Handbook of Functional Lipids by Casimir C. Akoh. 2005. https://books.google.co.in/books?isbn=1420039180
  7. The Lipid Handbook with CD-ROM, Third Edition by Frank D. Gunstone, John‎ L. Harwood, Albert‎ J. Dijkstra. 2007. https://books.google.co.in/books?isbn=1420009672