Engineering Chemistry

Lesson 22 Lipids

22.2   Functions of Lipids

22.2.1  They serve as fuel molecules. 1 gm of fat provides 9 kcal of energy.

22.2.2  Phospholipide and triglycerides are important to structure, composition and permeability of cell wall and cell membrane.

22.2.3  Lipoproteins help in transporting lipids in blood.

22.2.4  Skin waxes act as defense mechanism in avoiding thermal and physical shock.

22.2.5  Fats present in food help to carry fat soluble vitamins so that they are efficiently    absorbed.

22.2.6  Fats serve as vitamins, emulsifiers and flavour and aroma compounds.

22.3   Classification of Lipids

Lipids are classified into two groups, simple lipids and compound lipids.

Simple lipids include fatty acids, fats and oils (triglycerides of fatty acids) and waxes (esters of fatty acids with long chain monohydroxy alcohols).

All other lipids like phospholipids, steroids, caretenoids etc. are included in compound lipids.

22.4   Fatty Acids

Fatty acids are long chain carboxylic acids having general formula CH₃(CH₂)_nCOOH. Most of the fatty acids contain even number of Carbon atoms because of the way in which they are synthesized in living beings. But few fatty acids are present which have odd number of Carbon atoms. Most of the fatty acids found in nature are linear, but few branched chain and cyclic fatty acids can also be found.

22.4.1 Saturated Fatty Acids

Straight chain fatty acids containing 2 to 80 Carbon atoms are known, but in food those having 4 to 24 Carbon atoms are present. The largest share is of fatty acids containing 16 & 18 Carbon atoms.

Table 22.1 Saturated fatty acids found in foods

No. of C atoms	Name	Structure
4	Butyric	CH3(CH2)2COOH
5	Valeric	CH3(CH2)3COOH
6	Caproic	CH3(CH2)4COOH
8	Caprylic	CH3(CH2)6COOH
10	Capric	CH3(CH2)8COOH
12	Lauric	CH3(CH2)10COOH
14	Myristic	CH3(CH2)12COOH
16	Palmitic	CH3(CH2)14COOH
18	Stearic	CH3(CH2)16COOH
20	Arachidic	CH3(CH2)18COOH
22	Behenic	CH3(CH2)20COOH
24	Lignoceric	CH3(CH2)22COOH

 

Fatty acids with Carbon atoms 4-10 are present mainly in milk fat, those having 12-24 Carbon atoms are present in vegetable and animal fats & fatty acids having greater than 24 Carbon atoms are found in waxes. In food, lauric, myristic, palmitic and stearic are the most common acids which are present.

22.4.2 Unsaturated fatty acids

Unsaturated fatty acids are fatty acids which contain double bonds between Carbon atoms. They are subdivided into two types MUFA (Mono-unsaturated fatty acid) and PUFA (poly-unsaturated fatty acid). Most of the naturally occuring unsaturated fatty acids are cis-isomers.

Table 22.2 Unsaturated fatty acids found in food

Common Name	IUPAC Name	Structure
Myristoleic	9-Tetradecenoic acid	CH3(CH2)3CH=CH(CH2)7COOH
Palmitoleic	9-Hexadecenoic acid	CH3(CH2)5CH=CH(CH2)7COOH
Oleic	9-Octadecenoic acid	CH3(CH2)7CH=CH(CH2)7COOH
Linoleic	9,12-Octadecadienoic acid	CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
Linolenic	9,12,15-Octadecatrienoic acid	CH3(CH2CH=CH)3(CH2)7COOH
Arachidonic	5,8,11,14-Eicosatertaneoic acid	CH3(CH2)3(CH2CH=CH)4(CH2)3COOH

 

Oleic Acid is commonly found in all fats. It is present upto a concentration of 75% in Olive oil. Linoelic acid is also commonly found and is present to an extent of 60-80 % in safflowerseed oil. Linolenic acid is found upto an extent of 50-60 % in linseed oil. Arachidonic acid is found in animal sources.

22.4.3 Essential Fatty Acids

Certain fatty acids like lineolic acid are necessary for maintaining healthy conditions of animals. In plants, these acids are formed from oleic acid, but animals lack necessary enzymes for their synthesis. Therefore, they must be supplied from the diet and are known as essential fatty acids.

22.5 Triglycerides:

Triglycerides are carboxylic esters derived from glycerol. Glycerol contains 3 –OH groups which can form ester linkage with fatty acids. If one –OH group is esterified, the compound is called monoglyceride, if two are esterified diglyceride and if all three are esterified, the compound is designated as triglyceride.

Figure 22.1 Structures of Glycerol, monoglycerides, diglycerides & triglycerides

22.6   Characteristics of lipids

Fats do not have sharp melting points because they are mixture of compounds. As unsaturation of fatty acids increases, the specific heat increases. Viscosity decreases with increasing unsaturation and increases with number of C atoms in fatty acid chain. Solid-liquid ratio can be found out by measuring density of fat. The refractive index increases with increasing unsaturation or with increasing no. of C atoms in fatty acid chain.

22.7   Rancidity

Lipids undergo chemical changes which lead to undesirable odour and flavour. This process is known as rancidity. Two reactions are responsible for rancidity – hydrolysis and oxidation.

Hydrolytic rancidity (lipolytic rancidity) is rancidity developed by hydrolysis reactions in which the lipids are hydrolyzed to yield glycerol and free fatty acids. These reactions can be catalyzed by acids, bases, enzymes or heat. If the reaction is catalyzed by base, salts of fatty acids are obtained and the reaction is known as saponification. Other catalysts release the acids in the free form. Hydrolytic rancidity is important for flavours of milk products, which are due to presence of fatty acids having even number of Carbon atoms, from 4 to 12. Lipolysis depresses the smoke point of cooking oils and as a result, cooking quality of fat is lost, because the foods fried in such oils absorb greater amounts of fat, and the frying quality is also degraded.

Oxidative rancidity is rancidity developed when fats react with oxygen in air, or oxygen producing products. In oxidative rancidity, a large number of intermediates are formed and compounds produced by their condensation are responsible for rancidity. This rancidity is catalyzed by heat, light, ionizing radiatioons and enzymes. This type of rancidity can be prevented by adding substances called anti-oxidants. They function by reacting with intermediates which lead to spoilage of foods.

22.8   Phospholipids

In phospholipids or phosphoglycerides, one of the terminal –OH groups in glycerol is esterified by phosphoric acid and the remaining two by fatty acids. The phosphoric acid is in turn esterified by other compounds like choline, ethanolamine, serine or inositols. Phospholipid containing choline is called lecithin. It is a very good emulsifier and present in egg yolk.

Figure 22.2 Structure of Phospholipid

Phospholipids are components of cell membrane, and influence its structure and function. They help in transporting other lipids in the blood.

References & Further Reading:

1. Satyanarayana U. & Chakrapani U. (2011), “Biochemistry”. Books and Allied (P) Ltd. pp. 28-40.

3. N. Shakuntala Manay & M. Shadaksharaswamy (2001), “Foods: Facts and Principles”. New Age International Publishers, pp. 33-41.