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Properties and Uses of Oils and Fats

Definition: Oils and fats are glyceryl esters or glycerides of higher fatty acids. Those which are liquids at ordinary temperature are called oils & other are called fats.

 

Common Fatty acids in Naturally Occurring Oils and Fats:

 

	Lauric	Myristic	Palmitic	Stearic	Oleic	Linoleic	Linolenic
Melting Point	44 °C	54 °C	63 °C	69 °C	13 °C	-5 °C	-16 °C
Fats
butter (cow)	3	11	27	12	29	2	1
tallow		3	24	19	43	3	1
lard		2	26	14	44	10
Oils
canola  oil			4	2	62	22	10
coconut  oil	47	18	9	3	6	2
corn  oil			11	2	28	58	1
olive  oil			13	3	71	10	1
peanut  oil			11	2	48	32
soybean  oil			11	4	24	54	7

*Totals less than 100% indicate the presence of fatty acids with fewer than 12 carbon atoms or more than 18 carbon atoms.

†Coconut oil is highly saturated. It contains an unusually high percentage of the low-melting C8, C10, and C12 saturated fatty acids.

 

1.   Lauric acid (IUPAC name: dodecanoic acid), is a saturated fatty acid with a 12-carbon atom chain, thus having many properties of medium-chain fatty acids. It is a bright white, powdery solid with a faint odour of bay oil or soap. CH₃(CH₂)₁₀COOH.

 

2.   Myristic acid (IUPAC name: tetra decanoic acid) is a common saturated fatty acid with the molecular formula CH₃(CH₂)₁₂COOH. Its salts and esters are commonly referred to as myristates or tetradecanoates. The name of the acyl group derived from myristic acid is myristoyl or tetradecanoyl.

3.   Palmitic Acid (IUPAC Name: hexa decanoic acid) is a fatty acid with a 16-carbon chain. It is the most common saturated fatty acid found in animals, plants and microorganisms. Its chemical formula is CH₃(CH₂)₁₄COOH. It is a major component of palm oil from the fruit of Elaeis guineensis (oil palms), making up to 44% of total fats. Meats, cheeses, butter, and other dairy products also contain palmitic acid, amounting to 50–60% of total fats.

4.   Stearic (IUPAC name: octadecanoic acid) is a saturated fatty acid with an 18-carbon chain. It is a soft waxy solid with the formula CH₃(CH₂)₁₆COOH. The triglyceride derived from three molecules of stearic acid is called stearin. Stearic acid is a prevalent fatty-acid in nature, found in many animal and vegetable fats, but is usually higher in animal fat than vegetable fat.

5.   Oleic acid is a fatty acid that occurs naturally in various animal and vegetable fats and oils. It is an odourless, colourless oil, although commercial samples may be yellowish due to the presence of impurities. In chemical terms, oleic acid is classified as a monounsaturated omega-9 fatty acid, abbreviated with a lipid number of 18:1 cis-9, It has the formula CH₃−(CH₂)₇−CH=CH−(CH₂)₇−COOH. The name derives from the Latin word oleum, which means oil.

6.   Linoleic acid is a polyunsaturated, omega-6 fatty acid. It is a colourless liquid that is virtually insoluble in water but soluble in many organic solvents. It typically occurs in nature as a triglyceride (ester of glycerin) rather than as a free fatty acid. It is one of two essential fatty acids for humans, who must obtain it through their diet, and the most essential, because the body uses it as a base to make the others. Linoleic acid (LA) is an organic compound with the formula HOOC(CH₂)₇CH=CHCH₂CH=CH(CH₂)₄CH₃. Both alkene groups (−CH=CH−) are cis. It is a fatty acid sometimes denoted 18:2 (n-6) or 18:2.

7.   Linolenic acid is a type of naturally-occurring fatty acid. It can refer to either of two octadecatrienoic acids (i.e. with an 18-carbon chain and three double bonds, which are found in the cis configuration), is often found in vegetable oils. The two forms are:
(a) ALA (alpha Linolenic acid) = omega-3 fatty acid = unsaturation at C9, C12, C15 = Octadeca-9,12,15-trienoic acid

(b) GLA (gamma Linolenic acid) = omega-6 fatty acid = unsaturation at C6, C9, C12 = Octadeca-6,9,12-trienoic acid

 

 

Physical Properties:

          When pure, these are colourless, odourless, neutral liquids or solids. They are lighter than water and immiscible with it but dissolve in organic solvents, e.g., ether and benzene. When agitated with water in the presence of soap, gelatin, etc. (emulsifiers) they form emulsions.

Chemical Properties:

(i)           Hydrolysis: Oils and fats are hydrolysed when heated with water alone or in the presence of acids and yield glycerol and higher fatty acids. With alkalis they give glycerol and soap.

 In presence of Acid                                                       In presence of Base/Alkali

          (ii) Hydrogenation: Oils are glycerides of unsaturated fatty acids. These are changed to solid fats when hydrogen gas is passed into them under pressure in the presence of finely-divided nickel (catalyst). The unsaturated acid radicals change into saturated as a result of hydrogenation. For example:

 

         (iii) Drying: Certain oils like linseed oil change into hard solids on exposure to air. These are called Drying oils and find use in paint and varnish industry. Drying is catalysed by litharge and various other metallic oxides. Some other oils like cottonseed oil thicken slowly and contain a smaller percentage of the glycerides of unsaturated acids than the drying oils. These are called semidrying oils.

Drying involves oxidation, polymerization and colloidal gel formation. The mechanism of the process is, however, complicated and not definitely known.

         (iv) Rancidification: The unpleasant smell which fats and oils develop on long exposure to moist air is due to rancidity. This results from partial saponification (hydrolysis) which sets free strongly smelling fatty acids. For example, butter on hydrolysis yields volatile fatty acids having unpleasant odour. Another form of rancidity is due to oxidation of unsaturated fats promoted by heat and light. The oxidation yields aldehydes and acids which have a strong smell.

 

Uses of Oils and Fats

 

          These are largely used (i) as articles of food,

(ii) for toilet purposes,

(iii) in medicine,

(iv) as lubricants,

(v) as illuminants, and

(vi) in the manufacture of soap, glycerine and paints.

 

Hydrogenation of Oils

          Most of the housewives prefer solid fats to liquid oils for cooking purposes. Accordingly, millions of kilograms of groundnut oil or cottonseed oil are changed to solid edible fat (vegetable ghee) each year by hydrogenation in presence of a suitable catalyst. Hydrogenation converts only a part of the glycerides of unsaturated acids into those of saturated acids. Different steps involved in the actual manufacture of vegetable ghee are:
 

          (a) Removal of Free Acids: The oil is warmed in a pan and

treated with a calculated quantity of sodium hydroxide to neutralize

the free fatty acids. The salts formed as a result of neutralization

come up in the form of a scum along with some of the suspended

Matter.

         (b) Bleaching. The oil from the first tank is decanted into the

second and treated with animal charcoal at 353K or so. The colouring matter is adsorbed by animal charcoal and the oil is filtered.

         (c) Deodorising. The bleached oil is treated with superheated steam for deodorising.

(d) Hydrogenation or hardening of oil

The oil purified above is taken in an iron tank surrounded by a heating jacket at 423-473K. Some finely divided nickel is suspended in the oil and hydrogen gas is passed in under pressure. Finely divided nickel acts as a catalyst in hydrogenation

 

The hydrogenation is continued until a fat of the desired consistency is obtained. The hardened oil is taken out and freed from the catalyst by filtration. Some flavouring material somewhat resembling genuine ghee is then added to this before placing it on the market.  Figure: Hydrogenation Chamber for Oil

 

 

Analysis of Oils and Fats

          The composition and purity of a given fat is determined by means of a number of physical and chemical tests. Various physical tests for the determination of its physical constants such as melting point, specific gravity and refractive index. Various chemical tests which give an indication of the type of fatty acids present in the fat or oil are:

 

(1) Acid Value. It is the number of milligrams of potassium hydroxide required to neutralize 1 g of the fat or oil. The acid value indicates the amount of the free acid present in the fat or oil.

To determine acid value, a weighed quantity of the fat is dissolved in alcohol and titrated against a standard alkali using phenolphthalein as indicator.

 

(2) Saponification Value. It is the number of milligrams of potassium hydroxide required to neutralize the fatty acids resulting from the complete hydrolysis of 1 g of the oil or fat.

To determine the saponification value, a weighed quantity of the given fat is refluxed with a known volume of standard alcoholic potash solution. The unused alkali is then titrated against some standard acid. Saponification values of some common oils are:

Coconut oil — about 250
Olive oil— about 250

(3) Iodine Value. It is the number of grams of iodine which combine with 100 grams of oil or fat. It indicates the degree of unsaturation of acids in the fat or oil.

 

In Hubl's method of determination of the iodine value, a known weight

of oil or fat is dissolved in carbon tetrachloride and treated with a known volume of standard solution of iodine in ethanol in presence of mercuric chloride. The unused Iodine is titrated against a standard thiosulphate solution

Iodine values of some common oils are Coconut oil—10 ; Olive oil—88 ; Linseed oil—108

 

Classification of Oils and Fats

 

Classification of Oils—On the basis of iodine value

On basis of the iodine value the oils have been sub-divided into following three groups:

(a) Drying oils: These are oils with iodine values above 120. They harden slowly on exposure to air to form resinous solid. These are glycerides of highly unsaturated acids, e.g., linoleic and linolenic acid. Linseed oil is a typical drying oil and consists of: Linolenic acid ester (80%), Linoleic acid ester (15%) and Triolein (5%).

Tung oil (China-wood oil) is another example of an excellent drying oil.

Drying oils find use in paints and in the manufacture of oil cloth, rexin and linoleum (mixture of ground cork and boiled linseed oil rolled into sheet which hardens on standing).

(b) Semi-drying oils: These are oils with iodine values 90 to 120. They thicken very slowly when exposed to air. Cottonseed oil and sesame oil are examples of semi-drying oils.

(c) Non-drying oils: These have iodine values less than 90. They do not thicken when exposed to air. They consist of mainly triolein. Some examples of this class are olive oil, coconut oil and castor oil

 

Classification of Oils—On the Basis of Unsaturation of Fatty Acids present:

Fats and oils can be classified based on their degree of unsaturation into three main categories: saturated, monounsaturated, and polyunsaturated.

Category	Description	Examples
Saturated Fats	No double bonds in the fatty acid chains. Solid at room temperature.	Butter, Lard, Coconut Oil, Palm Oil
Monounsaturated Fats	One double bond in the fatty acid chains. Liquid at room temperature but may solidify when chilled.	Olive Oil, Canola Oil, Peanut Oil, Avocado Oil
Polyunsaturated Fats	Two or more double bonds in the fatty acid chains. Liquid at room temperature.	Sunflower Oil, Soybean Oil, Corn Oil, Fish Oils (like salmon and mackerel)

 

Classification of Oils—On the Basis of Origin
Oils and fats can be classified based on their origin into two main categories: animal-based and plant-based.

Distinction between Animal and Vegetable Fats—Animal fats contain cholesterol—an unsaturated alcohol, with molecular formula C₂₇H₄₆O.

Cholesterol forms rhombic plates (Melting Point 421K).
 Vegetable fats contain Phytosterol which crystallizes as needles (melting point 405K - 417K), This makes the distinction between animal and vegetable fats possible.

 

Aspect	Vegetable Oil	Animal Fat
Origin	Extracted from plants	Derived from animals
Composition	Mostly polyunsaturated fats	Mostly monounsaturated fats
Chemical Bonds	Contains one or more carbon double bonds (polyunsaturated)	Contains generally single carbon-carbon bonds (mono-unsaturated)
State at Room Temperature	Generally liquid	Generally solid
Hydrogenation	Can be hydrogenated to form vanaspati ghee	Not typically hydrogenated
Health Impact	Generally considered healthier, can lower cholesterol levels	Can increase blood cholesterol levels
Sources	Extracted from seeds (e.g., rapeseed, soybean, corn, sunflower, safflower, peanut)	Includes butter, ghee, lard, tallow, and other fatty acids