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Science, Maths & Technology

Vegetable Oils

Updated Wednesday, 9th August 2006

In the Rough Science programme ‘To the Lighthouse’, the Rough Scientists build a lighthouse, Ellen has the task of manufacturing the fuel for the lamps. Zanzibar is full of coconut trees and so an obvious source of oil is the coconut to make coconut oil. But it isn’t a simple task of just squeezing out the oil! To find out more about vegetable oils at molecular level and how one vegetable oil differs from one another and from animal fats, read the following extract from the second level OU course Our Chemical Environment (ST240).

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Coconuts Copyrighted  image Icon Copyright: Production team

Each of the triglyceride molecules is formed from glycerol and three carboxylic acids, the difference between the triglycerides of the three species results from each organism producing different carboxylic acids. The table here shows the structures of some of the common carboxylic acids, known as fatty acids, used in nature to make triglycerides, together with their names and origins.

Carbon atoms
Origin of name
Latin for butter is butyrium
smell of goats (Latin for goat is caper)
from laurel
from nutmeg (genus Myristica)
from palm oil
Greek for fat is stear
from oil (Latin, oleum)
from linseed oil
Latin for flax is linum

The chains are even because the building block from which fatty acids are made contains two carbons. So, however many building blocks are added together the chain always contains an even number of carbons.

Kate and Ellen Copyrighted  image Icon Copyright: Production team

Notice that some of the fatty acids are called saturated fatty acids and others are called unsaturated fatty acids. If a carbon atom is attached to four other atoms by single bonds it is known as a saturated carbon atom. However, a carbon atom can be attached to fewer than four other atoms, but still have four bonds to it. The carbon could be part of a double bond, where it is attached to only three other atoms. In such cases the carbon is known as an unsaturated carbon atom.

Each of the carbon atoms in the hydrocarbon chains is attached to four other atoms by single bonds. Thus, they are all saturated chains.

Acids such as palmitic and stearic are known as saturated fatty acids, and the triglycerides made from them give rise to saturated fats. Note that, using this nomenclature, we examine only the chains. The unsaturated carbon atom in the functional group is not considered. Oleic acid has one double bond in its hydrocarbon chain. The carbon atoms of this double bond are attached to only three other atoms and are thus unsaturated. Oleic acid is therefore known as a monounsaturated fatty acid. Linoleic acid contains two double bonds in its hydrocarbon chain and is known as a polyunsaturated fatty acid or PUFA. Any fatty acid containing two or more double bonds in its hydrocarbon chain is a PUFA.

Because the continuing chains attached to the two carbon atoms of the double bond are on the same side of the double bond, this is a cis compound. In fact, in this case, it is easier to look at the disposition of the hydrogens on the double bond. In cis compounds they are on the same side, whereas in trans compounds they are on opposite sides.

Most fatty acids in nature involve cis rather than trans double bonds.

The figure here shows the range of fatty acids that lead to the triglyceride molecules that make up a particular fat or oil. The data are presented as a series of histograms, where the relative heights of bands reflect the percentage composition of the different fatty acids that are used to make the fat. The percentages are based on the relative numbers of molecules of each of the fatty acids. Fats are an essential part of the diet, but, as we saw earlier, there have been recommendations that we should eat less fat in general, but in particular we should eat less fat derived from saturated fatty acids.

Histogram of the fatty acids used to make fats and oils.
Ellen Copyrighted  image Icon Copyright: Production team

Beef fat, butter, lard and hard margarine should be avoided because they contain a high level of triglycerides made from saturated fatty acids. Fats arising from polyunsaturated fatty acids, such as soft margarine or fish oils, are more acceptable.

The structure of a triglyceride molecule shows that it is made from three fatty acids, and the figure below shows that for a particular fat or oil, there is a large range of fatty acids used to make the triglycerides. Thus, rather than being a pure compound, a fat or oil is made up of a mixture of different triglyceride molecules, each derived from different combinations of three fatty acids. Although some of the triglycerides will be made from three identical fatty acids, the majority of triglyceride molecules are derived from two or three different fatty acids.

Histogram of the fatty acids used to make fats and oils.

Even if a fat were made from just five of the fatty acids then it would be possible to get up to 75 different triglyceride molecules, each containing a different permutation of three fatty acids from the five available. However, the way the fatty acids are combined to make the triglycerides is not random, and the proportion of each triglyceride molecule is controlled by the organism to give the fat the required properties.

In lard, there is a definite tendency for unsaturated fatty acids to occupy the outer positions; however, in cocoa butter they tend to occupy the central position. This type of structural difference and distribution affects such properties as the melting range: lard softens 20°C lower than cocoa butter. Notice we referred to a melting range rather than a melting temperature. Pure compounds, such as salt and sugar, have distinct melting temperatures. If the compound contains some impurities then the melting temperature is lowered and the substance melts over a range of temperature. Substances such as lard and cocoa butter, which contain a mixture of different molecules, melt over a wide range of temperature. At the bottom of the range the compounds with a low melting temperature melt, and as the temperature is raised so more of the compounds melt.

Most fats are soft and can spread, but do not flow – they have properties intermediate between those of solids and liquids. This again arises because fats are a mixture of a number of different triglycerides. A large proportion of these triglycerides have a melting temperature above room temperature and will tend to solidify. Hence, the mixture consists of solid particles in a matrix of viscous liquid oils made up of triglycerides with lower melting temperatures. A solid material is thus produced that is not rigid because the crystals can slide over each other. This gives rise to the spreading properties associated with a fat.

Coconut pulp Copyrighted  image Icon Copyright: Production team

As the fat is heated, the temperature will rise above the melting temperatures of more and more triglycerides and these will melt. Thus, there will be fewer solid particles. This means the fat gets runnier and eventually liquefies completely.

Lard is fairly solid whereas cooking oil is a liquid. Thus, most of the triglycerides in lard have a melting temperature higher than room temperature. However, in cooking oil most of the triglycerides have a melting temperature below room temperature.

Chocolate is made from cocoa, cocoa butter and sugar. Cocoa butter is made up of only a few different types of triglyceride molecule so can have a fairly sharp melting temperature. In fact it can solidify in six different forms. This means the molecules can pack together in different ways leading to different types of crystal. One of these forms melts at 33.81°C. Thus it melts in your mouth (36.91°C) not in your hand. This form is also smooth and glossy. To persuade it to solidify in this form the chocolate is cooled and maintained at just below 33.81°C. It is also stirred so the fat crystallizes into very small crystals, which gives chocolate its velvety texture.

If chocolate is subjected to fluctuations in temperature, as in a shop window, a bloom develops on the chocolate. This is not mould but is the fat crystallizing out in different crystalline forms. Since cocoa butter is such a difficult fat to crystallize in the desired form, chocolate substitutes have to be used in cooking.

Most people will know what fats or sugar physically look like. Proteins, however, are more difficult to identify because they occur in so many different forms. Most people would identify protein with eggs, cheese or beef, but even in lean beef there is only 20% by mass of protein, the rest being water and fat. In eggs the figure is 12%. Low fat soya flour has one of the highest levels of protein at 45%. Some proteins turn up in nature as the structural part of tissue, as in muscle, tendons and hair. Others play an important part in cells as the molecules that carry out and control many of the body’s functions.

Proteins are long-chain molecules, and are known as biopolymers. Proteins are made by joining together smaller compounds called amino acids. Alanine is a typical amino acid.





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