Fats and oils

Fats and oils

foods which contain a large proportion of fats and oils

Fats and oils have a bad reputation, however they are essential for normal healthy bodily functions. A layer of fat offers some protection to your organs and fats also act as insulator to help maintain an even body temperature and they also an energy store. Fats and oils are a valuable source of many essential fatty acids which the human body cannot make for itself, fats are also essential to ensure that vitamins A,D and E are absorbed into the body. Vitamins A,D and E are fat soluble and cannot be properly absorbed unless a small amount of fat is present in the diet. However too much fat in your diet; especially saturated fats, can raise your cholesterol, which increases the risk of heart disease.

A large percentage of the energy in our diet comes from fats and oils. Sources of fats and oils include:

triglyceride molecule

Differences between fats and oils

Fats and oils are esters of long chain carboxylic acids, called fatty acids and an alcohol called glycerol or propane-1,2,3-triol. The three hydroxyl groups (-OH) in the glycerol molecule can form ester linkages with three fatty acid molecules to form a molecule called a triglyceride. Fatty acids are long chain carboxylic acids which usually contain between 12 and 20 carbon atoms, these fatty acids can be saturated or unsaturated and even polyunsaturated, that is they contain more than one carbon carbon double bond (C=C) per molecule. Fats and oils have similar structures to each other but differ only in their melting points. Fats are solid at room temperature and they contain triglyceride molecules with a high percentage of saturated fatty acid molecules while oils are liquid at room temperature. Oils contain triglyceride molecules with a high percentage of unsaturated fatty acids. Solid fats tend to be found in animals while oils are found mainly in plants.

The structure of fats and oils

AS mentioned above fats and oils are esters which are formed when the triol (an alcohol with 3 hydroxyl groups) glycerol or propane-1,2,3-triol reacts with a long chain carboxylic acids or fatty acids. This is simply a condensation or esterification reaction where the three hydroxyl groups on the glycerol molecule react with the hydroxyl group (-OH) on a fatty acid. The ester or triester formed is often called a triglyceride, this is outlined below:

The fatty acids

Fatty acids are simply long chain carboxylic acids, usually with between 12-20 atoms of carbon. Some of these fatty acids are saturated and some are unsaturated and some are even polyunsaturated (contain more than one C=C bond per molecule). In any one particular triglyceride or triester the three fatty acids could be completely different long chain carboxylic acids or they could all be the same acid molecules e.g. Two common fatty acids found in fats and oils are hexadecanoic acid (also called palmatic acid) and octadecanoic acid (also called stearic acid). The structure of these two saturated fatty acids is shown below:

structure of stearic and palmatic acids-fatty acids

In the examples given above the fatty acids were all saturated, that is they contain only carbon carbon single covalent bonds (C-C), however fatty acids can also be unsaturated. Saturated long chain fatty acid molecules are able due to their shape to pack closely together, this means that there will be lots of intermolecular bonding between neighbouring molecules, this means they are likely to have higher melting points than maybe expected, for example stearic acid has a melting point of 70cC while palmitic acid has a melting point of 63cC. This means that these two fatty acids will be solids at room temperature.

Unsaturated fatty acids

You may recall that there is the possibility of stereoisomerisation with unsaturated molecules, that is they may exist as a pair of (see image below) geometric isomers. Fats and oils which occur naturally mainly contain the long chain fatty acids which have the cis or Z-isomer geometric arrangement. One of the features of the shape of the cis or Z-isomer is that due to its shape the molecules are unable to pack closely together, this will lead to a reduction in the amount of intermolecular bonding and this will lead to a reduction in the melting point of any molecule containing one or more of the unsaturated fatty acids with the cis or Z geometric arrangement.

The trans isomers are more linear in shape and are able to form more intermolecular bonding between neighbouring molecules. Oils are liquids simply because they contain a higher proportion of these unsaturated long chain fatty acids than do fats, which have a higher proportion of the saturated straight chain linear fatty acids.

unsaturated fatty acids can exist as a pair of geometric isomers

As an example consider the two molecules shown below; they are the cis and trans isomers of octadec-9-enoic acid (octa = 10, deca=8, en= C=C and 9 gives the position of the C=C). The main form of intermolecular bonding present in these two molecules will be hydrogen bonding; however these large molecules will also have significant amounts of Van der Waals bonding between the alkyl chains. The large differences in the shapes of these two isomers will have a large affect how closely they can pack together and so on the amount of Van der Waals intermolecular bonding that can occur.

the cis and trans isomers of octdeac-9-enoic acid have 
very different shapes which affacts the melting points of the two isomers

Hydrogenation of vegetable oils

The structure of a typical vegetable oil is shown below. You should be able to identify the three ester groups and the three long chain carboxylic acids or fatty acids which make up this particular vegetable oil. In this particular oil one of the long chain fatty acids is saturated, while one has a single site of unsaturation and the bottom fatty acid chain has multiple sites of unsaturation; that is it is polyunsaturated. structure of a typical vegetable oil The sites of unsaturation present in oils and fats can be reduced by simply adding high pressure hydrogen (H2) across the C=C. This would increase both the melting and boiling points of the fat or oil. Depending on the number of C=C which are reduced to C-C bonds by hydrogenation the resultant substance can be a solid or a semi-solid fat such are margarine. This hydrogenation of vegetable oils is often called hardening and it requires the use of a catalyst and a temperature of around 600C.

Long chain carboxylic acids such as those present in fats and oils as we have seen can be saturated, unsaturated or polyunsaturated. The addition of hydrogen or hardening of the oils makes use of a catalyst, platinum, palladium, nickel, rhenium and copper are all effective catalysts for hydrogenation. The actual hydrogenation process takes place on the surface of the chosen catalyst in a number of simple steps (as shown in the image below), these include:

image shows how a unsaturated molecule becomes hydrogenated on the 
surface of a catalyst

If a vegetable oil contains unsaturated or polyunsaturated long chain carboxylic acids it is possible to fully hydrogenate all the carbon carbon double bonds (C=C) to produce a saturated molecule, this saturated molecule is likely to be a solid fat. However by carefully controlling the reacting quantities of hydrogen gas it is possible to only partly hydrogenate the unsaturated molecule, this will produce "soft" fats or semi-solid fats such as margarine which being soft are very spreadable on your toast unlike say butter!

Trans fats

Natural unsaturated fatty acids have the cis or Z geometric arrangement. When polyunsaturated fats and oils are hydrogenated some of the sites of unsaturation, that is the carbon carbon double bonds are hydrogenated but some of the carbon carbon double bonds (C=C) are left intact. However these C=C which are left intact are altered from the cis or Z isomer to the trans or E-isomer. Many cheap and highly processed foods contain these partly hydrogenated fats or trans fats as they have become know. These partly hydrogenated oils and fats are inexpensive and less likely to spoil when compared to animal fats, so foods made with it have a longer shelf life. However these trans fats cannot be processed by the body properly and there is growing evidence that they can lead to a build up of cholesterol in the artery walls which can lead to heart disease, strokes and type 2 diabetes. Many of the fast foods, pastries, cakes, cookies and biscuits we readily "tuck into" everyday are rich in trans fats, the image below shows a montage of foods which contain a high proportion of trans fats.

images of foods rich in trans fats Lipoproteins are molecules found in the body that help in the movement of fats and cholesterol around the body. There are two main types of these lipoproteins: