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Classification and Structure of Fatty Acids
A fatty acid is made up of a hydrocarbon chain with a carboxyl group at one end and a methyl group at the other. The behaviour and the properties of fats depend largely on the structure and the configuration of the fatty acids that make up the triglyceride - in particular the length of the fatty acid chain together with the nature of the bonds between the carbon atoms in the chain.
The types of fatty acids may be classified as follows: |
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Saturated fatty acids
Saturated fatty acids contain their full complement of hydrogen atoms in the hydrocarbon chain. The carbon atoms are linked together by single bonds. If these fatty acids are used in a triglyceride structure, the fats are usually solid at room temperature. They do not combine easily with oxygen and are normally found in animal fats.
Unsaturated fatty acids
If some of the hydrogen atoms are missing and have been replaced by a double bond between the carbon atoms, then the fatty acid is said to be unsaturated. If there is one double bond, the fatty acid is known as a monounsaturated fatty acid. If there is more that one double bond then the fatty acid is known as a polyunsaturated fatty acid. When these fatty acids form part of a triglyceride structure the fat produced will be liquid at room temperature and will be more prone to oxidative rancidity.
Cis and Trans Fatty Acids
In polyunsaturated fatty acids, the hydrogen atoms can be arranged in one of two ways. One arrangement is called cis, the other is called trans.
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Cis Fatty Acids
In nature, most unsaturated fatty acids are CIS fatty acids. This means that the hydrogen atoms are on the same side of the double bond as shown.
Sources of Cis Fatty Acids
Found in all foods that contain some fat of oil.
Trans Fatty Acids
In TRANS fatty acids the two hydrogen atoms are on opposite sides of the double bond. A small amount of trans fatty acids occur in nature, in foods like beef, lamb, and dairy products. Conjugated linoleic acid (CLA) is an example of a naturally occurring trans fat found in dairy products and meat. Other trans fatty acids are formed during the industrial processing of oil e.g. hydrogenation. During hydrogenation some double bonds are converted from the cis to the trans configuration by a process calledisomerisation.
Significance of trans fatty acids in the diet
Interest in the effects of trans fatty acids on health have increased in the last few years as evidence has grown that they may have an effect on the incidence of Coronary Heart Disease (CHD). It has been shown through clinical tests that trans fatty acids tend to raise the levels of total cholesterol and *Low Density Lipoprotein (LDL) in the body which in turn increases the risk of developing Heart Disease. It has also been shown that high levels of trans fatty acids in the diet can also decrease levels of **High Density Lipoprotein (HDL) in the body. High Density Lipoproteins are thought to be a protective factor in the prevention of Coronary Heart Disease. (CHD)
*Lipoproteins are clusters of protein and lipids all tangled up together. These act as a means of carrying lipids, including cholesterol, around our blood. There are two main categories of lipoproteins.
*Low Density Lipoprotein (LDL) is the “bad guy” as it is associated with causing cholesterol to build up on the walls of the arteries. Risk factor in CHD.
**High Density Lipoprotein (HDL) is the “good guy” as it is associated with carrying cholesterol out of the blood system. Protective factor in CHD.
Consumers at risk of heart disease need to be careful therefore not to replace the saturated fats in their diet with foods high in hydrogenated oils, which is an easy mistake to make. Processed foods which are sold as low cholesterol and low in saturated fats often have a high level of trans unsaturated fatty acids. Unfortunately, trans fatty acid concentration does not have to be specified under current food labelling legislation, so it is difficult to know their levels in spreads and processed foods. Labelling legislation may be introduced soon to ensure that all food containing trans unsaturated fatty acids will be indicated on the label.
It is not clear if trans fats that occur naturally have the same effect on cholesterol and heart disease as those produced by hydrogenated vegetable oils. For example, *conjugated linoleic acid (CLA), found in meat and dairy products is currently the focus of much exciting research. This research is ongoing but it has shown so far in tests on animals that this trans fat helps to fight cancer and prevent arteriosclerosis.
It would appear therefore that the problem with trans fatty acids exists with the synthetic trans fatty acids, which are formed during hydrogenation.
*Conjugated linoleic acid (CLA) is a variant of the essential fatty acid Linoleic acid.
Common Sources of Trans Fatty Acids
Hard margarines, crisps and snacks, packaged desserts, cakes, pastries and processed foods.
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Sources of Lipids
The sources of lipids can be divided into three categories: |
 | Animal |
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| Plant |
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| Marine Sources |
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| Marine lipids are an important source of Omega-3 fatty acids. |
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Omega-3 Fatty Acids
An Omega-3 fatty acid has a double bond present between the third and fourth carbon atom from the methyl end. Oily fish are the richest source of Omega-3 e.g. mackerel, salmon, sardines. |
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Dietary Benefits of Omega-3 Fatty Acids
Studies have shown that heart attacks, strokes and other circulatory diseases are relatively rare in societies where fish forms a major part of the diet. Further research has indicated that people with a personal history or family history of heart disease may reduce their risk of suffering a heart attack by increasing their intake of Omega-3 fatty acids as these oils protect the cardiovascular system and reduce blood clotting tendencies.
Distribution of Fatty Acids in Food
There is no food that is composed of one fat type, all contain a mixture. This topic provides an ideal opportunity for students to actively investigate a number of commercial food products to find out the distribution of fat types in the product. A chart of information could be compiled for display in the classroom.
Here are some examples:
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| Vegetables Oils and Shortening | Polyunsaturated fatty acids | Monounsaturated fatty acids | Saturated fatty acids | Sunflower oil
| 66% | 20% | 10% | Olive Oil
| 8% | 74% | 13% | Soft tub marg.
| 31% | 47% | 17% | Stick margarine
| 66% | 20% | 10% | | | | | | | Animal fats | | | | Tuna fish
| 37% | 26% | 27% | Chicken fat
| 21% | 45% | 30% | Lard
| 11% | 45% | 40% | Butter
| 4% | 29% | 62% |
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