Vegetable oils are obtained from plants. They are important ingredients in many foods, and can be hardened through a chemical process to make, for example, margarine. They can also be used as fuels, for example as biodiesel.
Food additives are chemicals added by food manufacturers to certain foods, including vegetable oils, to improve their shelf-life, taste and appearance.
Vegetable oils
Vegetable oils are natural oils found in seeds, nuts and some fruit.
Extracting vegetable oils
The plant materials are crushed and pressed to squeeze the oil out. Olive oil is obtained this way. Sometimes the oil is more difficult to extract and has to bedissolved in a solvent. Once the oil is dissolved, the solvent is removed by distillation, and impurities such as water are also removed, to leave pure vegetable oil. Sunflower oil is obtained in this way.
Structure of vegetable oils
Molecules of vegetable oils consist of glycerol and fatty acids. Glycerol has three carbon atoms and fatty acids have long chains of carbon atoms.
In the diagram below you can see how three long chains of carbon atoms are attached to a glycerol molecule, with its three carbon atoms. Together they combine to make one molecule of vegetable oil.
Fatty acids
Hardening vegetable oils
Unsaturated vegetable oils tend to be liquid at room temperature, but they can also be 'hardened', through a chemical process called hydrogenation, to make them solid at room temperature.
Testing for unsaturation
The carbon-carbon double bonds in unsaturated oils can be detected using the elements bromine or iodine. These elements react with the double bonds in the oils, and the more double bonds there are, the more bromine or iodine is used up.
You can check for unsaturated fats using a simple test with bromine water. The test is similar to one used to differentiate alkenes from alkanes.
Bromine water is a dilute solution of bromine, which is normally orange-brown in colour. It becomes colourless when shaken with an alkene, or with unsaturated fats. When shaken with alkanes or saturated fats, its colour remains the same.
Hydrogenation
During hydrogenation, vegetable oils are hardened by reacting them with hydrogen gas at about 60ºC. A nickel catalyst is used to speed up the reaction. The double bonds are converted to single bonds by the hydrogenation. In this way unsaturated fats can be made into saturated fats.
Saturated vegetable oils are solid at room temperature, and have a higher melting point than unsaturated oils. This makes them suitable for making margarine, or for commercial use in the making of cakes and pastry.
The long fatty acid chains stop vegetable oils dissolving in water.
The fatty acids in some vegetable oils are saturated, and only have single bonds between their carbon atoms. Saturated oils tend to be solid at room temperature, and are sometimes called vegetable fats instead of oils. Lard is an example of a saturated oil.
The fatty acids in some vegetable oils are unsaturated, and have double bonds between some of their carbon atoms. Unsaturated oils tend to be liquid at room temperature, and are useful for frying food. They can be divided into two categories:
- Monounsaturated fats have one double bond in each fatty acid
- Polyunsaturated fats have many double bonds.
Hardening vegetable oils
Unsaturated vegetable oils tend to be liquid at room temperature, but they can also be 'hardened', through a chemical process called hydrogenation, to make them solid at room temperature.Testing for unsaturation
The carbon-carbon double bonds in unsaturated oils can be detected using the elements bromine or iodine. These elements react with the double bonds in the oils, and the more double bonds there are, the more bromine or iodine is used up.You can check for unsaturated fats using a simple test with bromine water. The test is similar to one used to differentiate alkenes from alkanes.Bromine water is a dilute solution of bromine, which is normally orange-brown in colour. It becomes colourless when shaken with an alkene, or with unsaturated fats. When shaken with alkanes or saturated fats, its colour remains the same.Hydrogenation
During hydrogenation, vegetable oils are hardened by reacting them with hydrogen gas at about 60ºC. A nickel catalyst is used to speed up the reaction. The double bonds are converted to single bonds by the hydrogenation. In this way unsaturated fats can be made into saturated fats.Saturated vegetable oils are solid at room temperature, and have a higher melting point than unsaturated oils. This makes them suitable for making margarine, or for commercial use in the making of cakes and pastry.Vegetable oils in food, fuel and emulsions and trans fats
Vegetable oils are important nutrients and provide a lot of energy. You must be careful not to eat excessive amounts to avoid becoming overweight.Vegetable oils are also used as fuels for vehicles. Some of this biodiesel is made from waste cooking oil and rapeseed oil. Such fuels are carbon neutral, which means that they release only as much carbon dioxide when they burn as was used to make the original oil by photosynthesis. This helps to reduce global warming. However, some people are concerned about whether it is ethical to use food crops in this way, instead of using them to feed hungry people.Emulsions
Vegetable oils do not dissolve in water. If a mixture of oil and water is shaken, then left to stand, eventually a layer of oil will form on the surface of the water.If an emulsifier is added to the oil and water, a mixture called an emulsion forms. Emulsions are more viscous than oil or water on their own, and contain tiny droplets of one of the liquids spread through the other liquid.Examples of oil droplets in water:- egg yolk
- milk
- ice cream
- salad cream
- mayonnaise
Examples of water droplets in oil:- margarine
- butter
- skin cream
- moisturising lotion
Trans fats
Partially hydrogenated vegetable oils may contain trans fats. These are thought to cause health problems such as heart disease in humans, and food manufacturers are being encouraged to reduce the amount of them in our food.Food additives
As we have seen, processed foods, including vegetable oils, may have chemicals added to them. These chemical additives have different jobs, including extending a food product’s shelf life and improving its taste and appearance. You can find additives listed on the ingredients label of such foods, and many of these additives have E numbers to identify them.Food additives and their use
type of additive example typical use colouring tartrazine (E102) orange colouring for soft drinks, sweets and sauces emulsifier lecithin (E322) allows oil and water mix to make margarine, ice cream and salad cream preservative benzoic acid (E210) used in many foods to stop harmful micro-organisms growing sweetener Aspartame (E951) low-calorie drinks and food Additives with an E number have been licensed by the European Union. Some are natural and some are artificial, but they have all been tested for safety and passed for use.The UK Food Standards Agency (FSA) has strict limits on the amount of colourings allowed in food. Some additives can lead people to have allergic reactions to them, and colourings are banned from use in baby foods.Food scares
A red dye called 'Sudan I' was quite recently banned for use in food, because it was thought to be a health risk. Some of the dye had been used in chilli powder before the ban came into force, and was later added in this form to some foods by mistake. Hundreds of food items with this ingredient had to be taken off supermarket shelves and destroyed as a result.
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