6.1 Extracting vegetable oil
Vegetable oils can be extracted from seeds, nuts and fruits by pressing or by distillation.
Vegetable oils provide nutrients and a lot of energy. They are important foods and can be used to make biofuels.
Unsaturated oils contain carbon-carbon double bonds (C=C) and so they decolourise bromine water.
Some oils are extracted from seeds, nuts and fruit by pressing them. Some oils are extracted by distilling the plant material mixed with water. This produces a mixture of oil and water from which the oil can be separated.
Vegetable oils release a lot of energy when they burn in air and so can be used as fuels. They are used to make biofuels such as biodiesel.
The molecules in vegetable oils have hydrocarbon chains. Those with carbon-carbon double bonds (C=C) are unsaturated. If there are several double bonds in each molecule, they are called polyunsaturated. Unsaturated oils react with bromine water, turning it from orange to colourless.
6.2 Cooking with vegetable oils
Vegetable oils are useful in cooking because of their high boiling points.
Cooking in oil increases the energy content of foods and changes the flavour, colour and texture of the food.
Vegetable oils can be hardened by reacting them with hydrogen at 60 degrees with a nickel catalyst. This makes them solids at room temperature that are suitable for spreading.
Unsaturated oils can be reacted with hydrogen so that some or all of the carbon-carbon double bonds become single bonds. This reaction is called hydrogenation and is done at about 60 degrees using a nickel catalyst. The hydrogenated oils have higher melting points because they are more saturated. The reaction is also called hardening because the hydrogenated oils are solids at room temperature. This means they can be used as spreads and to make pastries and cakes that require solid fats.
6.3 Everyday emulsions
Oils do not dissolve in water but oils and water can be used to produce emulsions. These have special properties.
Emulsions made from vegetable oils are used in many foods.
Emulsifiers stop oil and water from separating into layers.
Emulsifiers have molecules in which one part is hydrophobic and the other part is hydrophilic.
Oil and water do not mix and usually separate from each other, form in two layers. If we shake, stir or beat the liquids together, tiny droplets form that can be slow to separate. This type of mixture is called an emulsion.
Emulsions are opaque and thicker than the oil and water they are made from. This improves their texture, appearance and their ability to coat and stick to solids. Milk, cream, salad dressings and ice cream are examples of emulsions. Some water-based paints and many cosmetic creams are also emulsions.
Emulsifiers are substances that help stop the oil and water from separating into layers. Most emulsions contain emulsifiers to keep the emulsion stable.
Emulsifier molecules have a small hydrophilic part and a long hydrophobic part. The hydrophilic part or ‘head’ is attracted to water. The hydrophobic part or ‘tail’ is attracted to oil. The hydrophobic parts of many emulsifier molecules go into each oil droplet, and so the droplets become surrounded by the hydrophilic parts. This keeps the droplets apart in the water, preventing them from joining together and separating out.
6.4 Food issues
Vegetable oils are high in energy and provide nutrients.
Vegetable oils are believed to be better for health then saturated fats.
Emulsifiers improve the texture of foods enabling water and oil to mix. This makes fatty foods more palatable.
Vegetable oils contain unsaturated fats, that are believed to be better for you than saturated fats.
Animal fats and hydrogenated vegetable oils contain saturated fats and are used in many foods. Saturated fats have been linked to heart disease.
Emulsifiers stop oil and water spreading into layers. This makes foods smoother, creamier and more palatable. However, because they taste better and it is less obvious that they are high in fat, you may be tempted to eat more.
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