1.1 Oils do not dissolve in water. They can be used to
produce emulsions. If oil and water are shaken
together, tiny droplets of one liquid spread through the
other liquid, forming a mixture called an emulsion.
1.1.1 Emulsions are thicker than oil or water and have many uses
that depend on their special properties. They provide better
texture, coating ability and appearance, for example in salad
dressings, ice creams, cosmetics and paints.
22.214.171.124 There are two main types of emulsion:
oil droplets in water (milk, ice cream,
salad cream, mayonnaise); water
droplets in oil (margarine, butter, skin
cream, moisturising lotion).
126.96.36.199.1 If an emulsion is left to stand, eventually a
layer of oil will form on the surface of the
water. Emulsifiers are substances that
stabilise emulsions, stopping them separating
out. Egg yolk contains a natural emulsifier.
Mayonnaise is a stable emulsion of vegetable
oil and vinegar with egg yolk.
188.8.131.52.1.1 Emulsifier molecules have two different ends: a hydrophilic end -
'water-loving' - that forms chemical bonds with water but not with oils
and a hydrophobic end - 'water-hating' - that forms chemical bonds
with oils but not with water. The hydrophilic 'head' dissolves in the
water and the hydrophobic 'tail' dissolves in the oil. In this way, the
water and oil droplets become unable to separate out.
2 C1.6.1 Vegetable Oils
2.1 Some fruits, seeds and nuts are rich in oils that can
be extracted. The plant material is crushed and the oil
removed by pressing (olive oil) or in some cases by
distillation (sunflower oil). Water and other impurities
2.1.1 Vegetable oils are important
foods and fuels as they provide a
lot of energy and nutrients.
184.108.40.206 Vegetable oils have higher boiling points than water
and so can be used to cook foods at higher
temperatures than by boiling. This produces quicker
cooking and different flavours but increases the
energy that the food releases when it is eaten.
3 C1.6.3 Saturated And Unsaturated Oils
3.1 Vegetable oils that are unsaturated contain double
carbon–carbon bonds. These can be detected by reacting with
bromine water, turning the liquid from orange to colourless.
3.1.1 Saturated vegetable fats 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.
Unsaturated vegetable oils can be ‘hardened’ by reacting them with hydrogen, a reaction called hydrogenation.
220.127.116.11 During hydrogenation, vegetable oils which are
unsaturated are reacted with hydrogen gas at about
60ºC in the presence of a nickel catalyst. Hydrogen
adds to the carbon–carbon double bonds. The
hydrogenated oils have higher melting points so they
are solids at room temperature, making them useful
as spreads and in cakes and pastries.