What are fats?
Fats and oils are carbon compounds that store energy in living things. Chemically they are esters formed from glycerol and fatty acids. Fats give more than twice as much energy per gram as carbohydrates and also carry the fat-soluble vitamins A, D, E and K.
Saturated and unsaturated fats
The key difference is whether the fatty-acid chains contain carbon–carbon double bonds.
| Feature | Saturated | Unsaturated |
|---|---|---|
| C=C double bonds | None | One or more |
| State at room temp. | Usually solid | Usually liquid |
| Melting point | Higher | Lower |
| Main source | Animal fats | Vegetable oils |
Key idea
Saturated fat = no C=C double bonds, solid, from animals. Unsaturated fat = has C=C double bonds, liquid, from plants.
Testing and health
An unsaturated fat has double bonds, so it decolourises bromine water; a saturated fat does not, because it has no double bonds for the bromine to react with. This simple test lets us compare cooking oils in the laboratory. Eating too much saturated fat raises blood cholesterol and the risk of heart disease, so nutritionists advise using more unsaturated oils.
Fats are still an essential part of a balanced diet. They store energy efficiently, protect and cushion body organs, form part of every cell membrane, and provide the fat-soluble vitamins. The advice is not to avoid fat completely but to choose unsaturated fats and to eat fat in sensible amounts. Fats that are left in warm air too long can turn rancid as their molecules react with oxygen.
Example
Butter and ghee are solid saturated fats from animals, while the palm and corn oils in a Malaysian kitchen are liquid unsaturated oils.
Remember
Adding hydrogen (hydrogenation) turns liquid unsaturated oil into a more solid fat, which is how margarine is made.