Cooking food and additives
Cooking brings about chemical changes in food. The texture and taste changes when food is cooked. Baking powder contains sodium hydrogencarbonate. This breaks down when heated, releasing carbon dioxide that helps cake mixtures to rise during baking.
Food additives are included in food to improve their shelf-life, appearance and flavour. Antioxidants such as ascorbic acid prevent food from going off by reacting with oxygen. Emulsifiers help oil and water to mix - for example, in mayonnaise.
Cooking involves chemical changes:
For example, bread turns brown as it is toasted. Sugars in the bread break down to form carbon. This change needs heat energy from the toaster, and it cannot be reversed.
Meat and eggs are good sources of protein. The protein molecules change shape as a result of the heat energy they absorb. This is called denaturing and it is permanent. Denaturing causes changes in the appearance and texture of the meat and eggs when they are cooked. For example:
Potatoes are a good source of carbohydrate, mainly as a complex carbohydrate called starch. Raw potato is hard and has an unpleasant taste but it becomes softer and easier to digest when is cooked. This is beacuse:
Everything in food is made from chemicals. Some of these are natural, and some are artificial. Processed foods, including vegetable oils, may have chemicals added to them. These additives have different roles, including extending a product’s shelf-life and improving its taste and appearance.
The table below describes some of the main types of food additives.
|type of additive||reason for adding it|
|antioxidants||stop food from reacting with oxygen|
|colourings||improve the colour of food|
|flavour enhancers||improve the flavour of food|
|emulsifiers||help oil and water mix, and not separate out|
Additives with an E number have been licensed by the European Union. Some are natural, some artificial, but they have all been safety tested and passed for use.
Immiscible liquids do not mix together. For example, if you add oil to water, the oil floats on the surface of the water. Then if you shake the two together, tiny droplets of one liquid become spread through the other liquid, forming a mixture called an emulsion.
Mayonnaise and emulsion paints are emulsions. The table describes two other emulsions.
|type of emulsion||example||minor component||major component|
|water in oil||butter||water||fat|
|oil in water||milk||fat||water|
In an emulsion, the oil and water gradually separate out again. Tiny droplets join together until eventually the oil is floating on the water again. To stop the two liquids separating, we need a substance called an emulsifier.
Emulsifiers are molecules that have two different ends:
Lecithin is an emulsifier commonly used in foods. It is obtained from oil seeds and is a mixture of different substances. A molecular model of one of these substances is seen in the diagram.
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. The emulsion is stabilised.
Baking powder is used for baking cakes. It contains sodium hydrogencarbonate, which breaks down when heated to form carbon dioxide gas. The carbon dioxide helps to make the cake mixture rise, so that it is light and fluffy.
Here are the equations for the reaction:
sodium hydrogencarbonate → sodium carbonate + carbon dioxide + water
2NaHCO3 → Na2CO3 + CO2 + H2O
Carbon dioxide can be detected using a simple laboratory test. Limewater turns cloudy white when carbon dioxide is bubbled through it.