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 and chemical changes

Cooking involves chemical changes:

  • new substances are made
  • the process is irreversible
  • an energy change occurs.

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:

  • meat becomes firmer and turns from red to brown
  • egg white solidifies and becomes white instead of transparent.


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:

  • the cell walls break, leading to a softer texture
  • the starch grains in the cells swell and spread out.

Food additives

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.

Types of food additives

type of additivereason for adding it
antioxidantsstop food from reacting with oxygen
colouringsimprove the colour of food
flavour enhancersimprove the flavour of food
emulsifiershelp 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.

Emulsions and emulsifiers

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.

Two types of emulsion

type of emulsionexampleminor componentmajor component
water in oilbutterwaterfat
oil in watermilkfatwater

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:

  • a hydrophilic end - 'water-loving' - that forms chemical bonds with water but not with oils
  • a hydrophobic end - 'water-hating' - that forms chemical bonds with oils but not with water.

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.

Emulsifier molecules

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

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

sodium hydrogencarbonate is heated and passed through limewater, which turns cloudy

Thermal decomposition of sodium hydrogencarbonate

Testing for carbon dioxide

Carbon dioxide can be detected using a simple laboratory test. Limewater turns cloudy white when carbon dioxide is bubbled through it.

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