11: Cooking is Chemistry

Cooking is chemistry. When an egg is boiled the protein of the egg undergoes a chemical change. When you bake a cake you mix the ingredients together and heat them in an oven where they undergo change. Carbon dioxide gas is given off which helps the cake to rise.

In this section you can investigate mixtures of chemicals which are found in the average kitchen.

A. French dressing - a liquid suspended in a liquid

French dressing is made from oil and vinegar. Vinegar is mainly water but oil and water do not mix. If they are shaken together they will separate into layers once you let them stand. Liquids that do not mix are said to be immiscible. Here is how you can make French dressing.

What you will need

vinegar
cooking oil
a jar with a tight lid
stirring rod or whisk
a magnifying glass

What you do

  1. Put 10cm3 of vinegar in the jar. Pour in 20cm3 of oil and let the mixture stand for a few minutes.
  2. Where does the oil go? What does this tell you about the density of oil?
  3. Put the mixture in a beaker or bowl and beat it hard with a stirring rod or whisk for about four minutes.
  4. Quickly pour this back into the jar and look at the droplets with a magnifying glass. What did the beating do to the size of the oil droplets?

B. Mayonnaise - a stabilised suspension

When the oil and vinegar in French dressing separate, the oil droplets grow larger and larger until they come together to form a separate layer. When you make mayonnaise you stop the oil droplets coming together. If the oil droplets are kept from coming together they cannot form a layer. The result is a stable mixture of two immiscible liquids called an emulsion. The substance that keeps the liquids apart is called an emulsifying agent. Mayonnaise is an emulsion of oil and water. The emulsifying agent is egg yolk. Chefs say that it is a sign of a good cook if you can make mayonnaise. If you know some chemistry it is easy. Try it out for yourself.

What you will need

2 egg yolks
small bowl		cooking oil
whisk		vinegar

What you do

  1. WarningBiological Hazard
    Have all the ingredients at room temperature. The oil flows better and the eggs will emulsify better.
  2. Put the egg yolks and about 10cm3 of vinegar in the bowl and beat the mixture until the egg yolks are sticky.
  3. Add the rest of the vinegar and mix well.
  4. The next part is the tricky bit. What you are trying to do is spread the tiny oil droplets evenly through the egg yolks. The egg yolk will coat the oil droplets and prevent them from coming together.
    If you add the oil too quickly or too much oil is added at once, the droplets will come together before they can be coated with egg yolk and the mayonnaise will curdle or separate.
    Add the oil drop by drop while beating constantly. If you do not have an electric beater, you can make the emulsion by hand, with a friend to help you. One of you does the beating with a whisk, while the other adds the oil.

OTHER THINGS TO TRY
Making Strawberry Bombe.
Investigate the effect of adding gelatin to the recipe. See your teacher for details.
Warning
do not eat any foods in, or prepared in, a laboratory

Biological Hazard
there may a risk of Salmonella infection from raw eggs


Contents