3.3 Food tests

Summary: You may be able to guess by looking at some foods which nutrients they contain. For example, you may know that oily foods contain lipids. Scientists use food tests to find out which nutrients are in… Tags: science oxford-science student-book-7 biology Created: 2026-05-19 Last Updated: 2026-05-19

After this topic, you will be able to:

• name the chemicals used to test foods for starch, lipids, sugar, and protein
• state the positive result for each food test
• describe how to test foods for starch, lipids, sugar, and protein.

Think back

1. Name the nutrient that is used for growth and repair.
2. What is the main function of carbohydrates?
3. What is a balanced diet?

Key idea Food tests identify starch, lipids, sugar, and protein in foods.

Key words food test, starch, iodine, lipid, sugar, Benedict’s solution, protein, biuret solution

You may be able to guess by looking at some foods which nutrients they contain. For example, you may know that oily foods contain lipids. Scientists use food tests to find out which nutrients are in a food product.

How can you test foods?

A different chemical test exists for each type of nutrient. For most food tests, you will need a solution of the food. To prepare a food solution:

1. break up the food using a pestle and mortar
2. add a few drops of water and mix well.

You should use a special type of water called distilled water – this is pure water that contains no other chemical substances.

How do you test for starch?

To test for starch, you use iodine solution. Iodine solution is an orange-yellow liquid.

1. Add a few drops of iodine solution to the food solution.
2. If the solution turns a dark blue-black colour, like in Figure 1, the food contains starch.

Figure 1Iodine changes from orange-yellow to blue-black if starch is present.

A Potatoes contain starch. State the colour a potato would appear after iodine has been added.

How do you test for lipids?

To test for lipids in a solid piece of food, you use a piece of filter paper.

1. Rub some of the food onto a piece of filter paper.
2. Hold the paper up to the Light.
3. If the paper has gone translucent (that is, you can see through it), the food contains lipids.

To test for lipids in a food solution, you use ethanol. Ethanol is a colourless liquid.

1. Add a few drops of ethanol to the food solution.
2. Shake the test tube and leave for one minute.
3. Pour the ethanol into a test tube of water.
4. If the solution turns cloudy, like in Figure 2, the food contains lipids.

Figure 2Ethanol forms a cloudy solution if lipids are present.

B Describe how you would test a chip to see if it contains fat.

How do you test for sugar?

To test for simple sugars such as glucose, you use Benedict’s solution. Benedict’s solution is a blue liquid.

1. Add a few drops of Benedict’s solution to the food solution.
2. Heat the test tube in a water bath.
3. If the solution turns orange-red, like in Figure 3, the food contains sugar.

Figure 3Benedict’s solution turns orange-red if sugar is present.

C Identify which of these foods would turn orange-red if Benedict’s solution was added: vegetable oil, boiled sweets, flour.

How do you test for protein?

To test for protein, you use biuret solution. This is a pale blue liquid.

1. Add a few drops of biuret solution to your food solution.
2. If the solution turns purple, like in Figure 4, the food contains protein.

Figure 4Biuret solution turns purple if protein is present.

D Describe how you can tell that a solution made from fish contains protein.

Working scientifically Before carrying out a food test, you must complete a risk assessment. Biuret solution is an irritant. Suggest some actions you should take to avoid or reduce the risks of working with this chemical.

Stretch zone

1. Complete Table 1 using the phrases. Table 1Nutrient Colour change if nutrient present starch lipids sugar protein turns blue-blackturns orange-redmakes paper translucentturns purple
2. Describe how to prepare a food solution of a breakfast cereal.Name the chemical that would be used to find out if the cereal contains sugar.

Stretch zone

1. Explain which chemicals would produce a positive food test result for a chocolate cake.