How can we test a food sample to find out what nutrients it contains?
Carry out and interpret the standard food tests for starch, reducing sugar, protein and fat
A focused answer to the O-Level Biology practical outcome on food tests. The reagent, method and colour change for starch, reducing sugar, protein and fat, and how to write up a result for full marks.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
SEAB wants you to know the standard chemical tests used to find out which nutrients a food contains: starch, reducing sugar, protein and fat. For each you should know the reagent, the method (including whether to heat), and the colour or appearance change that shows a positive result, and you should be able to write up a result clearly.
The answer
Test for starch (iodine test)
Add a few drops of iodine solution to the sample, at room temperature. If starch is present, the orange-brown iodine turns blue-black. If no starch is present, it stays orange-brown.
Test for reducing sugar (Benedict's test)
Add Benedict's solution to the sample and heat it in a water bath. If a reducing sugar (such as glucose) is present, the blue solution changes to a brick-red precipitate (green or orange for smaller amounts). If none is present, it stays blue. Heating is essential.
Test for protein (biuret test)
Add biuret solution (or sodium hydroxide solution followed by a little copper sulfate solution) at room temperature. If protein is present, the blue colour changes to purple (violet). If no protein is present, it stays blue.
Test for fat (emulsion test)
Add ethanol (alcohol) to the sample and shake to dissolve any fat, then pour the mixture into a tube of water. If fat is present, a cloudy white emulsion forms. If no fat is present, the liquid stays clear.
Writing up a result
A good result statement names the test, the observation, and the conclusion: for example, "The sample turned blue-black with iodine, so it contains starch." Examiners want the colour change (from and to) and the matching conclusion.
Examples in context
Example 1. Testing a leaf for photosynthesis. A leaf is decolourised in hot ethanol and then tested with iodine. A blue-black colour shows starch has been made, evidence that photosynthesis has taken place in that part of the leaf.
Example 2. Checking a food label. A food claiming to be sugar-free can be tested with Benedict's solution. If it stays blue after heating, the claim is supported; a brick-red colour would show reducing sugar is present.
Try this
Q1. Name the reagent used to test for starch and the positive colour change. [2 marks]
- Cue. Iodine solution; it changes from orange-brown to blue-black.
Q2. State the positive result for the protein test and the reagent used. [2 marks]
- Cue. Biuret solution changes from blue to purple (violet) when protein is present.
Q3. Describe how to test a sample for fat. [2 marks]
- Cue. Add ethanol and shake to dissolve any fat, then pour into water; a cloudy white emulsion shows fat is present.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original6 marksFor each of the following tests, name the reagent used and state the positive (final) colour: (a) the test for starch, (b) the test for reducing sugar, (c) the test for protein.Show worked answer →
(a) Starch: reagent is iodine solution; a positive result turns from orange-brown to blue-black.
(b) Reducing sugar: reagent is Benedict's solution; the mixture is heated, and a positive result changes from blue to a brick-red (orange or green at lower amounts) precipitate.
(c) Protein: reagent is biuret solution (or sodium hydroxide then copper sulfate); a positive result changes from blue to purple (violet).
Markers reward the correct reagent and the correct positive colour change for each test. Note that Benedict's test must be heated; iodine and biuret are done at room temperature.
Original4 marksA student is given a sample of milk and asked to find out whether it contains fat and protein. Describe how the student would test for fat, and state the result expected if fat is present.Show worked answer →
To test for fat, the student carries out the emulsion test: add ethanol (alcohol) to the sample and shake to dissolve any fat, then pour the mixture into a tube of water.
If fat is present, a cloudy white emulsion forms in the water. If no fat is present, the mixture stays clear.
Markers reward the use of ethanol followed by adding water (the emulsion test) and the correct positive result of a cloudy white emulsion.
Related dot points
- Describe the elements, building blocks and roles of carbohydrates, fats and proteins
A focused answer to the O-Level Biology outcome on biological molecules. The elements and building blocks of carbohydrates, fats and proteins, and the role each plays in the body.
- Explain that enzymes are biological catalysts and describe their action using the lock and key model
A focused answer to the O-Level Biology outcome on enzymes. What a biological catalyst is, the lock and key model, the meaning of substrate, active site and specificity, and why enzymes matter in the body.
- Describe and explain the effects of temperature and pH on the rate of enzyme-controlled reactions
A focused answer to the O-Level Biology outcome on enzyme activity. The effect of temperature and pH on the rate of reaction, the meaning of the optimum, and what denaturing does to an enzyme's active site.
- Describe the human digestive system and the role of mechanical and chemical digestion
A focused answer to the O-Level Biology outcome on human digestion. The path of food through the gut, the difference between mechanical and chemical digestion, and the main digestive enzymes and what they break down.
- Use a light microscope to observe cells and calculate magnification and actual size
A focused answer to the O-Level Biology outcome on microscopy. Parts of the light microscope, how to prepare and view a slide, and how to calculate magnification, actual size and image size with worked numbers.