Edexcel IGCSE Biology · Spec 2.9
Food Tests
The Benedict's, iodine, Biuret and emulsion tests for sugars, starch, protein and fats, and their positive colour changes.
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Food Tests
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Explained
The four food tests, and the colours that matter
Four tests, four nutrients. For each one you need the reagent, what you do to it, and the colour change that counts as positive. Questions in this area are usually short and factual, which means the marks turn on getting the colours the right way round.
The tests
- Starch: add iodine solution. Orange brown to blue black.
- Glucose, and other reducing sugars: add Benedict's solution and heat in a water bath. Blue to brick red, through green and orange if there is less sugar present.
- Protein: add Biuret solution. Blue to purple, sometimes described as lilac.
- Fat: the emulsion test. Shake the sample with ethanol, then add water. A cloudy white emulsion forms.
The two that need something extra
Benedict's is the only test that requires heating. If a question describes adding Benedict's solution and observing no change, check whether the sample was heated at all, because without a water bath the test cannot work.
The emulsion test is the only one that does not rely on a reagent changing colour. The white cloudiness is fat coming out of solution when water is added, because fat dissolves in ethanol but not in water. Understanding that makes the result easy to remember and hard to confuse with the others.
Reading a negative result
A negative result is not "nothing happened". It is the reagent staying the colour it started as. Iodine that remains orange brown tells you starch is absent. Benedict's that stays blue tells you there is no reducing sugar.
Saying what colour the reagent remains is worth doing, because it shows you knew what the starting colour was.
What examiners say about this topic
Principal examiner reports for Edexcel International GCSE Biology show the starch test appearing most often inside photosynthesis experiments rather than as a question on its own. Learners are asked to predict and explain a result, and to label a diagram showing it.
One report describes learners correctly explaining that no starch would be present so the iodine would remain orange, because no carbon dioxide had been available for photosynthesis. That is the shape to aim for: state the colour, then give the reason it stayed that way.
Testing a food that contains several nutrients
Most real foods give more than one positive result, so questions often ask you to test the same sample several times. Split the sample first. Testing one portion, then adding a second reagent to the same tube, contaminates the result and the colours become impossible to read.
If asked to plan an investigation, say that you would divide the sample and carry out each test on a separate portion. It is a simple point and it is frequently worth a mark.
Spec 2.9
What you need to know
- Test foods for glucose and starch
- Test foods for protein and fat
- Recall the positive colour change for each test
Active recall
Quick check
Answer each question before opening the answer.
State the reagent and positive result for starch and for protein.
Starch — iodine solution turns blue-black. Protein — Biuret reagent turns from blue to purple.
How do you test for a reducing sugar and for a lipid?
Reducing sugar — Benedict's solution, heat; blue to brick-red. Lipid — ethanol emulsion test; a white/cloudy emulsion forms.
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