How can we test a food to find out which food molecules it contains?
Describe the food tests for starch, reducing sugars, proteins and fats, including the reagents used and the colour changes seen
A scaffolded answer to the N(A)-Level Biology outcome on food tests. The reagent, method and colour change for the starch, reducing sugar, protein and fat tests, set out as a clear practical reference.
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What this dot point is asking
This outcome wants you to know the four food tests: starch, reducing sugar, protein and fat. For each one you should be able to name the reagent (the chemical added), describe the method, and state the colour change that shows a positive result. These are common practical questions, and the marks come from precise reagents and precise colours, so learn them exactly.
The answer
The starch test (iodine test)
Add a few drops of iodine solution to the food sample. Iodine solution is orange-brown. If starch is present, it turns blue-black. If no starch is present, it stays orange-brown. No heating is needed.
The reducing sugar test (Benedict's test)
Add Benedict's solution to the food sample and heat it in a hot water bath. Benedict's solution is blue. If a reducing sugar (such as glucose) is present, the colour changes from blue to green, then orange, and finally brick-red. The more sugar, the further the colour change goes. If no reducing sugar is present, it stays blue.
The protein test (Biuret test)
Add Biuret solution to the food sample (this contains sodium hydroxide and copper sulfate). Biuret solution is blue. If protein is present, it turns purple (violet). If no protein is present, it stays blue. No heating is needed, but because sodium hydroxide is corrosive, eye protection should be worn.
The fat test (ethanol emulsion test)
Add ethanol to the food sample and shake, then pour the mixture into a tube of water. If fat is present, a white, cloudy (milky) emulsion forms. If no fat is present, the liquid stays clear.
Examples in context
Example 1. Checking a baby's milk for sugar. A nurse could add Benedict's solution to a sample of milk and heat it; a colour change towards orange or red shows reducing sugar (lactose breaks down to give reducing sugars). The further the colour change, the more sugar is present, which gives a rough idea of the amount.
Example 2. Testing a crisp for fat. Crushing a crisp with ethanol and then adding water gives a white, cloudy emulsion, showing the crisp contains fat. This simple test is why fatty foods give a positive ethanol test while a plain cracker with little fat does not.
Try this
Q1. State 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 if starch is present.
Q2. Explain why the Benedict's test must be heated. [1 mark]
- Cue. Benedict's solution only changes colour with a reducing sugar when it is heated; without heat it stays blue even if sugar is present.
Q3. Describe the test for fat and the positive result. [2 marks]
- Cue. Add ethanol to the sample, shake, then add water; a white, cloudy (milky) emulsion shows that 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.
Original4 marksDescribe how you would test a food sample for (a) starch and (b) reducing sugar. For each, state the reagent used and the positive colour change.Show worked answer →
(a) Starch test: add a few drops of iodine solution to the food sample. If starch is present, the orange-brown iodine turns blue-black. If no starch is present, it stays orange-brown.
(b) Reducing sugar test: add Benedict's solution to the food sample and heat it in a hot water bath. If reducing sugar is present, the blue Benedict's solution turns from blue through green and orange to a brick-red (orange-red) colour. If none is present, it stays blue.
What markers reward: the correct reagent (iodine; Benedict's solution), the need to heat in the Benedict's test, and the correct colour change (orange-brown to blue-black; blue to brick-red). Forgetting to heat the Benedict's test is a common lost mark.
Original3 marksA student tests a food for protein. State the reagent used, the colour change for a positive result, and one safety precaution needed.Show worked answer →
Reagent: Biuret solution (or sodium hydroxide solution followed by copper sulfate solution).
Positive colour change: blue to purple (violet/lilac) if protein is present.
Safety precaution: Biuret solution contains sodium hydroxide, which is corrosive, so wear eye protection (safety goggles) and handle it carefully, avoiding skin contact.
What markers reward: Biuret as the reagent, the blue to purple colour change, and a sensible safety point linked to the corrosive sodium hydroxide. A vague safety answer such as be careful does not score.
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