Lesson 3 of 8 / Biological molecules
Testing for starch and reducing sugars
What does a colour change actually show?
I can describe iodine and Benedict's tests and interpret their observations accurately.
I can describe and carry out the four specified food tests.Syllabus
Syllabus K224 / K225, Topic 3(b). Describe and carry out the four specified food tests.
About 5 min
Make a guess. You are not marked.
You add Benedict's solution to a bread extract and heat it. It stays blue. Does the bread contain no carbohydrate?
Show the answer
Starch does not react with Benedict's solution.
Benedict's detects reducing sugars only. Bread is mostly starch, so test a fresh portion with iodine.
The key idea
Iodine solution tests for starch. Heated Benedict's solution tests for reducing sugars. A result identifies what the test detects, not every carbohydrate present.
A reducing sugar is a sugar that reacts in Benedict's test; glucose and maltose are examples. Starch is a carbohydrate, but it is tested with iodine solution.
Explore the idea
Match the procedure to the observation
Method: Add iodine in potassium iodide solution to a separate portion of the sample.
Starch: The result is not yet available. Use the full procedure, including heating where required.
Illustrative result, not a simulated experiment. Use a known positive control and a suitable negative control. A negative result means the target was not detected under the test conditions.
Explanation
Make a food extract and use a fresh portion for each test. To test for starch, add iodine solution. If starch is there, the colour changes from yellow-brown to blue-black. If there is no starch, it stays yellow-brown.
To test for reducing sugars, add Benedict's solution and heat in a hot water bath. The solution starts blue. If reducing sugar is there, a coloured precipitate forms: green, yellow, orange or brick-red. The more sugar, the further along this list the colour goes.
A precipitate is a solid that forms in a liquid. Only compare colours if you used the same volumes, the same heating time and the same temperature. The colour gives a rough idea of the amount of sugar, not an exact value.
Use water as a negative control, which should give no colour change. Use a known starch or glucose solution as a positive control, which should change colour. The controls show that the method works. If the Benedict's mixture was not heated properly, a blue result proves nothing.
Be exact when you write a conclusion. A negative iodine test means no starch was found. A negative Benedict's test means no reducing sugar was found. Neither test shows that there is no carbohydrate at all.
Step by step
- 1
Choose the correct reagent
Iodine solution for starch; Benedict's solution for reducing sugars.
- 2
Include the necessary treatment
Benedict's test requires heating in a water bath. Follow the laboratory procedure for hot glass and liquids.
- 3
State observation before conclusion
For example: blue-black colour, therefore starch detected. Do not write only "positive".
Worked example
Two portions of one food extract
One portion turns blue-black with iodine. Another remains blue after a correctly controlled Benedict's test. What can be concluded?
One way to explain it
Starch is detected, but reducing sugar is not detected under the test conditions. The extract still contains carbohydrate because starch is a carbohydrate.
Why this answer works
- Treat each portion as a separate test.
- Match each observation to the substance detected.
- Avoid converting "no reducing sugar detected" into "no carbohydrate".
Watch out for this
A student says: "A blue Benedict's result means the sample has no carbohydrate." What is wrong with this?
Show the answer
Benedict's test detects reducing sugars, not every carbohydrate. Starch may still be present, so choose a separate appropriate test.
Check your understanding
Benedict's solution stays blue after being added to glucose solution, but the mixture was not heated. What is the best next step?
Show the answer
Repeat correctly with water-bath heating and appropriate controls.
Missing heating makes the method incomplete. A known positive control checks that the method and reagent work.
Exam-style questions
Check your understanding
Invented results for four foods: P: iodine blue-black, Benedict's blue. Q: iodine brown, Benedict's brick-red. R: iodine blue-black, Benedict's green. S: iodine brown, Benedict's blue. Which food contains starch and a small amount of reducing sugar?
Show the answer
R
Blue-black iodine shows starch, and a green Benedict's result shows a small amount of reducing sugar.
Quick recall
What does a colour change actually show?