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 interpret tests for starch, reducing sugars, protein and fat.Syllabus
Syllabus K325, Topic 3(c). Describe and interpret tests for starch, reducing sugars, protein and fat.
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
Prepare a suitable food extract, then use separate portions for separate tests. For starch, add iodine in potassium iodide solution. A change from yellow-brown to blue-black indicates starch. If no starch is detected, the iodine colour remains yellow-brown.
For reducing sugars, add Benedict's solution to the sample and heat it in a hot water bath using the instructed laboratory method. The solution begins blue. A positive result produces a green, yellow, orange or brick-red precipitate, depending on conditions and the amount of reducing sugar.
A precipitate is an insoluble solid formed in the liquid. Compare colours only when sample volumes, reagent volumes, heating time and temperature are controlled. The test is useful for approximate comparisons; it does not give an exact sugar concentration from a colour name alone.
Use water as a negative control and a known starch or glucose solution as an appropriate positive control. The controls help show whether the reagents and method work. A blue Benedict's mixture that was not heated properly is not reliable evidence that reducing sugar is absent.
Keep the conclusion specific. A negative iodine test means starch was not detected. A negative Benedict's result means reducing sugar was not detected under the conditions used. Neither test alone rules out every carbohydrate.
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?