Grade 9 · 🧬 Biology · Food and nutrition
Which foods contain starch?
Use a drop of iodine antiseptic from the pharmacy to test injera, potato, bread, sugar and more for starch.
Why this matters
Teff, maize, wheat and potato provide most of the energy Ethiopians eat each day, mostly in the form of starch. Nutritionists test foods to plan school meals and fight malnutrition. With one drop of iodine you can do the same test they use in the lab.
Watch the demo
Video coming soon
We're recording a demo for this experiment. Until then, follow the step-by-step guide below.
The science
Carbohydrates and starch
Starch is a carbohydrate made of many glucose units joined in long chains. Plants store energy as starch in seeds, roots and tubers.
The iodine test
Iodine solution is yellow-brown. When it meets starch, iodine molecules slip inside the coiled starch chains and the colour changes to blue-black. No starch means the colour stays yellow-brown.
You'll need
- Iodine antiseptic solution (povidone-iodine)↳ Sold in most pharmacies as a wound antiseptic
- Small pieces of food: injera, raw potato, bread, banana, boiled egg white, sugar, rice
- A white plate or a sheet of white paper under a plate
- A dropper or a straw
Safety first
- Iodine stains skin and clothes and must not be swallowed. Do not eat the tested food afterwards.
- Wash your hands after the experiment.
Step by step
0/4 doneWhat you should see
Potato, injera, bread and rice turn blue-black. Sugar and egg white stay yellow-brown. Ripe banana may show only a weak colour change.
Why it happens
Foods from seeds and tubers store energy as starch, which reacts with iodine to give the blue-black colour. Sugar is a simple carbohydrate, not starch, and egg white is mostly protein, so neither changes colour. As bananas ripen, their starch is converted into sugars, so the reaction becomes weaker.
Didn't work? Try this
Everything looks dark brown, so I can't tell the difference.
Your iodine is too concentrated. Dilute it with water and test again on a white background.
Sugar turned slightly dark.
Check that the spoon or plate didn't have starchy crumbs on it. Use a clean spot and test a fresh pinch.
Still stuck? Ask the lab assistant
Check your understanding
1. Iodine turns blue-black on a food. What does that show?
2. Why might a ripe banana react less than an unripe one?
Your lab notebook
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