Agriculture and Food 7–12 · Years 7–8

Testing foods for starch with iodine (microscale)

Food and agricultural practices

PracticalLow risk

The idea

Iodine forms a blue-black complex with starch, so a drop test on small food samples shows which foods store carbohydrate as starch and which do not.

What you need

clear plastic film (acetate sheet) over a printed grid
pea-sized samples of pasta, bread, porridge oats, potato, mushroom, apple, cheese and cooked chicken (the RSC list)
cornflour as a known starch control and water as a blank
iodine solution in potassium iodide (about 0.01 mol/L iodine) in a dropper bottle, or the RSC in-situ method with potassium iodide crystals and sodium hypochlorite solution (the RSC method uses 5% w/v available chlorine; read the label before substituting household chlorine bleach, which is usually weaker)
plastic dropping pipettes, eye protection, a bin for used samples

How to do it

Place each food sample, the cornflour control and a drop of water on its own square of the grid.
Predict for each food whether it contains starch, using what you know of the food or its nutrition information panel.
Add one drop of iodine solution to each sample and wait one minute.
Record blue-black (starch present) or yellow-brown (no starch detected) for each sample.
Test a raw and a cooked sample of the same food, such as potato or pasta, and compare how clear the result is.
Wipe the samples into the bin; do not wash them down the sink.

What you should see

Pasta, bread, oats, potato and the cornflour control turn blue-black; mushroom, cheese, cooked chicken and the water blank stay yellow-brown. Cooked starchy foods give a clearer result than some raw ones, because cooking frees more starch (as the RSC notes say). Apple is on the RSC list of non-starchy foods, and ripe apple that tastes sweet holds its carbohydrate mostly as sugars, but apple still holds some starch around harvest (growers use this iodine test to judge when to pick), so recently picked apple can stain partly blue-black; the RSC notes advise pre-testing each food. The learner knows it worked when the cornflour control is blue-black and the blank stays yellow-brown.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Every carbohydrate turns iodine blue-black.
The sweeter a food tastes, the more starch it contains.
Iodine solution tests for all nutrients.

Safety card

Low riskLearners carry it out

Hazards

iodine solution stains skin and irritates eyes
sodium hypochlorite solution (in-situ method) irritates eyes and skin
food samples in a laboratory are contaminated

Controls

eye protection throughout
reagents in dropper bottles, dispensed one drop at a time
learners do not taste or eat any sample in the laboratory

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1 (general information for all staff) and Volume 2 Appendix D (generic risk assessment advice for each chemical). Complete a CSIS or RiskAssess site-specific risk assessment before the lesson.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Food Technology Years 7–10 Syllabus (2019)FT4-3
Technology 7–8 Syllabus (2023), focus area Food and agricultural practicesTE4-SAF-01
Australian Curriculum v9AC9TDE8K05

Sources

The pages the author read to write this activity.

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