Detecting starch in food on a microscale

In this experiment, students conduct qualitative tests to find out whether different foodstuffs contain starch. Working on a microscale, students produce iodine in situ by adding potassium iodide crystals and sodium hypochlorite solution to small samples of various foods. They then note any colour change to blue-black, indicating that starch is present.

A quick and easy class experiment. It should be possible to test a range of foodstuffs in about ten minutes.

Equipment

Apparatus

• Eye protection
• Clear plastic film (eg acetate sheet as used for an overhead projector)
• Forceps (for handling foodstuffs)
• Paper towels

Chemicals

• Sodium hypochlorite solution, 5% w/v of available chlorine (IRRITANT), 10 cm³
• Potassium iodide crystals, allow 5–10 small crystals per group
• A range of foodstuffs, broken into small pieces, to include both starchy and non-starch-containing foods

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Sodium chlorate(I) solution (sodium hypochlorite), NaOCl(aq), (IRRITANT at concentration used) – see CLEAPSS Hazcard HC089. Note this is NOT sodium chlorate(V), NaClO₃. Sodium chlorate(I) solution can be purchased as such from chemical suppliers. However domestic chlorine-containing bleach solution is quite adequate for this experiment, preferably a cheap brand containing no added detergent or perfumes. Household ‘bleaches’ based on peroxide are becoming more widely available and do not contain chlorine; they should therefore not be used. The sodium chlorate(I) solution should be provided in such a way that students can add a single drop using a plastic dropping pipette. Plastic dropper bottles of capacity 30–60 cm³ would be suitable for this purpose.
• Potassium iodide crystals, KI(s) –  see CLEAPSS Hazcard HC047b.

Procedure

It is worth pre-testing the foodstuffs to check that they test correctly – that is, the starchy foods contain enough free starch to give a clear positive test, and the non-starchy foods have not been contaminated by starch-containing material. Note that the amount of free starch present in some uncooked foods may be small, and the test may work more reliably on cooked food.

Suggestions for foodstuffs for testing:

1. Place a small piece of each of the foods to be tested on the plastic sheet.
2. Place a small potassium iodide crystal on top of the piece of food.
3. Add one drop of bleach solution (sodium hypochlorite solution) and allow it to run over both crystal and food.
4. If an intense blue-black colour is seen, the food contains starch.
5. Clean the plastic sheet with a moistened paper towel.

Teaching notes

The chlorine available from the bleach solution reacts with potassium iodide to form potassium chloride and iodine. The iodine then forms an intense blue-black coloured complex with any starch present. If starch is not present, only the brown colour of iodine in the presence of iodide ions will be seen. The nature of the coloured complex is beyond the level of the students, but note that it is an unstable substance from which the iodine can be easily removed by, for example, sodium thiosulfate.

Each group can be allocated a selection from the range of available foodstuffs, perhaps two starchy foods, and two non-starchy. The class results can then be pooled.