Detecting starch in food

Read our standard health & safety guidance

Lesson organisation

A quick and easy class experiment. It should be possible to test a range of foodstuffs in about 10 minutes.

Apparatus and chemicals

Eye protection

Each working group needs:

Clear plastic film (e.g. acetate sheet as used for an overhead projector)
Forceps (for handling foodstuffs)

Access to:

Paper towels

Sodium chlorate(I) solution (Sodium hypochlorite), 5% w/v of available chlorine (Irritant at this concentration), 10 cm³ (see notes 1 and 2)
Potassium iodide crystals (Low hazard), allow 5-10 small crystals per group

A range of foodstuffs, broken into small pieces, to include both starchy and non-starch containing foods (see note 3)

Technical notes

Sodium chlorate(I) solution (Sodium hypochlorite) (Irritant at concentration used) Note: this is NaOCl, NOT sodium chlorate(V), NaClO₃. Refer to CLEAPSS Hazcard 89
Potassium iodide crystals (Low hazard) Refer to CLEAPSS Hazcard 47B

1 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.

2 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 cm³ - 60 cm³ would be suitable for this purpose.

3 Suggestions for foodstuffs for testing:

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.

Procedure

HEALTH & SAFETY: Wear eye protection.

a Place a small piece of each of the foods to be tested on the plastic sheet.

b Place a small potassium iodide crystal on top of the piece of food.

c Add one drop of bleach solution (sodium chlorate(I) solution) and allow it to run over both crystal and food.

d If an intense blue-black colour is seen, the food contains starch.

e 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.

Health & Safety checked, April 2008

Web Links

This German website (but with English language version available) performs the test on a much larger scale, and with a different procedure. However it does explain the nature of the blue-black complex, and also shows a short video of their procedure.

http://www.uni-regensburg.de/Fakultaeten/nat_Fak_IV/Organische_Chemie/Didaktik/Keusch/D-Starch-e.htm

(Website accessed June 2008)