Food Test for Starch (Iodine Test)

Aim

To test a food sample for the presence of starch using iodine solution, and to identify which foods contain starch.

What it tests for

The iodine test is used to detect the presence of starch, a carbohydrate stored in staple plant foods such as rice, potato, and bread. Iodine solution reacts with starch molecules to form a distinctive colour complex, so the test gives a quick, reliable result without any heating.

Variables

• Manipulated variable: the type of food sample tested, for example rice, potato, apple, and glucose solution.
• Responding variable: the colour of the sample after iodine solution is added, whether it turns blue-black or stays orange-brown.
• Controlled variables: the volume of iodine solution added, the amount of food sample used, and the temperature (room temperature throughout).

Materials and apparatus

• Food samples to be tested
• Iodine solution (iodine in potassium iodide solution)
• Distilled water as a control
• A white dimple tile, or test tubes and a test-tube rack
• A dropper or teat pipette
• A spatula, knife, or mortar and pestle for preparing solid foods
• Labels for each sample

Reagent and method

1. Prepare a small sample of the food. If it is solid, cut a thin slice or crush it; if testing a liquid, use it directly.
2. Place the food sample on a white dimple tile or in a test tube.
3. Add 2 to 3 drops of iodine solution (iodine in potassium iodide solution) onto the sample.
4. Observe and record any colour change immediately.
5. Compare the result with a control, such as distilled water, which should show no colour change.

Positive result

If starch is present, the orange-brown iodine solution changes to blue-black (sometimes described as blue-violet). The intensity of the blue-black colour gives a rough indication of how much starch is present, though the test is not used to measure exact concentration.

Expected results

Each food can be recorded as starch-positive or starch-negative from its colour after iodine is added.

Inference and conclusion

A colour change from orange-brown to blue-black shows that starch is present, because iodine molecules fit into the coiled starch molecules to form a blue-black complex. A sample that stays orange-brown contains little or no starch.

The control confirms that the colour change is caused by starch in the food and not by the iodine solution itself. Because the test reacts only with starch and not with sugars, it can be used alongside the Benedict's test to distinguish the two carbohydrates in a food sample.

Paper 3-style questions

Question 1 (hypothesis). A student thinks a slice of bread contains starch. State a suitable hypothesis.

Model answer. Bread contains starch, so iodine solution added to it turns from orange-brown to blue-black.

Question 2 (variables). A student tests rice, apple and glucose solution. State the manipulated, responding and one controlled variable.

Model answer. Manipulated variable: the type of food sample. Responding variable: the colour after iodine is added (blue-black or orange-brown).

Controlled variable: the volume of iodine solution (also the amount of sample and the temperature).

Question 3 (recording results). Design a table to record whether each of three foods contains starch. Model answer. A table with the headings Food sample, Colour after adding iodine and Presence of starch, with one row per food, records the results clearly.

Question 4 (inference). The potato turned blue-black but the apple stayed orange-brown. What can you infer?

Model answer. The potato contains starch because iodine turned blue-black, while the apple contains little or no starch because the iodine stayed orange-brown.

Safety

• Wear safety goggles, because iodine solution can irritate the eyes and skin.
• Avoid getting iodine solution on skin or clothing, as it stains; wash any splashes with water at once.
• Use a dropper carefully to add iodine solution and keep it away from your mouth.
• Label samples clearly and dispose of used samples as directed by the teacher.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions