Food Tests (Cambridge International IGCSE Biology)

Exam board Cambridge International · IGCSEUpdated 2026-08-25

Spec point — 4.1.2 · Describe the use of: (a) iodine solution test for starch (b) Benedict’s solution test for reducing sugars (c) biuret test for proteins (d) ethanol emulsion test for fats and oils (e) DCPIP test for vitamin C

Food tests

Food test for glucose (a reducing sugar)

Add Benedict's solution into sample solution in test tube
Heat at 80 °C in a water bath for 5 minutes
Take test tube out of water bath and observe the colour
A positive test will show a colour change from blue to orange or brick red

Diagram of Benedict's test for sugars. A test tube with solution and Benedict's solution is heated in a water bath over a flame to observe colour change.

*A positive test for glucose will show a colour change from blue to orange or brick red *

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Food test for starch

Iodine is used to test for the presence or absence of starch in a food sample

Diagram showing starch turns blue-black with iodine, maltose doesn't change. Amylase breaks starch into glucose; maltose remains unchanged.

We can use iodine to test for the presence or absence of starch in a food sample

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Add drops of iodine solution to the food sample
A positive test will show a colour change from orange-brown to blue-black

Steps of a starch test on a potato: whole, peeled, iodine applied turning brown, and colour turns blue-black indicating starch presence.

A positive test for starch will show a colour change from orange-brown to blue-black

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Food test for protein

Add drops of Biuret solution to the food sample
A positive test will show a colour change from blue to violet / purple

Test tubes illustrating a Biuret test. Left tube with Biuret solution drips into food sample, middle shows positive result (purple), right shows negative (blue).

A positive test for protein will show a colour change from blue to violet / purple

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Food test for lipids

Food sample is mixed with 2 cm³of ethanol and shaken
The ethanol is added to an equal volume of cold water
A positive test will show a cloudy emulsion forming

Four test tubes show ethanol and water mixtures. One is cloudy for a positive emulsion test, and one is clear for a negative result.

A positive test for lipids will show a cloudy emulsion forming

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Food test for vitamin C

Add 1 cm³of DCPIP solution to a test tube
Add a small amount of food sample (as a solution)
A positive test will show the blue colour of the dye disappearing

Pipette adding sample to blue DCPIP in test tube, showing colour change to clear for positive vitamin C test, or remaining blue for negative result.

A positive test for vitamin C will show the blue colour of the dye disappearing

Use this image

Exam Hint When describing food tests in exam answers, make sure you give the starting colour of the solution and the colour it changes to for a positive result.

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Food Tests (Cambridge International IGCSE Biology)

Exam board Cambridge International · IGCSEUpdated 2026-08-25

Spec point — 4.1.2 · Describe the use of: (a) iodine solution test for starch (b) Benedict’s solution test for reducing sugars (c) biuret test for proteins (d) ethanol emulsion test for fats and oils (e) DCPIP test for vitamin C

Food tests

Food test for glucose (a reducing sugar)

Add Benedict's solution into sample solution in test tube
Heat at 80 °C in a water bath for 5 minutes
Take test tube out of water bath and observe the colour
A positive test will show a colour change from blue to orange or brick red

Diagram of Benedict's test for sugars. A test tube with solution and Benedict's solution is heated in a water bath over a flame to observe colour change.

*A positive test for glucose will show a colour change from blue to orange or brick red *

Use this image

Food test for starch

Iodine is used to test for the presence or absence of starch in a food sample

Diagram showing starch turns blue-black with iodine, maltose doesn't change. Amylase breaks starch into glucose; maltose remains unchanged.

We can use iodine to test for the presence or absence of starch in a food sample

Use this image

Add drops of iodine solution to the food sample
A positive test will show a colour change from orange-brown to blue-black

Steps of a starch test on a potato: whole, peeled, iodine applied turning brown, and colour turns blue-black indicating starch presence.

A positive test for starch will show a colour change from orange-brown to blue-black

Use this image

Food test for protein

Add drops of Biuret solution to the food sample
A positive test will show a colour change from blue to violet / purple

Test tubes illustrating a Biuret test. Left tube with Biuret solution drips into food sample, middle shows positive result (purple), right shows negative (blue).

A positive test for protein will show a colour change from blue to violet / purple

Use this image

Food test for lipids

Food sample is mixed with 2 cm³of ethanol and shaken
The ethanol is added to an equal volume of cold water
A positive test will show a cloudy emulsion forming

Four test tubes show ethanol and water mixtures. One is cloudy for a positive emulsion test, and one is clear for a negative result.

A positive test for lipids will show a cloudy emulsion forming

Use this image

Food test for vitamin C

Add 1 cm³of DCPIP solution to a test tube
Add a small amount of food sample (as a solution)
A positive test will show the blue colour of the dye disappearing

Pipette adding sample to blue DCPIP in test tube, showing colour change to clear for positive vitamin C test, or remaining blue for negative result.

A positive test for vitamin C will show the blue colour of the dye disappearing

Use this image

Exam Hint When describing food tests in exam answers, make sure you give the starting colour of the solution and the colour it changes to for a positive result.

Was this revision note helpful?

Build on this topic

Exam QuestionsBiological molecules FlashcardsBiological molecules