Food Tests (Cambridge O Level Biology)

Test for starch using iodine

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

Iodine test for starch diagram

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Iodine turns blue-black in the presence of starch

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

Placing iodine on a potato diagram

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Testing a potato to prove the presence of starch

Test for glucose (a reducing sugar)

• Add Benedict's solution into sample solution in test tube
• Heat at 60-70 °C in 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

The Benedict's test for glucose diagram

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The Benedict's solution turns brick red when heated in the presence of glucose

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

The Biuret test for protein diagram

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Biuret solution turns purple in the presence of protein

Test for lipids

• Food sample is mixed with 2cm³ of ethanol and shaken
• The ethanol is added to an equal volume of cold water
• A positive test will show a cloudy emulsion forming

The ethanol emulsion test for lipids

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The ethanol emulsion test shows a cloudy white positive result