Food Tests (CIE IGCSE Biology: Co-ordinated Sciences (Double Award))

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

Use this image

The Benedict's test for glucose

 Test for starch using iodine

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

Use this image

The iodine test for starch

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

Use this image

Testing a potato to prove the presence of starch

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

Use this image

The Biuret test for 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

  

  Use this image

  The ethanol test for lipids

Test for vitamin C

• Add 1cm³ of 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

Use this image

The DCPIP test for vitamin C