Food Tests (WJEC GCSE Biology: Combined Science)

Preparing a sample

• Before you can carry out any of the food tests described below, you may need to prepare a food sample first (especially for solid foods to be tested)
• To do this:
  • Break up the food using a pestle and mortar
  • Transfer to a test tube and add distilled water
  • Mix the food with the water by stirring with a glass rod
  • Filter the mixture using a funnel and filter paper, collecting the solution
  • Proceed with the food tests

Test for starch using iodine solution

• We can use iodine solution to test for the presence or absence of starch in a food sample
• Add drops of iodine solution to the food sample
• A positive test will show a colour change from orange/brown to blue/black

The iodine solution test for the starch diagram

In the presence of starch, the iodine solution will turn from brown to blue/black

Test for glucose

• We can use Benedict's reagent to test for the presence or absence of simple reducing sugars, such as glucose, in a food sample
• Add Benedict's reagent to the sample solution in a test tube
• Heat in a boiling water bath for 5 minutes
• Take the test tube out of the water bath and observe the colour
• A positive test will show a colour change from blue to brick red

Benedict's test for glucose diagram

The steps to carry out Benedict's test for glucose

Test for protein

• We can use biuret solution to test for the presence or absence of protein in a food sample
• Add drops of Biuret solution to the food sample
• A positive test will show a colour change from blue to violet

The biuret test for protein diagram

The steps to carry out the biuret test for protein

Food test results table

Food test	Colour of reagent	Positive test result	Negative test result
Iodine for starch	Brown	Blue/black	Brown
Benedict's for glucose	Blue	Brick red	Blue
Biuret for protein	Blue	Violet	Blue