Food Tests (WJEC GCSE Biology)

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Testing for the Presence of Starch, Glucose & Protein

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

Testing for starch in solution

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 Benedict's test for glucose

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 Biuret test for protein

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

Examiner Tip

Note that the positive result of the iodine test is blue/black. This does not mean blue OR black. You must refer to the colour as "blue/black".

This is the same with the positive Benedict's test result: it is not "red" it is "brick red".

You will not get the mark if you do not use the correct term. 

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Emma

Author: Emma

Expertise: Biology

Prior to working at SME, Emma was a Biology teacher for 5 years. During those years she taught three different GCSE exam boards and two A-Level exam boards, gaining a wide range of teaching expertise in the subject. Emma particularly enjoys learning about ecology and conservation. Emma is passionate about making her students achieve the highest possible grades in their exams by creating amazing revision resources!