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
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 |
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.