Practical 3 (Biomolecules) - Food Tests for Starch, Glucose, Lipids and Proteins

Today during Biology Practical, we conducted food tests on some solutions.

Aim of experiment: To conduct food tests on known and unknown food samples.

The food tests conducted on the unknown food samples were to find out the composition of these samples.

This is the summary of our results:

 Solution 1 is glucose solution, solution 2 is egg albumen (which is rich in proteins), solution 3 is starch suspension and solution 4 is oil. As shown in the table above, the Benedict Test is the test for reducing sugar, Iodine test is the test for starch, Biuret's Test is the test for proteins and lastly the ethanol emulsion test is the test for lipids. The tests conducted on solutions 1, 2, 3 and 4 proves the components in the solutions.

We were also given unknown solutions A and B, where we had to conduct all the tests on. Based on the table, it is apparent that solution A had large amounts of reducing sugar but no starch, protein and lipid present. The contents of solution B is opposite of those in solution A (ie. starch, protein and lipids were present but reducing sugar was absent).

These are the photos taken during the tests.

Benedict's test

 Set-up of water bath (heating the water to boiling point with a bunsen burner)

State of the solution B, A and 1 (from left to right) when nearing the end of the experiment 

Note: colour seen in this photo is not the final colour (ie. it was taken a little to early and we have not shaken the test-tubes to see the final colour)

As can be seen from the photo, solution B remained blue after the test which required us to add 2cm3 of Benedict's Solution into all 3 solutions and heat them up in a boiling water bath. Since solution B remained blue, it indicates that no reducing sugar. Solution A turned brick-red in colour by the end of the experiment which means that a large amount of reducing sugar is present in solution A. Lastly, solution 1, which was glucose solution turned orange in colour, indicating moderate amount of reducing sugar present in the solution. 

Iodine Test

From left to right: Solution B, A and 3 

Colour of solutions after the test 

As seen from the photo, solution B and 3 turned blue-black in colour after iodine was added. This indicates that both solution B and 3 had starch content. However, solution A remained yellow-brown in colour after iodine was added; hence, there is no starch in solution A. 

 Biuret's Test

 From left to right: Solution B, A and 2 after NaOH was added, but before CuSO4 was added

From left to right: Solution B, A and 2 after CuSO4 was added

As can be seen from the photo, solutions B and 2 turned violet in colour, indicating that protein is present. However, solution A remained clear, which means that protein is absent.

Ethanol Emulsion test 

From left to right: Solutions A, B and 4 after ethanol and water was added. 

From the photograph, it is apparent that solutions B and 4 had a white emulsion after ethanol and water was added (and the test tube was shaken); hence, lipid is present in solutions B and 4. However, solution A remained clear (did not have a white emulsion) after the test, meaning that lipid was absent in solution A. 

Moreover, we realised that solution 4 was warm after the test.