Food Tests

There are different tests we can use to identify carbohydrates, proteins and lipids in food.

For all the tests, you must first follow these steps:

1. Use a mortar and pestle to break up your food.

2. Place the crushed food into a beaker with distilled water.

3. Stir the mixture with a glass rod until some of the food dissolves.

4. Use a funnel and filter paper to filter the solution, removing any remaining solid bits of food.

Now you can use the final solution for the food tests.

Benedict’s Test for Reducing Sugars

Benedict’s solution is a deep-blue, alkaline chemical reagent, used to detect reducing sugars. Monosaccharides and some disaccharides are reducing sugars.

Illustrative diagram explaining the process of using Benedict's solution to determine sugar concentration. On the left, three steps are shown:A test tube with blue Benedict's solution.The test sample being added to the solution.Heating of the mixture. Above, labels indicate that Benedict's solution contains Copper sulphate pentahydrate, Sodium carbonate, and Sodium citrate.On the right, in step 4, five test tubes display results ranging from blue to red, indicating sugar concentrations from 'no sugar' to 'high'. The colour progression is: blue for 'no sugar', green for 'traceable', yellow for 'small', orange for 'moderate', and red for 'high'.

1. Using a Bunsen burner, set up a water bath and heat it to a minimum of 80 degrees Celsius.

2. Add some of the test sample to a test tube containing Benedict’s solution.

3. Put the test tube in the water bath for 5 minutes.

4. Record any colour change in your results.

If no reducing sugars are present in your sample, the solution will remain blue. However, if they are present, it can change to a range of colours (green, yellow-orange, or brick red) depending on the concentration.

Iodine Test for Starch

Detecting starch or complex carbohydrates in an organic sample:

Illustrative diagram explaining the iodine test for the presence of starch. The image depicts droppers adding a few drops of iodine solution (composed of iodine and potassium iodine) to both a solid sample on a dish and a liquid sample in a test tube. Observing the colour change will determine the result:Positive result: The sample changes to a deep blue colour, indicating the presence of starch.Negative result: There is no colour change in the sample, indicating the absence of starch.

1. Place some of your test sample into a test tube

2. Add a few drops of iodine solution

3. Record any colour change in your results

If the colour changes to deep blue/black then your sample contains starch.

Biuret Test for Proteins

The Biuret test shows the presence of peptide bonds, which are the bonds that hold amino acids together in proteins. In the test, NaOH and CuSO4 are used.

Diagram illustrating the biuret test for the presence of proteins. The process is divided into three steps:Step 1: Sodium hydroxide (NaOH) is added to a test sample, turning it blue.Step 2: A few drops of copper sulfate (CuSO₄) are added to the blue solution.Step 3: The result is observed. A negative result shows no colour change, while a positive result turns the solution violet, indicating the presence of proteins. A detailed chemical structure shows how copper ions (Cu²⁺) form a complex with peptide bonds. The more proteins present, the more intense the violet colour change.

1. Add an equal volume of sodium hydroxide (NaOH) to your test sample in a test tube.

2. Add two drops of copper sulphate (CuSO4 ) solution and stir for a few minutes.

3. Record any colour changes in your results.

If your food sample contains proteins then the solution will change from blue to purple.

Emulsion Test for Lipids

Lipids are insoluble in water. However, they can be dissolved in an organic solvent like ethanol.

A step-by-step illustration detailing the test for lipids in a solution.In 'step 1', a test sample is shown being introduced into a test tube containing ethanol using a dropper.For 'step 2', the test tube is depicted as being shaken, represented by wavy lines around it.By 'step 3', distilled water is poured into the heated test tube containing the solution, causing a reaction.Finally, in 'step 4', the outcome of the test is shown. The appearance of a white cloudy emulsion in the tube indicates the presence of lipids in the solution.

1. Add some of the test sample to a test tube containing ethanol.

2. Shake it until it’s mixed well.

3. Add an equal volume of distilled water.

4. Record any observations in your results.

A white cloudy emulsion will form if your test sample contains lipids. If there is no cloudy white emulsion, then there are no lipids in your sample.

You’ve used 10 of your 10 free revision notes for the month

Sign up to get unlimited access to revision notes, quizzes, audio lessons and more

Sign up