Food Tests (Carbohydrates, lipids and proteins)

Carbohydrates

Testing for reducing sugars

1. Label some test tubes for the substances to be tested
2. Add 2cm² of test solution to labelled test tubes
3. Add 10 drops of Benedict's reagent (this is around 0.5cm³) to each test solution, without the dropper pipette touching the inside of the test tube
4. Half fill a 400cm³ beaker with boiling water
5. Use the test tube holder to transfer the test tube into the hot water. Observe the colour changes in the test tube over 2 minutes
6. Return the test tube to the test tube rack. Make a note of your observations
7. Repeat the test for all your samples

If there is a reducing sugar present, there should be a colour change. Benedict's reagent is blue and will change to a green, yellow, orange, brown or brick-red suspension.

However, a green solution indicates that there is a low concentration of reducing sugars.

Testing for non-reducing sugars

NOTE: This food test should only be carried out after a negative result for Benedict's test for reducing sugars

1. Label the test tubes
2. Use a pasteur pipette to dispense 2cm³ of a fresh sample into a test tube
3. Use a dropper pipette to add 10 drops of dilute hydrochloric acid (HCl)to the sample
4. Half fill a 400cm³ beaker will boiling water
5. Use a test tube holder to transfer the test solution to the beaker
6. Heat for two minutes - this is the stage where acid hydrolysis may occur
7. Cool the test tube in cold water (in a beaker or under a cold tap)
8. With a spatula add sodium hydrogencarbonate to the solution until fizzing stops.
9. Add 10 drops of Benedict's reagent (approx. 0.5cm³) to the test solution
10. Refill a 400cm³ beaker with boiling water
11. Heat the test solution for two minutes
12. Observe the colour changes in the test tubes over two minutes of heating at boiling point. Record your observations in a table

A colour change from blue to brick red, brown, orange, yellow or green indicates that a non-reducing sugar is present.

Testing for starch

1. Label individual dimples on the dimple tile with foods to be tested
2. Use a clean pasteur pipette to dispense a few drops of the test solution into a labelled dimple
3. Use the dropper pipette to add two drops of iodine solution (iodine in potassium iodide) to the food samples

If starch is present then the solution should change from a yellow iodine colour, to a blue-black colour

Lipids

1. Add 2ml of ethanol to each food sample or until the food sample is covered
2. Shake vigorously
3. Allow it to settle in a test tube rack for two to three minutes
4. Add 2ml of distilled water to a fresh test tube
5. Carefully decant the ethanol from the first test tube into the second test tube of water (the food must stay in the first test tube)

If a lipid is present, a cloudy white suspension forms at the top of the solution - this is called an emulsion.

Proteins

1. Label the test tubes with the food materials that you plan to test for protein.
2. Add the sample of food to each tube, then add 2cm² of distilled water. Put the clean bung in and shake carefully at least 10 times.
3. Add 10 drops of biuret reagent to the test solution without the pipette touching the inside of the test tube.
4. Observe the test solution for a purple/violet or blue colour. Record your observations and deductions from the biuret test in a table.

If a protein is present there will be a colour change from blue to violet or purple.

The colour change is due to two or more peptide bonds in a compound.