Practical - Testing for Food Molecules (OCR GCSE Biology A (Gateway))

Food Tests

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 glucose (a reducing sugar)

• Add Benedict's solution 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 orange / brick red

The Benedict's test for glucose

Test for starch using iodine

• We can use iodine 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

In the presence of starch, iodine will turn from brown to blue-black

Test for protein

• Add drops of Biuret solution to the food sample

• A positive test will show a colour change from blue to violet / purple

The Biuret test for protein

Test for lipids

• Mix the food sample with 4cm³ of ethanol and shake

• Allow time for the sample to dissolve in the ethanol

• Strain the ethanol solution into another test tube

• Add the ethanol solution to an equal volume of cold distilled water (4cm³)

• A positive test will show a cloudy emulsion forming

The ethanol test for lipids

Food Test Results Table

Important hazards

• Whilst carrying out this practical you should try to identify the main hazards and be thinking of ways to reduce harm

• Biuret solution contains copper (II) sulfate which is dangerous particularly if it gets in the eyes, so always wear goggles

• Iodine is also an irritant to the eyes

• Sodium hydroxide in biuret solution is corrosive, if any chemicals get onto your skin wash your hands immediately

• Ethanol is highly flammable; keep it away from any Bunsen burner

• The Bunsen burner itself is a hazard due to the open flame