Food tests

• The aim of food tests is to conduct qualitative chemical tests for starch, reducing sugars, proteins, and lipids.
• The Iodine test for starch involves putting some of the food sample into a test tube, adding a few drops of iodine solution, and observing if a change in colour occurs from brown to blue-black.
• The Benedict’s test for reducing sugars involves adding an equal volume or excess of Benedict’s solution to the food sample in a test tube, placing it in a hot water bath for a few minutes, and observing if a brick red precipitate is formed if reducing sugar is present, or if the solution remains blue if reducing sugar is absent.
• The test for protein involves adding a few drops of Biuret’s reagent (sodium hydroxide and copper (II) sulphate) to the food sample in a test tube, shaking the solution to mix, and waiting for a few minutes.
• The test for lipids involves adding a few cm 3 of ethanol to the food sample, pouring this mixture into a test tube of equal volumes of distilled water, and observing if a white emulsion is formed on the surface of the mixture, which is called the emulsion test.
• Colour change of Benedict’s test and Biuret test may be subtle and difficult to judge if the concentration of the tested molecule is low.
• Safety precautions for the Benedict’s test include tying hair back and wearing safety goggles when using a Bunsen burner and hot water bath.
• Biuret solution contains copper sulphate (poisonous) and sodium hydroxide (corrosive), and should be handled with care.
• Ethanol is highly flammable, and should be kept away from flames.