CP 3: Food Tests

• What states can the test substance be in for testing?

  Solid or liquid
• Where should you place a small amount of the test substance for testing?

  On a white tile or in a test tube
• What is the color of iodine solution before adding it to the test substance?

  Yellow orange
• What indicates a positive result in the iodine test?

  A blue-black color

  
• What does a negative result in the iodine test look like?

  A yellow orange color that does not change
• What are the steps in the iodine test?

  1. Place about 1cm³ of the test substance in a test tube.
  2. Add an equal volume (about 1cm³) of biuret solution.
  3. Mix by swirling the tube.
• What indicates a positive result in the biuret test?

  A violet/purple/lilac color
• What does a negative result in the biuret test look like?

  No change to the blue color of copper sulfate
• What are the steps in the ethanol emulsion test?

  1. Mix and crush the material with a glass rod in ethanol.
  2. Filter or decant the ethanol into a second test tube with cold water.
  3. Discard the solid residue.

  
• What indicates a positive result in the ethanol emulsion test?

  A milky emulsion forming in the water

  
• What does a negative result in the ethanol emulsion test look like?

  No milky emulsion
• What are the steps in the Benedict's test?

  1. Place about 1cm³ of a solution in a test tube.
  2. Add an equal volume (about 1cm³) of Benedict's solution.
  3. Boil or heat to 80°C in a water bath for 2 minutes.
• What indicates a positive result in the Benedict's test?

  The Benedict's reagent will turn red through a series of colors
• What does a negative result in the Benedict's test look like?

  No change to the blue color
• What is the color change sequence in the Benedict's test?

  • Blue → Green → Yellow → Orange → Brick red
• What is the aim of food tests?

  To conduct qualitative chemical tests for starch, reducing sugars, proteins, and lipids.
• What does the term "reducing sugars" mean?

  A molecule that can react with other molecules by giving electrons
• What equipment is needed for the iodine test for starch?

  • Food sample
  • A test tube
  • Iodine solution (0.01 mol/dm³)
  • Pipettes
• What are the exam tips for food tests?

  • Reducing Sugars (Benedict’s, Heat) – Red-orange
  • Proteins (Biuret, cold) – Purple
  • Lipids (ethanol and water, cold) – Look Cloudy
• What is the first step in the iodine test for starch?

  Put some of the food sample into a test tube.
• What temperature needs to be applied in the Benedict's test?

  80°C
• What color change indicates a positive result for starch testing?

  Brown to blue/black

  
• What indicates the presence of starch in the iodine test?

  The solution turns from brown to blue-black.
• What color change indicates a positive result for reducing sugar testing?

  Blue to orange/red

  
• What equipment is needed for the Benedict's test for reducing sugars?

  • Food sample
  • A test tube
  • Benedict's solution
  • Hot water bath
  • Thermometer
  • Pipettes
• What is the procedure for the Benedict's test for reducing sugars?

  Add Benedict's solution to the food sample and place it in a hot water bath.
• What is the procedure for testing non-reducing sugars?

  Boil with hydrochloric acid, cool, neutralize, and repeat Benedict's test

  
• What color change indicates the presence of reducing sugars in the Benedict's test?

  A brick red precipitate is formed.
• What color change indicates a positive result for protein testing?

  Blue to lilac

  
• What equipment is needed to test for protein?

  • A test tube
  • A 10cm³ measuring cylinder
  • Biuret solution
• What indicates a positive result for lipid testing?

  Cloudy emulsion forms near the top of the water

  
• What is the expected color change in the protein test if protein is present?

  The solution turns from blue to purple.
• What is the aim of practicing food tests?

  • To determine the presence of different food groups in samples.

  
• What should you record after performing food tests?

  • Record a positive or negative result in a table.
• What equipment is needed for the lipid test?

  • Food sample
  • Test tube
  • Ethanol
  • Distilled water
• What indicates the presence of lipids in the lipid test?

  A white emulsion is formed on the surface of the mixture.
• What is the emulsion test used for?

  To test for the presence of lipids.
• What are some sources of error in the Benedict's test and Biuret test?

  • Subtle color changes may be difficult to judge.
  • Low concentration of the tested molecule can affect results.
• What safety precautions should be taken during the Benedict's test?

  • Tie hair back and wear safety goggles.
  • Handle Biuret solution with care due to its poisonous and corrosive nature.
  • Keep ethanol solution away from flames as it is highly flammable.
• What problems might arise during food testing experiments?

  • Contamination of samples
  • Inaccurate measurements
  • Misinterpretation of color changes