Food Tests - Required Practical

• All of the chemicals used in food tests are potentially hazardous , so safety goggles must be worn
• Describe the test for starch

  1. Take the food sample, and grind it with distilled water using a mortar and pestle to make a paste  
  2. Transfer the paste to a beaker and add more distilled water  
  3. Stir so the chemicals in the food dissolve in the water  
  4. Filter the solution to remove suspended food particles  
  5. Place 2cm ³of food solution into a test tube and add a few drops of iodine solution  
  6. If starch is present, the iodine solution will turn blue-black  
  7. If no starch is present, the iodine solution remains orange 
• Describe the test for sugars?

  1. Take the food sample, and grind it with distilled water using a mortar and pestle to make a paste  
  2. Transfer the paste to a beaker and add more distilled water  
  3. Stir so the chemicals in the food dissolve in the water  
  4. Filter the solution to remove suspended food particles  
  5. Place 2cm ³of food solution into a test tube and add Benedict’s reagent – then heat in a water bath for 2 minutes  
  6. If sugar is present, the solution changes colour  
• The colour of the Benedict's solution after a sugar test gives us an approximate idea of the amount of sugar present , but it cannot tell us the exact amount
• The colour of the Benedict's solution indicates the amount of sugar present in the solution :

  • Green indicates that there is a small amount of sugar  
  • Yellow tells us that there is more sugar present  
  • Brick-red tells us that there is a lot of sugar present 
• The Benedict's test only works for certain sugars like such as glucose - scientists call these reducing-sugars
• The Benedict's test will not work for non-reducing sugars such as sucrose
• Describe the test for protein?

  1. Take the food sample, and grind it with distilled water using a mortar and pestle to make a paste  
  2. Transfer the paste to a beaker and add more distilled water  
  3. Stir so the chemicals in the food dissolve in the water  
  4. Filter the solution to remove suspended food particles  
  5. Place 2cm ³of food solution into a test tube and add Biuret reagent / solution  
  6. If protein is present, the solution will turn purple  
• Describe the test for lipids?

  1. Take the food sample, and grind it with distilled water using a mortar and pestle to make a paste  
  2. Transfer 2cm³ of the food solution to a test tube  
  3. Add a few drops of distilled water and a few drops of ethanol to the food solution  
  4. Gently shake the solution  
  5. If lipids are present, a white cloudy emulsion forms 
• When testing for lipids , you need to remember that ethanol is highly flammable - so no naked flames should be present
• When testing for lipids , you should not filter the solution - this is because lipid molecules can stick to the filter paper
• When testing for lipids , you need to remember that ethanol is highly flammable - so no naked flames should be present