Required practical food test

• Carrying out chemical tests for carbohydrates, proteins and lipids

  1. Grind food sample with distilled water using mortar and pestle to make a paste

  2. Transfer paste to beaker and add more distilled water

  3. Stir to dissolve chemicals

  4. Filter solution to remove suspended food particles
• Carbohydrates

  Include starch and sugars such as glucose
• Test for starch

  1. Place 2cm3 of food solution in test tube

  2. Add a few drops of iodine solution

  3. Blue-black colour indicates presence of starch
• Test for sugars (e.g. glucose)

  1. Place 2cm3 of food solution in test tube

  2. Add 10 drops of Benedict's solution

  3. Heat test tube in hot water bath for 5 minutes

  4. Colour change indicates amount of reducing sugars present
• Benedict's test

  Only works for reducing sugars, not non-reducing sugars like sucrose
• Test for proteins

  1. Place 2cm3 of food solution in test tube

  2. Add 2cm3 of Biuret solution

  3. Purple/lilac colour indicates presence of proteins
• Test for lipids/fats

  1. Grind food with distilled water using mortar and pestle

  2. Transfer 2cm3 of solution to test tube

  3. Add a few drops of distilled water and ethanol

  4. Shake gently

  5. White cloudy emulsion indicates presence of lipids
• All chemicals used in these tests are potentially hazardous, so safety goggles must be worn
• Enzymes


  Usually large protein molecules that speed up reactions
• Amylase


  • Helps break down starch into smaller maltose molecules
• Without an enzyme holding the large molecule in position

  The reaction would be extremely slow
• The activity of salivary amylase is affected by temperature
• Measuring the effect of temperature on amylase activity

  1. Place a measured amount of starch solution into three test tubes

  2. Immerse the test tubes in water at 5, 20, and 35 degrees Celsius

  3. Add the same amount of salivary amylase to each test tube

  4. After 1 minute, add a drop of each solution to a reddish-brown iodine solution

  5. Repeat the iodine test after another minute

  6. After 3 minutes, starch only remains in the coldest solution
• Temperature effect on amylase activity

  • At low temperatures, it may take several minutes before a change is observed
  • At higher temperatures, the amylase converts all the starch into sugars faster
• Enzymes are made of protein
• If the surrounding temperature is too high


  The protein molecules denature and the shape changes, affecting enzyme activity
• If the amylase and starch solution is kept at 60 degrees Celsius, the enzyme activity will be reduced due to denaturation