RP3 - FOOD TEST

• Food tests

  1. Prepare food sample

  2. Crush food with mortar and pestle

  3. Dissolve in distilled water

  4. Filter solution
• Sugars


  Small units that all carbohydrates are made of
• Types of sugars

  • Reducing sugars
  • Non-reducing sugars
• Benedict's test for reducing sugars

  1. Prepare food sample

  2. Transfer 5cm3 to test tube

  3. Add 10 drops benedict solution

  4. Heat in 75°C water bath for 5 minutes

  5. Observe colour change
• Starch


  A type of carbohydrate that is a larger and more complex molecule than sugars
• Iodine test for starch

  1. Take 5cm3 food sample

  2. Add iodine solution

  3. Observe colour change
• Protein

  Tested using the Biuret test
• Biuret test for proteins

  1. Take 2cm3 food sample

  2. Add 2cm3 Biuret reagent

  3. Observe colour change
• Lipids

  Tested using the Sudan 3 test
• Sudan 3 test for lipids

  1. Take 5cm3 food sample

  2. Add 3 drops Sudan 3 stain

  3. Observe red layer
• Emulsion test for lipids (Edexcel)


  1. Take food sample

  2. Add 2cm3 ethanol, shake vigorously

  3. Add equal volume distilled water

  4. Observe cloudy white colour
• Benedict's solution itself in blue, but if reducing sugars are present then it will change colour depending on the concentration of reducing sugar.

  Green indicates a low concentration of reducing sugar, orange indicates a medium concentration and brick-red indicates a high concentration.

  
• Required practical activity 3: Use qualitative reagents to test for a range of carbohydrates, lipids and proteins


  1. Grind up food

  2. Add Benedict's reagent

  3. Heat (above 65°C) in a water bath

  4. Observe colour change (blue to green/yellow/orange/brick red) for sugar test

  5. Grind up food

  6. Add iodine solution

  7. Observe colour change (orange/brown to blue-black) for starch test

  8. Grind up food

  9. Add ethanol, then add water

  10. Observe milky/cloudy white solution for lipid test

  11. Grind up food

  12. Add Biuret reagent

  13. Observe colour change (blue to purple/lilac) for protein test
• Sugar test


  • Grind up food
  • Add Benedict's reagent
  • Sugar turns solution from blue to green/yellow/orange/brick red
• Starch test


  • Grind up food
  • Add iodine solution
  • Starch turns solution from orange/brown to blue-black
• Lipid test


  • Grind up food
  • Add ethanol, then add water
  • Fat turns solution milky/cloudy white
• Protein test


  • Grind up food
  • Add Biuret reagent
  • Protein turns solution from blue to purple/lilac
• Cells have a nucleus, such as plant and animal cells
• Safety precautions


  • Wear goggles to protect eyes
  • Clean up spills immediately as Biuret is an irritant/corrosive/poisonous
• Method to compare sugar content in mycoprotein and meat sausages

  1. Grind up each sausage and add to test tubes with water

  2. Add same mass of each sausage

  3. Add same volume of Benedict's solution to each test tube

  4. Place test tubes in a heated water bath for same length of time

  5. Repeat test for each sample

  6. Record colour change - green/yellow/orange/brown/red if sugar present (or blue if no sugar present)
• The closer the colour to red, the more sugar present
• Title: Food tests
• The iodine test for starch


  1. Put some of the food sample into a test tube

  2. Add a few drops of iodine solution to the food sample using a pipette

  3. If starch is present, the solution turns from brown to blue-black
• Equipment for iodine test


  • Food sample
  • A test tube
  • Iodine solution (0.01 mol/dm3)
  • Pipettes
• The Benedict's test for reducing sugars


  1. Add an equal volume or excess of Benedict's solution to the food sample in a test tube

  2. Place in a hot water bath for a few minutes

  3. If reducing sugar is present, a brick red precipitate is formed. If reducing sugar is absent, the solution remains blue
• Equipment for Benedict's test


  • Food sample
  • A test tube
  • Benedict's solution
  • Hot water bath
  • Thermometer
  • Pipettes
• Test for protein


  1. Add a few drops of Biuret's reagent (sodium hydroxide and copper (II) sulphate) to the food sample in a test tube

  2. Shake the solution to mix and wait for a few minutes

  3. If protein is present, the solution turns from blue to purple
• Equipment for protein test


  • A test tube
  • 10cm3 measuring cylinder
  • Biuret solution
• Test for lipids


  1. Add a few cm3 of ethanol to the food sample

  2. Pour this mixture into a test tube of equal volumes of distilled water

  3. If lipids are present, a white emulsion is formed on the surface of the mixture
• Equipment for lipid test


  • Food sample
  • Test tube
  • Ethanol
  • Distilled water
• 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: Tie hair back and wear safety goggles when performing the Benedict's test using a Bunsen burner and hot water bath
• Safety precautions: Handle Biuret solution with care. It contains copper sulphate (poisonous) and sodium hydroxide (corrosive). Wash immediately if comes into contact with skin and wipe away any spills to surfaces