Food Tests for GCSE Biology Practicals

Overview

• This lecture covers different food tests required for GCSE Biology practicals.
• Tests covered include:
  • Benedict’s Test (for sugars)
  • Iodine Test (for starch)
  • Biuret Test (for proteins)
  • Sudan III Test (for lipids)
  • Emulsion Test (for lipids; specific to Edexcel course)

Preparing Food Samples

• Step 1: Break the food using a water and pestle.
• Step 2: Place the crushed food in a beaker with distilled water and stir with a glass rod until some dissolves.
• Step 3: Filter the solution using a funnel lined with filter paper to get a clean sample for testing.

Benedict’s Test for Sugars

• Purpose: Tests for reducing sugars.
• Procedure:
  • Prepare food sample.
  • Transfer 5 cm³ of the sample to a test tube.
  • Prepare a water bath and set it to 75°C.
  • Add ~10 drops of Benedict’s solution to the test tube.
  • Place the tube in the water bath for 5 minutes.
  • Results:
    • Solution stays blue if no reducing sugars.
    • Color changes to green, yellow, or brick-red depending on sugar concentration.

Iodine Test for Starch

• Purpose: Identifies presence of starch.
• Procedure:
  • Take 5 cm³ of the food sample.
  • Add a few drops of iodine solution.
  • Results:
    • Initial brown-orange color.
    • Changes to blue-black if starch is present.

Biuret Test for Proteins

• Purpose: Detects presence of proteins.
• Procedure:
  • Add 2 cm³ of the sample to a test tube.
  • Add 2 cm³ of Biuret solution.
  • Gently shake the mixture.
  • Results:
    • Original blue solution.
    • Changes to pink or purple if protein present.
  • Note for Edexcel: Use potassium hydroxide followed by copper sulfate instead of Biuret solution.

Sudan III Test for Lipids

• Purpose: Tests for lipids.
• Procedure:
  • Use 5 cm³ of unfiltered food sample.
  • Add 3 drops of Sudan III stain solution.
  • Gently shake the test tube.
  • Results:
    • Lipids stain red.
    • A bright red layer forms at the top if lipids are present.

Emulsion Test for Lipids (Edexcel)

• Purpose: Alternative test for lipids.
• Procedure:
  • Add 2 cm³ of ethanol to food sample and shake.
  • Add an equal volume of distilled water.
  • Results:
    • Cloudy white color indicates presence of lipids.
    • No cloudiness means no lipids present.

Conclusion

• These are all the tests needed for the food test practical in GCSE Biology.
• Useful for practical exams and understanding food composition.