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GCSE Biology Food Tests Overview

May 28, 2025

GCSE Biology Practicals: Food Tests

Introduction

  • Overview of food tests for GCSE Biology practicals.
  • Tests covered:
    • 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

  1. Break down food item using a mortar and pestle.
  2. Mix with distilled water in a beaker; stir with a glass rod.
  3. Filter the solution using a funnel lined with filter paper to remove solids.
    • Resulting solution is used for tests.

Benedict's Test for Sugars

  • Purpose: Tests for reducing sugars.
  • Procedure:
    1. Prepare food sample.
    2. Transfer 5 cm³ of sample to a test tube.
    3. Prepare a water bath at 75°C.
    4. Add ~10 drops of Benedict's solution to the test tube.
    5. Place test tube in water bath for 5 minutes.
      • Use test tube holder for safety.
  • Results:
    • Solution stays blue: No reducing sugars.
    • Changes to green, yellow, or brick red: Presence of reducing sugars.

Iodine Test for Starch

  • Purpose: Detects starch presence.
  • Procedure:
    1. Take 5 cm³ of sample.
    2. Add a few drops of iodine solution.
    3. Shake gently.
  • Results:
    • Browny-orange to blue-black color change: Starch present.

Biuret Test for Proteins

  • Purpose: Tests for protein.
  • Procedure:
    1. Add 2 cm³ of sample to a test tube.
    2. Add 2 cm³ of Biuret solution.
    3. Shake gently.
  • Results:
    • Blue to pink/purple change: Protein present.
  • Edexcel Specific:
    • Add potassium hydroxide, then copper sulfate.

Sudan III Test for Lipids

  • Purpose: Detects lipids.
  • Procedure:
    1. Use 5 cm³ of food sample (unfiltered).
    2. Add 3 drops of Sudan III stain solution.
    3. Shake gently.
  • Results:
    • Bright red layer on top: Lipids present.

Emulsion Test for Lipids (Edexcel)

  • Purpose: Alternative lipid test for Edexcel students.
  • Procedure:
    1. Add 2 cm³ of ethanol to sample and shake.
    2. Add equal volume of distilled water.
  • Results:
    • Cloudy white color: Lipids present.

Additional Resources

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Full transcript