Food Tests (Copy)

Food Tests – Expanded Section (O Level & IGCSE Biology Alternate to Practical)

Overview

Food tests are used to identify the presence of key nutrients in food samples. In practical exams, you may be asked to:

• Describe how to carry out the tests
• Identify positive and negative results
• Interpret results from data or color changes
• Design a table to record observations

1. Test for Starch

• Reagent: Iodine solution (yellow-brown)
• Procedure:
  1. Place food sample in a spotting tile or test tube.
  2. Add a few drops of iodine.
• Positive Result: Blue-black color
• Negative Result: Stays yellow-brown

2. Test for Reducing Sugars (e.g., glucose, maltose)

• Reagent: Benedict’s solution
• Procedure:
  1. Add Benedict’s solution to the sample.
  2. Heat in a hot water bath (~80°C) for 5 minutes.
• Positive Result:
  • Green → Yellow → Orange → Brick-red precipitate (depends on amount)
• Negative Result: Solution stays blue

Important Notes:

• Must heat for result to occur
• Benedict’s does not test for non-reducing sugars (e.g. sucrose unless hydrolyzed)

3. Test for Proteins

• Reagent: Biuret solution (sodium hydroxide + copper sulfate)
• Procedure:
  1. Add a few drops of sodium hydroxide.
  2. Add copper sulfate drop by drop, mix gently.
• Positive Result: Purple/lilac color
• Negative Result: Blue (no change)

4. Test for Lipids (Fats and Oils)

• Reagent: Ethanol and water (emulsion test)
• Procedure:
  1. Add ethanol to food sample and shake well.
  2. Add water and shake again.
• Positive Result: Cloudy white emulsion
• Negative Result: Solution remains clear

Important Notes:

• Test tube must be dry before starting
• Ethanol is flammable — heat should not be used

5. Test for Vitamin C (Ascorbic Acid)

(Less common but may appear)

• Reagent: DCPIP solution (blue dye)
• Procedure:
  1. Add food extract drop by drop to DCPIP.
• Positive Result: DCPIP is decolorized (blue → colorless)
• Negative Result: Stays blue

How to Present Food Test Results in Exams

Control Test

• Always test distilled water with each reagent to act as a control.
• Expected result = no color change.
• Proves reagents themselves are not contaminated and results are reliable.

Common Errors to Avoid

• Not heating Benedict’s test
• Forgetting to shake ethanol test
• Using excessive reagent, leading to unclear results
• Not comparing with a control
• Confusing positive color with the original reagent color

Marking Points

Quick Revision Table