Biological Tests for Starch, Glucose, and Proteins

Starch Test

• Iodine solution is used to test for starch.
• Place a small sample of the food on a spotting tile.
• Add 2-3 drops of iodine solution to the food sample.
• Observe the colour change.
• Negative result (no starch): orange/brown
• Positive result (starch present): blue/black

Glucose Test

• Benedict's solution is used to test for glucose (simple sugars).
• Prepare a sample of food.
• Put 5 ml of food sample into a test tube.
• Add 5 ml of Benedict's solution, mix thoroughly.
• Place the test tube in a water bath at 80°C for 10 minutes.
• Record the colour change after 10 minutes.
• Negative result (no glucose): blue
• Positive results:
  • Low glucose concentration: green/yellow
  • Medium glucose concentration: orange
  • High glucose concentration: brick red

Protein Test

• Biuret's test is used.
• Prepare a 5 ml sample of the food.
• Add 5 ml of Biuret's solution.
• Mix the solutions thoroughly.
• Observe the colour change.
• Negative result (no protein): solution remains blue.
• Positive result (protein present): solution turns purple.

Lipid Test

• Emulsion test is used to detect lipids.
• Place a small spatula of the food sample in a test tube.
• Add 5 ml of ethanol and shake until the food dissolves (to make it soluble).
• Add 5 ml of water and mix.
• Observe the result.
• Positive result (lipids present): solution turns cloudy white.
• Negative result (no lipids): solution remains clear.

Food Preparation for Testing

• Prepare the food sample (example: bread).
• Place the food sample into a mortar and add a small amount of water.
• Grind the food using a pestle.
• Add 5 ml of water and mix thoroughly.
• Fold filter paper and place it into a funnel.
• Filter the food sample into a small beaker.