Chemical Tests for Carbohydrates, Proteins, and Lipids

Introduction

• Focus on chemical tests for carbohydrates, proteins, and lipids.
• Important practical with details on chemicals and positive results.
• Safety: Wear goggles as chemicals are hazardous.

Preparation of Food Sample

• Grind food sample with distilled water using a mortar and pestle to make a paste.
• Transfer paste to a beaker and add more distilled water.
• Stir to dissolve chemicals and food in water.
• Filter solution to remove suspended particles.

Testing for Carbohydrates

Starch Test

• Use 2 cm³ of food solution in a test tube.
• Add a few drops of iodine solution (orange color).
• Positive Result: Iodine turns blue-black (starch present).
• Negative Result: Iodine remains orange.

Sugar Test (e.g., Glucose)

• Use 2 cm³ of food solution and add 10 drops of Benedict's solution (blue color).
• Place test tube in a beaker half-filled with hot water for 5 minutes.
• Positive Result: Benedict's solution changes color.
  • Green: Small amount of sugar.
  • Yellow: More sugar present.
  • Brick red: Lot of sugar present.
• Note: Benedict's test works for reducing sugars like glucose, not non-reducing sugars like sucrose.

Testing for Proteins

• Use 2 cm³ of food solution and add 2 cm³ of Biuret solution (blue color).
• Positive Result: Biuret solution changes from blue to purple/lilac if protein is present.

Testing for Lipids (Fats)

• Note: Do not filter solution due to lipid molecules sticking to filter paper.
• Transfer 2 cm³ of food solution to a test tube.
• Add a few drops of distilled water and ethanol.
• Gently shake solution.
• Positive Result: White cloudy emulsion forms if lipids are present.
• Safety: Ethanol is highly flammable; avoid naked flames.

Conclusion

• For more questions and exercises, refer to the provided revision workbook.

• Disclaimer: Always follow safety protocols while handling chemicals.