Food Tests
Why Do We Test for Nutrients?
Food contains biological molecules like carbohydrates, proteins, and lipids. Different chemical tests are used to identify these nutrients.
💡 Key Concept: Each nutrient reacts differently with specific reagents, producing a colour change to indicate its presence.
Required Practical: Food Tests
Nutrient | Test Used | Positive Result |
Starch (Carbohydrate) | Iodine Test | Blue-black colour |
Reducing Sugars (e.g., Glucose) | Benedict’s Test | Brick red (after heating) |
Proteins | Biuret Test | Purple/lilac colour |
Lipids (Fats & Oils) | Emulsion Test (Ethanol Test) | Cloudy white emulsion |
How to Perform the Food Tests
Iodine Test (for Starch)
✔ Add iodine solution to the food sample.
✔ Positive result: Turns blue-black if starch is present.
Benedict’s Test (for Reducing Sugars like Glucose)
✔ Add Benedict’s solution to the food sample.
✔ Heat in a water bath (75°C) for 5 minutes.
✔ Positive result: Turns from blue → green → yellow → brick red (depending on sugar concentration).
💡 Important: This test requires heat to work!
Biuret Test (for Proteins)
✔ Add Biuret solution (a mix of sodium hydroxide and copper sulfate).
✔ Positive result: Turns purple/lilac if protein is present.
💡 Important: This test does not require heating.
Emulsion Test (for Lipids)
✔ Add ethanol to the food sample and shake.
✔ Pour into water and mix.
✔ Positive result: A cloudy white layer forms if lipids are present.
💡 Why? Lipids dissolve in ethanol but not in water, forming an emulsion.
Questions
- What solution is used to test for starch?
- What is a positive result for Benedict’s test?
- What colour does Biuret solution turn if protein is present?
- Which test requires heating?
- How do you test for lipids?
Summary
- Different food tests help identify carbohydrates, proteins, and lipids.
- Iodine detects starch (blue-black).
- Benedict’s test detects reducing sugars (brick red after heating).
- Biuret test detects proteins (purple).
- Ethanol test detects lipids (cloudy white emulsion).