Overview
This lesson covers essential GCSE biology practical food tests: Benedict's test for reducing sugars, iodine test for starch, biuret test for proteins, and Sudan 3 and emulsion tests for lipids.
Food Sample Preparation
- Break up food sample using a mortar and pestle to crush it thoroughly.
- Transfer crushed food to a beaker and add distilled water, then stir with glass rod.
- Filter the solution through filter paper in a funnel to remove solid particles.
- Use the filtered liquid solution for all food testing procedures.
Summary of Food Tests
| Test Name | Tests For | Key Reagent | Positive Result |
|---|
| Benedict's | Reducing sugars | Benedict's solution | Blue → green/yellow/brick red |
| Iodine | Starch | Iodine solution | Brown-orange → blue-black |
| Biuret | Proteins | Biuret solution | Blue → pink/purple |
| Sudan 3 | Lipids | Sudan 3 stain | Bright red layer forms on top |
| Emulsion | Lipids (Edexcel) | Ethanol + water | Cloudy white appearance |
Benedict's Test (Reducing Sugars)
- Sugars are small carbohydrate units; Benedict's test detects only reducing sugars.
- Transfer 5 cm³ of food sample to test tube.
- Set water bath to 75°C before starting the test.
- Add 10 drops of Benedict's solution to the test tube.
- Use test tube holder; point tube away from people for safety.
- Place tube in water bath for 5 minutes.
- Blue color indicates no reducing sugars present.
- Green, yellow, or brick red indicates reducing sugars; intensity depends on concentration.
Iodine Test (Starch)
- Starch is a large, complex carbohydrate molecule requiring simpler testing method.
- Add 5 cm³ of food sample to test tube.
- Add a few drops of iodine solution; mixture turns brown-orange initially.
- Gently shake the test tube to mix contents.
- Blue-black color indicates starch is present in the sample.
- Brown-orange color persists if no starch is present.
Biuret Test (Proteins)
- Use 2 cm³ of food sample in test tube (smaller volume than other tests).
- Add 2 cm³ of biuret solution; mixture turns blue.
- Gently shake the test tube to ensure thorough mixing.
- Pink or purple color indicates protein is present.
- Blue color persists if no protein is present.
- Edexcel-specific method: add potassium hydroxide first, then copper sulfate separately.
Lipid Tests
Sudan 3 Test
- Use 5 cm³ of unfiltered food sample (filtering step not required).
- Add 3 drops of Sudan 3 stain solution to test tube.
- Gently shake to mix the contents thoroughly.
- Bright red layer forms on top if lipids present (lipids separate from water).
Emulsion Test (Edexcel)
- Add 2 cm³ of ethanol to food sample.
- Shake vigorously to mix thoroughly.
- Add equal volume of distilled water to mixture.
- Cloudy white appearance indicates lipids are present.
- Clear solution indicates no lipids present.