Overview
This lecture covers the main food tests required for GCSE Biology practicals, explaining the methods and results for detecting sugars, starch, proteins, and lipids.
Food Sample Preparation
- Break up the food using a mortar and pestle.
- Place the crushed food in a beaker with distilled water and mix.
- Filter the mixture using filter paper in a funnel to obtain a liquid sample.
Benedict's Test for Reducing Sugars
- Add 5 cm³ of the food sample to a test tube.
- Add about 10 drops of Benedict's solution to the sample.
- Place the test tube in a 75°C water bath for 5 minutes using a test tube holder.
- A color change from blue to green, yellow, or brick red indicates reducing sugars are present.
Iodine Test for Starch
- Add 5 cm³ of the food sample to a test tube.
- Add a few drops of iodine solution (browny-orange color), then shake gently.
- A color change from browny-orange to blue-black indicates starch is present.
Biuret Test for Proteins
- Add 2 cm³ of food sample to a test tube.
- Add 2 cm³ of Biuret solution (turns blue), then shake gently.
- If protein is present, the solution turns pink or purple.
- For Edexcel: Add potassium hydroxide, then copper sulfate, instead of Biuret reagent.
Sudan III Test for Lipids
- Add 5 cm³ of unfiltered food sample to a test tube.
- Add 3 drops of Sudan III stain and shake gently.
- A bright red layer forming at the top indicates the presence of lipids.
Emulsion Test for Lipids (Edexcel)
- Add 2 cm³ of food sample to a test tube.
- Add 2 cm³ of ethanol and shake vigorously.
- Add an equal volume of distilled water.
- A cloudy white color indicates lipids are present.
Key Terms & Definitions
- Reducing Sugars — Simple carbohydrates detected by the Benedict's test.
- Starch — A large, complex carbohydrate detected by the iodine test.
- Proteins — Large biological molecules detected by the Biuret test.
- Lipids — Fats and oils detected by Sudan III and Emulsion tests.
Action Items / Next Steps
- Review the procedure and expected results for each food test.
- Practice preparing food samples and performing each test.
- If on Edexcel, learn both the Sudan III and Emulsion tests for lipids.