Overview
This lecture reviews key food tests required for GCSE Biology practicals, including procedures and indicators for detecting sugars, starch, proteins, and lipids in food samples.
Preparing a Food Sample
- Break up the food using a mortar and pestle.
- Add the crushed food to a beaker with distilled water and stir.
- Filter the mixture using filter paper in a funnel to remove solids.
- Use the filtered solution as your food sample for tests.
Benedict's Test for Reducing Sugars
- Transfer 5 cm³ of the food sample to a test tube.
- Add 10 drops of Benedict's solution.
- Place the tube in a 75°C water bath for 5 minutes using a test tube holder.
- A color change to green, yellow, or brick red indicates reducing sugars are present; no change means none are present.
Iodine Test for Starch
- Add a few drops of iodine solution to 5 cm³ of the food sample.
- Shake the solution gently.
- A color change from browny-orange to blue-black indicates starch is present.
Biuret Test for Proteins
- Add 2 cm³ of food sample and 2 cm³ of Biuret solution to a test tube.
- Gently shake the mixture.
- A color change from blue to pink or purple indicates protein is present.
- Edexcel requirement: add potassium hydroxide, then copper sulfate, instead of just Biuret solution.
Sudan III Test for Lipids
- Use 5 cm³ of unfiltered food sample.
- Add three drops of Sudan III stain and shake gently.
- A bright red layer on top indicates lipids are present.
Emulsion Test for Lipids (Edexcel Only)
- Add 2 cm³ of ethanol to the food sample and shake vigorously.
- Add an equal volume of distilled water.
- A cloudy white color indicates the presence of lipids.
Key Terms & Definitions
- Reducing sugar — a type of sugar that reacts in the Benedict's test.
- Starch — a large, complex carbohydrate detected by the iodine test.
- Protein — a biological molecule detected by the Biuret test.
- Lipid — fat or oil, detected by the Sudan III or Emulsion test.
- Filtration — separating solid particles from a liquid using filter paper.
Action Items / Next Steps
- Practice preparing food samples and performing each test.
- Review Edexcel-specific steps if required.
- Complete any related practical questions or past paper practice.