Food tests — required practical

AQA GCSE Biology

Test a food sample for reducing sugars, starch, protein and lipids.

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Method

  1. Reducing sugars: add Benedict’s solution and heat in a water bath at ~75 °C — blue → green/yellow/brick-red if present.
  2. Starch: add iodine solution — orange-brown → blue-black if present.
  3. Protein: add Biuret solution — blue → purple/lilac if present.
  4. Lipids: shake the sample with ethanol then add water — a cloudy white emulsion forms if present.

Variables

  • Independent variable: The food sample tested
  • Dependent variable: The colour change (presence/absence of each nutrient)
  • Control variables: Volume of reagent, temperature/time for Benedict’s

Results & analysis

Record the final colour for each test and state which nutrients are present. Benedict’s is semi-quantitative — the more reducing sugar, the further the colour moves towards brick-red.

Risk assessment

  • Benedict’s and Biuret are irritants — wear eye protection.
  • Ethanol is flammable — no naked flames (use a water bath for Benedict’s).

Improving accuracy & reliability

  • Use the same volumes each time.
  • Rinse apparatus between tests to avoid contamination.
Exam tip: Learn each reagent, its colour change and the safety point — a 6-marker often asks you to describe testing for all four nutrients.

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