Food tests: identifying nutrients
Learn the four standard food tests on known samples, then work out which nutrients an unknown food contains.
Objectives
- Carry out tests for starch, reducing sugar, protein and fat.
- Recognise a positive and a negative result for each.
- Identify the nutrients in an unknown sample.
Theory
Starch: iodine solution turns from yellow-brown to blue-black. Reducing sugar (such as glucose): Benedict’s solution, heated in a water bath, changes from blue through green to a brick-red precipitate. Protein: the biuret test turns from blue to purple. Fat: shaken with ethanol and poured into water, fat gives a cloudy white emulsion. A control with distilled water shows what a negative result looks like.
Procedure
- Test glucose solution with Benedict’s solution to see a positive result.
- Test distilled water with any reagent to see a negative result.
- Test food X with iodine solution.
- Test food X with the other three reagents.
Precautions
- Use a clean tube for every test.
- Heat Benedict’s solution in a water bath, never directly.
- Ethanol is flammable: keep it away from flames.
- Subject
- Biology
- Grade
- 8–10
- Chapter
- Nutrition (Nepal National Curriculum (CDC), grade 9)
- Difficulty
- Easy
- Duration
- 20 min
- Safety level
- Low
- Version
- 1.0
Apparatus
- Test tubes
- Food samples
- Iodine solution
- Benedict’s solution
- Biuret reagents
- Ethanol
- Water bath
- Droppers
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