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Food tests

Test for starch

  1. Crush the food sample
  2. Mix the food sample in a test-tube with some warm water to make a suspension.
  3. Add 3 or 4 drops of iodine solution.
  4. If starch is present, the iodine turns dark blue colour. If it is absent, the iodine remains brown.

Test for glucose

  1. Crush the food sample and add it to water
  2. Heat the solution of the food sample with an equal volume of Benedict’s solution in a test-tube. The heating is done by placing the test-tube in a beaker of boiling water.
  3. If the glucose is present, the solution will change from clear blue to cloudy green, then yellow and finally to a red precipitate (deposit) of copper(I) oxide.

Test for protein (Biuret test)

  1. Crush the food sample and mix it with water, 5 cm3 dilute sodium hydroxide (CAUTION: this solution is caustic), followed by 5 cm3 1% copper sulphate solution.
  2. If protein is present, the solution turns purple colour.

Test for fat

  • Crush the food sample and mix it with about 5 cm3 ethanol in a dry test-tube.
  • Leave it for about 5 mins.
  • Pour this solution into a test-tube containing a few cm3 water.
  • If fats are present, a milky white emulsion will form.
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