Describe the tests used to identify the presence of starch, reducing sugars, non-reducing sugars, and proteins in a food sample.

Here's how to test for the food substances:

• i) Starch or carbohydrate

  • Test: Iodine test
  • Procedure: Add a few drops of iodine solution to the food sample.
  • Positive Result: A blue-black colour indicates the presence of starch.

• ii) Reducing sugar

  • Test: Benedict's test
  • Procedure: Add Benedict's solution to the food sample and heat the mixture gently in a water bath.
  • Positive Result: A colour change from blue to green, yellow, orange, or brick-red precipitate indicates the presence of a reducing sugar.

• iii) Non-reducing sugars

  • Test: Hydrolysis followed by Benedict's test
  • Procedure:
    1. Add dilute hydrochloric acid to the food sample and boil gently for a few minutes to hydrolyze the non-reducing sugar into reducing sugars.
    2. Neutralize the solution with sodium bicarbonate or sodium hydroxide.
    3. Then, perform the Benedict's test as described above (add Benedict's solution and heat).
  • Positive Result: A colour change from blue to green, yellow, orange, or brick-red precipitate after hydrolysis and Benedict's test indicates the presence of a non-reducing sugar.

• iv) Proteins

  • Test: Biuret test
  • Procedure: Add sodium hydroxide solution to the food sample, then add a few drops of dilute copper(II) sulphate solution. Shake gently.
  • Positive Result: A purple (violet) colour indicates the presence of protein.

• v) Fats and oils

  • Test: Emulsion test or Translucency test
  • Procedure (Emulsion test): Add ethanol to the food sample, shake well, then pour the ethanol into a test tube containing water.
  • Positive Result (Emulsion test): A white emulsion (cloudy appearance) indicates the presence of fats or oils.
  • Procedure (Translucency test): Rub a small amount of the food sample onto a piece of filter paper and let it dry.
  • Positive Result (Translucency test): A translucent spot on the paper indicates the presence of fats or oils.