2.1.1

Tests for Sugars & Starch

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Benedict's Test for Sugars

Benedict’s solution (also known as Benedict's reagent or the Benedict’s test) can be used as a test for reducing and non-reducing sugars.

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Reducing sugars

  • All monosaccharides are reducing sugars.
    • E.g. Glucose, galactose and fructose.
  • Some disaccharides are reducing sugars.
    • E.g. Lactose and maltose.
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Test for reducing sugars

  • Benedict's solution can be reduced by reducing sugars.
  • Benedict's solution is a clear blue liquid that changes colour and gives a precipitate depending on how much it is reduced.
    • Step 1: Place 2 ml of the substance in a boiling tube (substance must be in liquid form).
    • Step 2: Add 10 drops of Benedict's solution.
    • Step 3: Place in a boiling water bath for 3-5 minutes.
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Results of the Benedict's test

  • Blue solution → no reducing sugar.
  • Green/yellow precipitate → traces of reducing sugar.
  • Orange/red precipitate → moderate amounts of reducing sugar.
  • Brick red precipitate → large amount of reducing sugar.
  • The faster the colour change occurs, the greater the concentration of reducing sugar present.
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Non-reducing sugars

  • Non-reducing sugars will show a negative result to the Benedict’s test. A second test is needed to determine if non-reducing sugar is present.
  • Sucrose is a non-reducing sugar. It is a disaccharide made up of glucose and fructose joined by a glycosidic bond.
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Test for non-reducing sugars

  • Step 1: Boil in dilute HCl (to hydrolyse the non-reducing sugar).
  • Step 2: Neutralise the solution by adding sodium bicarbonate.
  • Step 3: Repeat the Benedict’s test.
    • The result will now be positive if a non-reducing sugar is present.
    • If the solution remains blue, then no sugar is present.

The Iodine Test for Starch

A common test for the presence of starch, particularly in food testing, is to add iodine. Iodine is commonly used as a name for a solution of potassium iodide.

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Step 1

  • Place a small sample into the dimple of a spotting tile or into a boiling tube.
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Step 2

  • Add a few drops of iodine and observe any colour change.
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Results

  • If starch is present, the solution will change colour from orange → blue-black.

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