Tests for Sugars & Starch
Biology: Cambridge International A Level, topic 2.1.1
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.
Reducing sugars
- All monosaccharides are reducing sugars.
- E.g. Glucose, galactose and fructose.
- Some disaccharides are reducing sugars.
- E.g. Lactose and maltose.
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.
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.
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.
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.
Step 1
- Place a small sample into the dimple of a spotting tile or into a boiling tube.
Step 2
- Add a few drops of iodine and observe any colour change.
Results
- If starch is present, the solution will change colour from orange → blue-black.
Test yourself on Tests for Sugars & Starch
What can the Benedict’s test be used to test?
Test for Non-Reducing Sugars
Which of the following describes Benedict's solution?
If starch is present, what colour change is seen during the iodine test?
What is used to test for starch in food testing?