2.2

Question 1

How is the starch test carried out?

Answer 1

Sample will be in potassium iodide solution.

Iodine solution will be added to the sample containing potassium iodide.

This will will cause a colour change from yellow-brown to blue-black.

This colour change is caused by the the iodine forming a triiodide when dissolved in potassium iodide solution. The triiodide ion slips into middle of amylose helix.

Question 2

What are the two ways you can test for reducing sugars?

Answer 2

• Benedicts solution

- Test strip

Question 3

How do you carry out test using Benedict’s solution?

Answer 3

Sample will be suspended in Benedict’s solution (alkaline copper (II) sulfate).

Mixture heated for 3 minutes at 80 degrees Celsius.

Reducing sugars will become reduced donating an electron causing Cu2+ to become Cu+.

This will from an orange-red precipitate called copper oxide (Cu2O)

The colour changed that will be observed is Blue to Green to Yellow to Orange-Red.

If excess Benedict’s solution is used the intensity of the red colour of the precipitate will be proportional to the concentration of the sugar in the solution.

If not a lot of precipitate is formed then a green colour is observed. If there is a lot an orange-red colour is observed.

Question 4

How to test reducing sugars using test strip?

Answer 4

Test strip is dipped into sample causing it change colour.

Colour observed is compared to calibration card provided with test strip.

Allows identification if reducing sugar present or not.

Question 5

How to test for non-reducing sugars?

Answer 5

1) Test for reducing sugars to ensure none are present.
2) Take sample and boil with hydrochloric acid in order to hydrolyse sucrose into glucose and fructose
3) Cool sample and neutralise using hydrogen-carbonate solution.
4) Test for reducing sugars

Positive result will produce a green-yellow-orange-red colour.