Hint: Starch which is a form of complex carbohydrate is composed of a long and branched-chain of glucose monomers. It is found in various foods such as flour. Salivary amylase in saliva is an enzyme for digestion of carbohydrates.
Aim: To understand the action of saliva on flour.
Apparatus: Test tubes, Flour, saliva, droppers, water
- Take three test tubes. Fill it with water. Mark it ‘A’, ‘B’ and ‘C’.
- Add a pinch of flour in the test tube ‘A’ and ‘B’.
- Shake the two test tubes to mix them well.
- Take a few drops out of each test tube on a watch glass. On each watch glass, add 1-2 drops of tincture of iodine.
- Observe the change in colour from hazy white to a blue-black colour ‘A’ and ‘B’. This confirms the presence of starch in those test tubes. No change in colour observed in ‘C’.
- Add a teaspoon of saliva in the test tubes ‘A’ and ‘C’
- Add a few drops of iodine tincture in all the three test tubes. Make sure all the three test tubes contain the same amount of solution.
- Leave the test tubes for 45 minutes.
-Observe all the three test tubes.
Observation: The colour of the test tube ‘A’ changed on the addition of saliva from deep blue-black to colourless. no change in colour was observed test tube’ B’. In the test tube,’ A’ the colour remained light orange-brown.
Note: The linear and helical form of starch is known as amylose while the branched-chain form is known as amylopectin. A tincture of iodine is composed of iodine and potassium iodide. It slips into helical spaces of amylose. This iodine- starch solution imparted the characteristic colour of blue-black to ‘A’ and ‘B’ which contained flour-water-iodine solution while only orange-brown colour in the test tube’ C’. Later after the addition of saliva led to the disappearance of colour in ‘A’ and no change in colour of test tube ‘C’. ‘B’ remained unchanged as saliva was not added. This is due to the presence of salivary amylase in saliva. It breaks down starch into smaller subunits. No long helices of amylose mean no binding of iodine in between its helices. Thus, the blue-black colour fades into transparency in ‘A’ as the action of salivary amylase proceeds.