CBSE Class 9 Answered

1) Testing for the presence of starch (complex sugar) -

(Lugol's reagent (iodine solution) changes from yellowish-brown to dark purple/black.)

Put a small piece of the sample food item into a test tube. Add some water and mash it well. Also set up a control, a test tube containing a liquid that does not contain starch (i.e. water). Now add 3-4 drops of iodine. A bluish black color indicates a positive test for starch. The control will not change colour.

 2) Test for the presence of proteins -

Buiret test - Buiret solution is a blue liquid that changes to purple when proteins are present and to pink in the presence of short chains of polypeptides. The copper atom of the biuret solution reacts with the peptide bonds to cause the color change

Put a small piece of the sample food item into a test tube. If it is not a liquid, add some water and mash it well. Also set up a control, a test tube containing a liquid that does not contain protein (i.e. water). Add about 2ml of Biuret reagent to the test and control tube. If positive, the Biuret solution will turn purple or pink  in the test tube containing the food sample. The control will remain blue.