VPB 112: General Veterinary Biochemistry (1+1)

Reactions of polysaccharides ( Starch)

EXERCISE-9: REACTIONS OF POLYSACCHARIDES (STARCH)

• Most important polysaccharides of biological interest are starch, glycogen, and cellulose.

The following experiments are done using starch as a polysaccharide specimen

• Molisch’s Test: (explained earlier)
• Iodine Test
  • To 2 mL of starch solution, add a drop of dil. iodine.
  • A deep blue color is developed. It disappears on warming and reappears on cooling.
  • The color is due to the adsorption of iodine on the colloidal molecules of starch.
  • On warming, the iodine is liberated and the color disappears.
  • On cooling the readsorption of iodine, the blue color appears again.

• Sodium Iodide Test
  • To 2 mL of starch solution, add 2 drops of 10% iodine NaOH and then 2 drops of dil. Iodine.
  • The blue color is not produced.
  • Now, add 2 drops of glacial acetic acid into the same tube.
  • Now, the blue color appears. It is due to the liberation of the free iodine from NaI molecules.
• Acid hydrolysis of Starch
  • Take 5 mL of starch solution in a test tube and add 4-5 drops of Conc. HCl and mix well.
  • Keep the test tube in a boiling water bath.
  • Take out 0.5 mL at frequent intervals and perform both Iodine and Benedict’s test.
  • Observe the color change every time.

Time (minutes)	Iodine Test	Benedict’s Test	Inference
1.5	Blue	Negative (Blue)	Starch
5.0	Blue - Violet	Very Slight Red	Amylodextrin
8.0	Red	Distinct Red	Erythrodextrin
12.0	Brownish Yellow	Marked Red	Achrodextrin
20.0	Negative (yellow)	Red	Maltose / Glucose

Experiment	Observation	Inference