Reactions of starch

Reactions of starch

1. Reactions of starch Gandham . Rajeev

2. Experiment: Molisch’s test: 1 ml starch + 2 or 3 drops of Molisch’s reagent. Mix well & add 1-2 ml Conc. Sulphuric acid along the sides of the test tube without shaking. Observation: A reddish violet ring at the junction of two liquids. Inference: Starch is a carbohydrate. Reactions of starch

3. Molisch’s test

4. Composition of Molisch’s reagent: 1% α - naphthol in 95% ethanol. Principle: Carbohydrates when treated with Conc. Sulphuric acid undergo dehydration to give furfural or furfural derivatives. These compounds condense with α - naphthol to form reddish violet ring.

5. General test for all carbohydrates Excess α - naphthol & impurities in reagent give green colored ring. Molisch’s test is given by carbohydrates with at least 5 carbons. Trioses & tetroses do not answer this test. Interaction of acid & water produces heat & can cause charring of carbohydrates (due to precipitation of carbon ) resulting in the formation of black ring. Acid should be layered very slowly. Note

6. Experiment: 2 ml starch + add 2-3 drops of iodine solution. Observation: Deep blue color Inference: Starch is a polysaccharide Iodine test

7. Principle: The amylose component of starch has helical structure & when treated with iodine solution, the iodine gets trapped inside the helical structure giving the solution blue color, which indicates the presence of starch.

8. Experiment: 5 ml Benedict’s reagent + add 8 drops of starch solution . Boil it for 2 mints. Observation: No colored precipitate. Inference: Starch is a non-reducing sugar. Benedict’s test

9. Benedict’s test

10. Experiment: 5 ml starch + add 4 drops of Conc. HCl + keep in boiling water bath for 10 min + cool under running water + neutralize by adding 5 drops of 10% sodium carbonate, till the effervescence ceases. Inference: The solution is hydrolysed Acid hydrolysis

11. Experiment: 5 ml Benedict’s reagent + 8 drops of hydrolysate of starch, mix + boil for 2 min. Observation: Brick red precipitate. Inference: Hydrolysate contains reducing sugar. Benedict’s test after acid hydrolysis

12. Benedict’s test

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