Food Tests

[ccyy1993]

1. How is the blue-black colouration formed when iodine is added to starch?

2. How is the brick-red colouration formed when Benedict's solution is added to a solution containing reducing sugars?

3. Why will over-boiling the mixture of Benedict's solution and the test solution result in false results?

4. Why must you add sodium hydroxide and copper sulphate in the Biuret Test?

5. Why must you add water to a solution suspected to contain lipids before adding Sudan IV?

[gen2]

2 Benedict's reagent contains blue copper(II) sulphate (CuSO4) � 5H2O which is reduced to red copper(I) oxide.

[gen2]

1 When starch is mixed with iodine in water, an intensely colored starch/iodine complex is formed. Many of the details of the reaction are still unknown. But it seems that the iodine (in the form of I5- ions) gets stuck in the coils of beta amylose molecules (beta amylose is a soluble starch). The starch forces the iodine atoms into a linear arrangement in the central groove of the amylose coil. There is some transfer of charge between the starch and the iodine. That changes the way electrons are confined, and so, changes spacing of the energy levels. The iodine/starch complex has energy level spacings that are just so for absorbing visible light- giving the complex its intense blue color.

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