Testing for Carbohydrates

Benedict's Test

Aim:
To test a sample of food to determine whether it contains simple sugars.

Equipment:
Bunsen burner,
test tube tongs,
Benedict's Solution,
a sample of food.

Method:
1. Place about 2mL of the sample into a test tube and then add 5 drops of Benedict's solution.
2. Heat with a Bunsen burner until it changes colour. Do not boil.

Results:
A positive test for a simple sugar results in Benedict's Solution changing from a blue colour to an orange colour.

Discussion:
Benedict's Solution is used to test for simple carbohydrates. Benedict's test identifies the monosaccharide's and disaccharides (reducing sugars), which have aldehyde functional groups or free ketone.

Conclusion:
Benedict's Test is used to test for simple sugars (simple carbohydrates). If there are simple carbohydrates present in a food sample, Benedict's solution will change colour, from blue, to orange.

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Iodine Test

Aim:

To test if a sample of food contains complex sugars.

Equipment:

Test tube,

food sample,

iodine solution.

Method:

1. Place about 2mL of the sample in a test tube.

2. Add 3-5 drops of iodine and mix.

Results:

If complex sugars are present, the iodine solution will change from a brown colour to a blue/black colour.

Discussion:

Iodine is used to test for the presence of starch, which is a complex sugar (complex carbohydrate). Starch turns into a very dar black colour once iodine is added, because of an intermolecular charge-transfer complex. If starch isn't present, a brown colour will remain.

Conclusion:

Iodine will turn black in the presence of starch.