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Activity 2 Identification tests of the food components

The food we eat is a mixture of many organic substances: lipids, carbohydrates and proteins.
How can we identify them in a foodstuff ?

1 Identification test for proteins

A- Biuret test (performed at cold temperature)

Pour 5 ml of albumin solution (egg white diluted in 100 ml of water)
in a test tube.
Add 2 ml of caustic soda NaOH solution at 40% and few drops of
copper sulfate CuSO at 1%.
4
Agitate the solution. A violet coloration appears.
This reaction identifies the presence of proteins or polypeptides but
never the presence of amino acids.

Doc 1- The Biuret test
B- Coagulation test allows us to detect the
presence of proteins or
Heat an albumin solution in a test tube. Under the effect of heat the polypeptides by
obtaining a violet
albumin coagulates forming white small clots. Similarly, when we heat
coloration.
milk (70°C), a small film forms on the surface: this is the milk albumin.
This test is used to identify proteins except gelatin.

2 Identification test for carbohydrates

A- Glucose paper strips

The paper strips are impregnated with a
reagent that changes color according to the
concentration of glucose found in the
solution.

Dip the strip in the solution for 3 seconds.
Compare the obtained color to the standard of
colors found on the box (Doc. 2).

Doc 2- Determining the presence and the
concentration of glucose using a paper strip.

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