2ND TERM

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SUGGESTED PRACTICALS
FOOD TESTS
The materials / nutrients present in food can be identified using various types of tests. These are:
(1) Test for carbohydrates
(a) To test for simple sugar e.g. glucose and fructose.
Put a small quantity of glucose solution in a test tube.
Add an equal amount of benedict solution.
Boil the mixture for 4 - 6 minutes. A bricks-red or orange precipitate indicates the presence glucose.
(b) To test for complex sugar e.g. sucrose, maltose or lactose
Put a small quantity of the sucrose solution in a test-tube
Add a few drops of dilute hydrochloric acid to the solution, (This hydrolyses the complex sugar to simple sugar)
Place the test-tube in a boiling water bath for a few minutes.
Add a few drops of dilute caustic soda (to neutralize the excess acid).
Add an equal amount of feeling’s solution and place the test-tube in a boiling bath. An orange-red precipitate / yellow precipitate indicate the presence of sucrose.
( c) To test for starch
-Boil a sample of the starch material (e.g. yam / rice)
-Add a few drops of dilute iodine solution to it. A blue-black colouration indicates
the presence of starch.

(2) To test for proteins
(a) Biuret test
-Take a small quantity of fresh milk, egg white solution or malt extract in a test-tube.
-Add a few drops of water and 1cm³ of dilute sodium hydroxide.
-Carefully add 1% of copper II sulphate solution in drops. Shake the mixture thoroughly after each drop.
-A violet, purple, violet colour indicates the presence of protein
-A pink colour indicates the presence of peptones.
(b) Millon’s test
-Put 3cm³ of egg white / colloidal solution of a protein into a test-tube
-Add 3cm³ of millon’s reagent and warm the mixture in a water bath for a few minutes.
-A deep red colour or precipitate shows the presence of protein.
(c) Xanthoproteic test
-Put 2cm³ of egg white or milk solution in a test-tube.
-Carefully add about 1cm³ concentrated trioxonitrate (v) acids. A white precipitate forms which turns yellow on heating.
-Cool the contents and add about 3cm³ of ammonium hydroxide solution. Heat the solution and allow it to cool. The colour of the precipitate deepens to orange indicating the presence of protein.
(3) To test for fats and oils
(a) Translucent mark test
Drop oil on a spot, on a piece of paper or rub the surface of a fatty food against the surface of a piece of white paper. A translucent mark shows the presence of fat.
(b) Sudan III test
-Add a few drops of sudan (iii) solution to some oil in a test-tube. A red colouration is obtained
-Boil the solution. A black precipitate is formed.