Biochemical Tests

Question 1

What happens when a reducing sugar is heated with Benedict’s reagent

Answer 1

It forms an insoluble red precipitate of copper (II) oxide

Question 2

Describe the biochemical test for a reducing sugar

Answer 2

1) Add 2cm^3 of the food sample to a test tube. If the sample is not already in liquid form, first grind it up in water.
2) Add an equal volume of Benedict’s reagent.
3) Heat the mixture in a gently boiling water bath for five minutes.
4) If a reducing sugar is present, the solution turns orange-brown

Question 3

What must happen in order for a non-reducing to be detected

Answer 3

It must first be hydrolysed into its monosacharide components

Question 4

Describe the test for a non-reducing sugar

Answer 4

1) If the sample is not already in liquid form, it must first be ground up in water
2) Add 2cm^3 of the food sample to 2cm^3 of Benedict’s in a test tube and filter.
3) Place the solution in a boiling water bath for five minutes and if the Benedict’s reagent does not change colour (remains blue), then a reducing sugar is not present.
4) Add another 2cm^3 of the food sample to 2cm^3 dilute hydrochloric acid in a test tube and place in a gently boiling water bath for five minutes.
5) Slowly add some sodium hydrogencarbonate solution to neutralise the hydrochloric acid as Benedict’s will not work in acidic conditions.
6) Test the solution with PH paper to ensure that it is alkaline.
7) Re-test the resulting solution by heating 2cm^3 of the solution with an equal volume of Benedict’s in a gently boiling water bath
8) If a non-reducing sugar was present in the original food sample, the solution will now turn orange-brown.

Question 5

Describe the test for starch

Answer 5

1) Place 2cm^3 of the sample being tested into a test tube
2) Add two drops of iodine solution and shake or stir.
3) If starch is present, the solution turns blue-black.

Question 6

Describe the test for lipids

Answer 6

1) Take a completely dry and grease-free test tube.
2) Add 5cm^3 of ethanol to 2cm^3 of the sample being tested.
3) Shake the tube to dissolve any lipid in the sample
4) Add 5cm^3 of water and shake gently.
5) A cloudy-white emulsion indicated the presence of a lipid.
6) As a control repeat the method but just add water to the ethanol instead of the sample and the solution should remain clear.

Question 7

Describe the test for proteins

Answer 7

1) Place a sample of the solution being tested into a test tube and add an equal volume of sodium hydroxide at room temperature.
2) Add a few drops of dilute copper (II) sulfate solution and mix gently.
3) If a protein is present, the solution turns from blue to purple.