Biochemical Tests

Biochemical Tests

1. Reducing sugars

   • 1) Add Benedict's reagent (blue) to the sample and heat it, making sure the solution doesn't boil
   • 2) If there is a reducing sugar present, it will form a coloured precipitate. The colour changes from blue-> green-> yellow-> orange-> brick red
   • The higher the concentration of reducing sugar, the further the colour change goes, and the heavier the precipitate will be

2. Non-reducing sugars

   • 1) Boil the test solution with dilute hydrochloric acid and then neutralise it with sodium hydrogencarbonate
   • 2) Carry out the Benedict's test as you would for a reducing sugar
   • The test for a reducing sugar is done as the positive result for a non-reducing sugar may show even if only a reducing sugar is present

3. Starch

   • 1) Add iodine dissolved in potassium iodide solution to the test solution
   • 2) If starch is present, the sample changes colour from browny-orange to a dark, blue-black colour. If not, the colour will stay browny-orange

4. Proteins

   • 1) Add a few drops of sodium hydroxide solution to the test sample and then add copper (II) sulphate solution
   • 2) If protein is present, a purple layer forms. If not, the solution will stay blue. Both colours are quite pale

5. Lipids

   • 1) Add ethanol to the sample and shake for about a minute. Pour the solution into water
   • 2) If a lipid is present, the solution turns milky. The more lipid there is, the more noticeable this will be. If there is no lipid, the solution will stay clear

6. Glucose

   • 1) Make up several glucose solutions of different concentrations. There should be the same volume of each
   • 2) Do a Benedict's test on each solution. Use the amount of Benedict's solution for each; large enough to react and so there is still some left over
   • 3) Remove any precipitate from the solutions and either leave the test tubes for 24 hours or centrifuge them
   • 4) Use a colorimeter to measure the absorbance of the Benedict's solution remaining in each tube
   • 5) Use the results to make a calibration curve, showing absorbance against glucose concentration
   • Use the same process to find the concentration of an unknown solution using the calibration curve