1.3.4 Tests for Biological Molecules

• A wide range of biological molecules can be tested for using simple chemical tests
  • This allows the biological molecules within a sample (such as a food substance) to be determined
• Reducing sugars include simple sugars like glucose that are just made from one unit (monomer), and some sugars made from two units joined together like maltose
  • The presence of reducing sugars can be tested for with a Benedict's test

Test for a reducing sugar

• Add Benedict's reagent (which is blue) to the sample solution in a test tube
• Heat in a water bath that has been set at 75^•C for 5 minutes
• Take the test tube out of the water bath and observe the colour
• A positive test will show a colour change from blue to orange/dark red
  • An orange/dark red precipitate (solid particles suspended in the solution) will form
• A negative test will remain blue
• The amount of colour change is an indication of the amount of reducing sugar present
  • From: blue → green → yellow → orange → red

The Benedict's test for glucose

• We can use iodine to test for the presence or absence of starch in a food sample
• Add drops of iodine solution to the food sample
• A positive test will show a colour change from orange-brown to blue-black
• The colour remains orange-brown if there is no starch present

In the presence of starch, iodine will turn from brown to blue-black

• The biuret test can be used to identify if any protein is present in a sample  
• Add drops of biuret solution (a mixture of sodium hydroxide and copper sulphate) to the food sample
• A positive test will show a colour change from blue to violet / purple
• If there is no protein present, the solution will stay blue

The biuret test for protein

• An emulsion test can be used to identify if any lipids are present in a sample  
• Mix the food sample with 4cm³ of ethanol and shake
• Allow time for the sample to dissolve in the ethanol
• Strain the ethanol solution into another test tube
• Add the ethanol solution to an equal volume of cold distilled water (4cm³)
• A positive test will show a cloudy emulsion forming
  • The more lipid is present, the stronger the milky colour will be

The ethanol test for lipids

Food Test Results Table

Practical investigation safety tips

• If you were carrying out these tests you should try to identify the main hazards and think of ways to reduce harm
• Biuret solution contains copper (II) sulfate which is dangerous particularly if it gets in the eyes, so always wear goggles
• Iodine is also an irritant to the eyes
• Sodium hydroxide in biuret solution is corrosive, if any chemicals get onto your skin wash your hands immediately
• Ethanol is highly flammable; keep it away from any Bunsen burner
• The Bunsen burner itself is a hazard due to the open flame

Worked Example

Exam Tip