Testing for biological molecules

• testing for starch

  • add iodine solution to sample
  • if starch present- colour change
  • yellow/brown-black/blue
• test for reducing sugars

  • reduce or give electrons to other molecules
  • heat reducing sugar with benedicts solution
  • colour change from blue-green-yellow-orange-red
  • benedicts solution contains copper ions which are reduced to copper forming orange-red copper oxide
• test for non reducing sugars

  • hydrolyse bond first to 'free up' reducing groups
  • test for reducing sugars to check there are none
  • take separate sample and boil it with HCL to hydrolyse sucrose into glucose and fructose
  • cool solution and use sodium hydrogen carbonate solution to neutralise it.
  • positive result (green-yellow-orange-red) indicates non reducing sugar
• test for lipids: emulsion test

  • take sample and mix thoroughly with ethanol
  • filter
  • pour solution into water and test tube
  • cloudy white emulsion indicates lipids
• test for proteins: biuret test

  • if protein present light blue-lilac colour change
  • biuret A (sodium hydroxide) add first
  • biuret B (copper sulfate) which is added next
  • place solution of food to be tested on a spotting tile
  • lilac colour indicates protein is present