Biochemical testing

• How to test for starch

  Add iodine reagent

  positive- turn from yellow/brown to blue/black
• Reducing sugars e.g glucose

  Add benedicts reagent and heat in water bath

  positive- turn from blue to green, yellow, orange or brick red (the closer to brick red the higher the concentration)

  - the coulour change begins at top of solution due to convection currents - molecules have more KE at top so more succesfull collisions an faster reaction
• Non reducing sugars e.g sucrose

  1. negative benedcts test

  2. add sugar and acid to test tube and boil for at least 2 mins in water bath - acid hydrolysis - hydrolyses glycocidic bonds to form monomers

  3. cool solution and add base to neutralise

  4. Add benedicts reagent and heat

  5. positive- turn from blue to green, yellow, orange or brick red (the closer to brick red the higher the concentration)
• What is a reducing and non reducing sugar

  reducing - can reduce the copper sulphate (blue) in benedicts reagent to cooper oxide (brick red)

  non-reducing - the reducing group is used up n the glycocidic bond so cant reduce the copper sulphate, acid hydrolysis breaks the glycosidic bond so the reducing group is exposed allowing for +ve result
• Protein

  1. add biuret reagent to protein solution

  2. positive - turns from blue to lilac
• Lipids e.g oil

  1. disolve in ethanol (shake)

  2. add distilled water and shake

  3. positive- white emulsion forms