Biochemical Tests

• 𝙏𝙀𝙎𝙏 𝙁𝙊𝙍 𝙎𝙏𝘼𝙍𝘾𝙃:

  1. Add iodine;
  2. A positive test = solution turns from orange to blue/black.
• 𝙏𝙀𝙎𝙏 𝙁𝙊𝙍 𝙍𝙀𝘿𝙐𝘾𝙄𝙉𝙂 𝙎𝙐𝙂𝘼𝙍𝙎:

  1. Add Benedict's reagent and heat;
  2. A positive test = solution turns from blue to green, yellow, orange, or brick red (the more brick red the higher the concentration of reducing sugar).
• The colour change observed during the reducing sugar test occurs from the top of the solution downwards because the convection currents in the solution mean that the hottest particles (with the most kinetic energy, more collisions, faster) are at the surface.
• 𝙏𝙀𝙎𝙏 𝙁𝙊𝙍 𝙉𝙊𝙉-𝙍𝙀𝘿𝙐𝘾𝙄𝙉𝙂 𝙎𝙐𝙂𝘼𝙍𝙎:

  1. Following a negative Benedict's test (where the reagent remains blue);
  2. Add HCl and boil;
  3. Cool the solution then add an alkali to neutralise;
  4. Add Benedict's reagent and heat;
  5. A positive test = solution turns from blue to green, yellow, orange, and brick red (the more brick red the higher the concentration of non-reducing sugar).
• Reducing sugars include:

  • glucose
  • fructose
  • galactose
  • lactose
  • maltose
• Non-reducing sugars include:

  • sucrose
• REDUCING SUGARS:

  Reduce the copper sulphate (blue) in Benedict's reagent to copper oxide (brick red).
• NON-REDUCING SUGARS:

  The reducing group is involved in the glycosidic bond in sucrose, and therefore sucrose cannot reduce copper sulphate to copper oxide.

  When sucrose is hydrolysed (through boiling with acid) the glycosidic bond is broken and therefore the reducing group becomes exposed. A positive result is achieved with Benedict's reagent following hydrolysis.
• 𝙏𝙀𝙎𝙏 𝙁𝙊𝙍 𝙋𝙍𝙊𝙏𝙀𝙄𝙉𝙎:

  1. Add biuret;
  2. A positive test = solution turns from blue to purple.
• 𝙏𝙀𝙎𝙏 𝙁𝙊𝙍 𝙇𝙄𝙋𝙄𝘿𝙎:

  1. Dissolve the sample in ethanol;
  2. Add distilled water;
  3. A positive test = a white emulsion forms
• Test for monosaccharides and disaccharides (except sucrose) = Benedict's test
• Test for sucrose (n-r sugar) = acid hydrolysis + Benedict's test
• Test for starch - iodine
• Test for proteins - biuret
• Test for lipids - emulsion test
• 𝘼𝙇𝙒𝘼𝙔𝙎 𝙎𝙏𝘼𝙏𝙀:

  • the reagent to add
  • the conditions (e.g. temperature, mix, alkali, etc)
  • the colour change to indicate the presence of a positive result