Biochemical Tests

• Identifying Reducing Sugars:

  1. add 1/4 spatula of sugar to a test tube
  2. add 2cm³ of Benedict's solution
  3. heat in a water bath and observe colour changes

  Results:

  brick red colour if reducing sugars are present
• Identifying Non-Reducing Sugars:

  1. add 1/4 spatula of sugar to a test tube
  2. add 2cm³ HCl and heat in a water bath
  3. cool in a beaker and add sodium bicarbonate until it stops effervescing
  4. add 2cm³ of Benedict's solution
  5. heat in a water bath and observe colour changes

  Results:

  brick red colour if non-reducing sugars are present
• A student investigated the difference in the reducing sugar content of two fruit juices. He performed a biochemical test on each fruit juice using Benedict's solution. He then used a colorimeter with each test result.

  How can the results from the colorimeter identify the fruit juice containing the higher sugar content?

  The fruit juice with a higher concentration of reducing sugar will have a higher absorbance (produces a darker red colour).
• Describe a biochemical test you could use with a solution to confirm that amylase affects starch in agar?

  Heat in Benedict's solution to test for the presence of a reducing sugar; should produce a red precipitate.
• 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
• A biochemical test for reducing sugar produces a negative result with raffinose solution. Describe a biochemical test to show that raffinose solution contains a non-reducing sugar.

  • heat with acid and neutralise;
  • heat with Benedict's solution;
  • red precipitate/colour;
• A precipitate is produced in a positive result for reducing sugar in a Benedict’s test. A precipitate is solid matter suspended in solution. A student carried out the Benedict’s test.

  Suggest a method, other than using a colorimeter, that this student could use to measure the quantity of reducing sugar in a solution.

  • filter and dry the precipitate;
  • find mass/weight;