Testing for sugars

• Reducing sugars 


  1. add access Benedict’s reagent to the sample
  2. heat in a water bath above 80 degrees
• Hazards 


  • Benedict’s is an irritant- goggles, wash hands on contact
  • hot water can cause burns
  • non reducing - HCL and NaOH can be corrosive
• Non-reducing 


  1. add dilute hydrochloric acid and heat in a water bath to break the bonds
  2. add an alkali to neutralise
  3. add excess Benedict’s to the sample
  4. heat in a water bath above 80
• Blue - none

  green - trace

  yellow - low

  orange - moderate

  brick red - high
• Quantitative measurements - sugar 


  1. Filter the solution and weigh the precipitate
  2. remove the precipitate and use a colorimeter to measure the absorbance of the remaining Benedict’s
• testing for startch 


  Add iodine, if starch is present the solution will turn from brown/yellow to blue/black
• Test for proteins 


  1. Add excess biurets reagent to sample
  2. shake and leave at room temperature
  3. if present a purple precipitate forms
• Quantitative measurements - proteins 


  Chromatography can be used to identify amino acids in a mixture
• Test for lipids 


  1. Add ethanol to the sample
  2. add water to the sample and shake for a minute
  3. if present will form a milky white emulsion