Carbohydrates

• Monosaccharides

  • are sweet tasting soluble substances with general formula (CH2O)n Where n is any number from 1-7
  • Examples of monosaccharides are glucose, galactose, fructose.
• A monosaccharide is a reducing sugar - a reducing sugar is a sugar that donates electrons to another chemical, in this case, benedict's reagent.

  1. Add 2cm3 of food sample in a test tube in liquid form if not grind food sample in water
  2. Add equal volume of benedict's reagent
  3. Heat mixture in water bath for 5 minutes
  4. Positive results = solution turns to blue black TO orange/brown/red 
• Benedict's reagent is an alkaline solution of copper oxide and when reducing sugar is heated with it it forms a red precipitate because, sugar donates electrons to reduce copper sulphate(blue) to copper oxide(red)
• • Glucose and glucose make maltose
  • Glucose and galactose make lactose
  • Glucose and fructose make sucrose
• Condensation

  When monosaccharides join, a molecule of water is removed. This is a condensation reaction which forms GLYCOSIDIC BONDS.
• hydrolysis


  When water is added to a disaccharide under suitable condition, this breaks the glycosidic bonds, releasing the constituent monosaccharides.
• Tests for non-reducing sugars

  1. Add 2cm3 of food sample to 2cm3 of benedict's reagent in a test tube and filter

  2. Place the test tube in a gently boiling water bath for 5 mins

  3. If no colour change a reducing sugar isn't present

  4. Add another 2cm3 of food sample to 2cm3 dilute hydrochloric acid in a test tube and place in a gently boiling water bath for 5 mins

  5. Slowly add sodium hydrogencarbonate solution to test tube to neutralise the hydrochloric acid (benedicts doesn't work in acidic conditions)

  6. Test pH paper to check if solution is alkaline

  7. Retest resulting solution by heating it with 2cm3 of benedict's reagent in gently boiling water for 5 mins
• Sucrose


  A non-reducing sugar
• If non-reducing sugar is present in the original sample

  Benedict's/reagents will now turn orange/brown = reducing sugars produced from hydrolysis of non-reducing sugars