1.3 carbohydrates - disaccharides and polysaccharides

• Glucose joined to Glucose forms maltose.
• Glucose joined to fructose forms sucrose
• Glucose joined to galactose forms lactose
• Examples of disaccharides:

  1. Maltose
  2. sucrose
  3. lactose
• when monosaccharides join, a molecule of water is removed and a glycosidic bond is formed.
• when water is added a disaccharide, it breaks the glycosidic bond.
• in a condensation reaction, the glycosidic bond is formed
• in a hydrolysis reaction, the glycosidic bond is broken
• Testing for non-reducing sugars:

  1. Do Benedict’s test
  2. add more of the food sample
  3. add dilute hydrochloric acid to hydrolyse any disaccharides
  4. add sodium hydrogen carbonate to neutralise the acid
  5. retest for benedict’s reagent
• As polysaccharides are very large molecules, they are insoluble
• Test for starch:

  1. add 2cm of sample into a test tube
  2. add 2 drops of iodine solution and shake
  3. positive result is blue-black